libbpf: Split CO-RE logic into relo_core.c.

Move CO-RE logic into separate file.
The internal interface between libbpf and CO-RE is through
bpf_core_apply_relo_insn() function and few structs defined in relo_core.h.

Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/Build

···
 libbpf-y := libbpf.o bpf.o nlattr.o btf.o libbpf_errno.o str_error.o \
 	    netlink.o bpf_prog_linfo.o libbpf_probes.o xsk.o hashmap.o \
-	    btf_dump.o ringbuf.o strset.o linker.o gen_loader.o
+	    btf_dump.o ringbuf.o strset.o linker.o gen_loader.o relo_core.o

tools/lib/bpf/libbpf.c

···
 	       insn->off == 0;
 }
 
-static bool is_ldimm64_insn(struct bpf_insn *insn)
-{
-	return insn->code == (BPF_LD | BPF_IMM | BPF_DW);
-}
-
 static bool is_call_insn(const struct bpf_insn *insn)
 {
 	return insn->code == (BPF_JMP | BPF_CALL);
···
 	return err;
 }
 
-#define BPF_CORE_SPEC_MAX_LEN 64
-
-/* represents BPF CO-RE field or array element accessor */
-struct bpf_core_accessor {
-	__u32 type_id;		/* struct/union type or array element type */
-	__u32 idx;		/* field index or array index */
-	const char *name;	/* field name or NULL for array accessor */
-};
-
-struct bpf_core_spec {
-	const struct btf *btf;
-	/* high-level spec: named fields and array indices only */
-	struct bpf_core_accessor spec[BPF_CORE_SPEC_MAX_LEN];
-	/* original unresolved (no skip_mods_or_typedefs) root type ID */
-	__u32 root_type_id;
-	/* CO-RE relocation kind */
-	enum bpf_core_relo_kind relo_kind;
-	/* high-level spec length */
-	int len;
-	/* raw, low-level spec: 1-to-1 with accessor spec string */
-	int raw_spec[BPF_CORE_SPEC_MAX_LEN];
-	/* raw spec length */
-	int raw_len;
-	/* field bit offset represented by spec */
-	__u32 bit_offset;
-};
-
-static bool str_is_empty(const char *s)
-{
-	return !s || !s[0];
-}
-
-static bool is_flex_arr(const struct btf *btf,
-			const struct bpf_core_accessor *acc,
-			const struct btf_array *arr)
-{
-	const struct btf_type *t;
-
-	/* not a flexible array, if not inside a struct or has non-zero size */
-	if (!acc->name || arr->nelems > 0)
-		return false;
-
-	/* has to be the last member of enclosing struct */
-	t = btf__type_by_id(btf, acc->type_id);
-	return acc->idx == btf_vlen(t) - 1;
-}
-
-static const char *core_relo_kind_str(enum bpf_core_relo_kind kind)
-{
-	switch (kind) {
-	case BPF_FIELD_BYTE_OFFSET: return "byte_off";
-	case BPF_FIELD_BYTE_SIZE: return "byte_sz";
-	case BPF_FIELD_EXISTS: return "field_exists";
-	case BPF_FIELD_SIGNED: return "signed";
-	case BPF_FIELD_LSHIFT_U64: return "lshift_u64";
-	case BPF_FIELD_RSHIFT_U64: return "rshift_u64";
-	case BPF_TYPE_ID_LOCAL: return "local_type_id";
-	case BPF_TYPE_ID_TARGET: return "target_type_id";
-	case BPF_TYPE_EXISTS: return "type_exists";
-	case BPF_TYPE_SIZE: return "type_size";
-	case BPF_ENUMVAL_EXISTS: return "enumval_exists";
-	case BPF_ENUMVAL_VALUE: return "enumval_value";
-	default: return "unknown";
-	}
-}
-
-static bool core_relo_is_field_based(enum bpf_core_relo_kind kind)
-{
-	switch (kind) {
-	case BPF_FIELD_BYTE_OFFSET:
-	case BPF_FIELD_BYTE_SIZE:
-	case BPF_FIELD_EXISTS:
-	case BPF_FIELD_SIGNED:
-	case BPF_FIELD_LSHIFT_U64:
-	case BPF_FIELD_RSHIFT_U64:
-		return true;
-	default:
-		return false;
-	}
-}
-
-static bool core_relo_is_type_based(enum bpf_core_relo_kind kind)
-{
-	switch (kind) {
-	case BPF_TYPE_ID_LOCAL:
-	case BPF_TYPE_ID_TARGET:
-	case BPF_TYPE_EXISTS:
-	case BPF_TYPE_SIZE:
-		return true;
-	default:
-		return false;
-	}
-}
-
-static bool core_relo_is_enumval_based(enum bpf_core_relo_kind kind)
-{
-	switch (kind) {
-	case BPF_ENUMVAL_EXISTS:
-	case BPF_ENUMVAL_VALUE:
-		return true;
-	default:
-		return false;
-	}
-}
-
-/*
- * Turn bpf_core_relo into a low- and high-level spec representation,
- * validating correctness along the way, as well as calculating resulting
- * field bit offset, specified by accessor string. Low-level spec captures
- * every single level of nestedness, including traversing anonymous
- * struct/union members. High-level one only captures semantically meaningful
- * "turning points": named fields and array indicies.
- * E.g., for this case:
- *
- *   struct sample {
- *       int __unimportant;
- *       struct {
- *           int __1;
- *           int __2;
- *           int a[7];
- *       };
- *   };
- *
- *   struct sample *s = ...;
- *
- *   int x = &s->a[3]; // access string = '0:1:2:3'
- *
- * Low-level spec has 1:1 mapping with each element of access string (it's
- * just a parsed access string representation): [0, 1, 2, 3].
- *
- * High-level spec will capture only 3 points:
- *   - intial zero-index access by pointer (&s->... is the same as &s[0]...);
- *   - field 'a' access (corresponds to '2' in low-level spec);
- *   - array element #3 access (corresponds to '3' in low-level spec).
- *
- * Type-based relocations (TYPE_EXISTS/TYPE_SIZE,
- * TYPE_ID_LOCAL/TYPE_ID_TARGET) don't capture any field information. Their
- * spec and raw_spec are kept empty.
- *
- * Enum value-based relocations (ENUMVAL_EXISTS/ENUMVAL_VALUE) use access
- * string to specify enumerator's value index that need to be relocated.
- */
-static int bpf_core_parse_spec(const struct btf *btf,
-			       __u32 type_id,
-			       const char *spec_str,
-			       enum bpf_core_relo_kind relo_kind,
-			       struct bpf_core_spec *spec)
-{
-	int access_idx, parsed_len, i;
-	struct bpf_core_accessor *acc;
-	const struct btf_type *t;
-	const char *name;
-	__u32 id;
-	__s64 sz;
-
-	if (str_is_empty(spec_str) || *spec_str == ':')
-		return -EINVAL;
-
-	memset(spec, 0, sizeof(*spec));
-	spec->btf = btf;
-	spec->root_type_id = type_id;
-	spec->relo_kind = relo_kind;
-
-	/* type-based relocations don't have a field access string */
-	if (core_relo_is_type_based(relo_kind)) {
-		if (strcmp(spec_str, "0"))
-			return -EINVAL;
-		return 0;
-	}
-
-	/* parse spec_str="0:1:2:3:4" into array raw_spec=[0, 1, 2, 3, 4] */
-	while (*spec_str) {
-		if (*spec_str == ':')
-			++spec_str;
-		if (sscanf(spec_str, "%d%n", &access_idx, &parsed_len) != 1)
-			return -EINVAL;
-		if (spec->raw_len == BPF_CORE_SPEC_MAX_LEN)
-			return -E2BIG;
-		spec_str += parsed_len;
-		spec->raw_spec[spec->raw_len++] = access_idx;
-	}
-
-	if (spec->raw_len == 0)
-		return -EINVAL;
-
-	t = skip_mods_and_typedefs(btf, type_id, &id);
-	if (!t)
-		return -EINVAL;
-
-	access_idx = spec->raw_spec[0];
-	acc = &spec->spec[0];
-	acc->type_id = id;
-	acc->idx = access_idx;
-	spec->len++;
-
-	if (core_relo_is_enumval_based(relo_kind)) {
-		if (!btf_is_enum(t) || spec->raw_len > 1 || access_idx >= btf_vlen(t))
-			return -EINVAL;
-
-		/* record enumerator name in a first accessor */
-		acc->name = btf__name_by_offset(btf, btf_enum(t)[access_idx].name_off);
-		return 0;
-	}
-
-	if (!core_relo_is_field_based(relo_kind))
-		return -EINVAL;
-
-	sz = btf__resolve_size(btf, id);
-	if (sz < 0)
-		return sz;
-	spec->bit_offset = access_idx * sz * 8;
-
-	for (i = 1; i < spec->raw_len; i++) {
-		t = skip_mods_and_typedefs(btf, id, &id);
-		if (!t)
-			return -EINVAL;
-
-		access_idx = spec->raw_spec[i];
-		acc = &spec->spec[spec->len];
-
-		if (btf_is_composite(t)) {
-			const struct btf_member *m;
-			__u32 bit_offset;
-
-			if (access_idx >= btf_vlen(t))
-				return -EINVAL;
-
-			bit_offset = btf_member_bit_offset(t, access_idx);
-			spec->bit_offset += bit_offset;
-
-			m = btf_members(t) + access_idx;
-			if (m->name_off) {
-				name = btf__name_by_offset(btf, m->name_off);
-				if (str_is_empty(name))
-					return -EINVAL;
-
-				acc->type_id = id;
-				acc->idx = access_idx;
-				acc->name = name;
-				spec->len++;
-			}
-
-			id = m->type;
-		} else if (btf_is_array(t)) {
-			const struct btf_array *a = btf_array(t);
-			bool flex;
-
-			t = skip_mods_and_typedefs(btf, a->type, &id);
-			if (!t)
-				return -EINVAL;
-
-			flex = is_flex_arr(btf, acc - 1, a);
-			if (!flex && access_idx >= a->nelems)
-				return -EINVAL;
-
-			spec->spec[spec->len].type_id = id;
-			spec->spec[spec->len].idx = access_idx;
-			spec->len++;
-
-			sz = btf__resolve_size(btf, id);
-			if (sz < 0)
-				return sz;
-			spec->bit_offset += access_idx * sz * 8;
-		} else {
-			pr_warn("relo for [%u] %s (at idx %d) captures type [%d] of unexpected kind %s\n",
-				type_id, spec_str, i, id, btf_kind_str(t));
-			return -EINVAL;
-		}
-	}
-
-	return 0;
-}
-
 static bool bpf_core_is_flavor_sep(const char *s)
 {
 	/* check X___Y name pattern, where X and Y are not underscores */
···
  * before last triple underscore. Struct name part after last triple
  * underscore is ignored by BPF CO-RE relocation during relocation matching.
  */
-static size_t bpf_core_essential_name_len(const char *name)
+size_t bpf_core_essential_name_len(const char *name)
 {
 	size_t n = strlen(name);
 	int i;
···
 	return ERR_PTR(err);
 }
 
-/* Check two types for compatibility for the purpose of field access
- * relocation. const/volatile/restrict and typedefs are skipped to ensure we
- * are relocating semantically compatible entities:
- *   - any two STRUCTs/UNIONs are compatible and can be mixed;
- *   - any two FWDs are compatible, if their names match (modulo flavor suffix);
- *   - any two PTRs are always compatible;
- *   - for ENUMs, names should be the same (ignoring flavor suffix) or at
- *     least one of enums should be anonymous;
- *   - for ENUMs, check sizes, names are ignored;
- *   - for INT, size and signedness are ignored;
- *   - any two FLOATs are always compatible;
- *   - for ARRAY, dimensionality is ignored, element types are checked for
- *     compatibility recursively;
- *   - everything else shouldn't be ever a target of relocation.
- * These rules are not set in stone and probably will be adjusted as we get
- * more experience with using BPF CO-RE relocations.
- */
-static int bpf_core_fields_are_compat(const struct btf *local_btf,
-				      __u32 local_id,
-				      const struct btf *targ_btf,
-				      __u32 targ_id)
-{
-	const struct btf_type *local_type, *targ_type;
-
-recur:
-	local_type = skip_mods_and_typedefs(local_btf, local_id, &local_id);
-	targ_type = skip_mods_and_typedefs(targ_btf, targ_id, &targ_id);
-	if (!local_type || !targ_type)
-		return -EINVAL;
-
-	if (btf_is_composite(local_type) && btf_is_composite(targ_type))
-		return 1;
-	if (btf_kind(local_type) != btf_kind(targ_type))
-		return 0;
-
-	switch (btf_kind(local_type)) {
-	case BTF_KIND_PTR:
-	case BTF_KIND_FLOAT:
-		return 1;
-	case BTF_KIND_FWD:
-	case BTF_KIND_ENUM: {
-		const char *local_name, *targ_name;
-		size_t local_len, targ_len;
-
-		local_name = btf__name_by_offset(local_btf,
-						 local_type->name_off);
-		targ_name = btf__name_by_offset(targ_btf, targ_type->name_off);
-		local_len = bpf_core_essential_name_len(local_name);
-		targ_len = bpf_core_essential_name_len(targ_name);
-		/* one of them is anonymous or both w/ same flavor-less names */
-		return local_len == 0 || targ_len == 0 ||
-		       (local_len == targ_len &&
-			strncmp(local_name, targ_name, local_len) == 0);
-	}
-	case BTF_KIND_INT:
-		/* just reject deprecated bitfield-like integers; all other
-		 * integers are by default compatible between each other
-		 */
-		return btf_int_offset(local_type) == 0 &&
-		       btf_int_offset(targ_type) == 0;
-	case BTF_KIND_ARRAY:
-		local_id = btf_array(local_type)->type;
-		targ_id = btf_array(targ_type)->type;
-		goto recur;
-	default:
-		pr_warn("unexpected kind %d relocated, local [%d], target [%d]\n",
-			btf_kind(local_type), local_id, targ_id);
-		return 0;
-	}
-}
-
-/*
- * Given single high-level named field accessor in local type, find
- * corresponding high-level accessor for a target type. Along the way,
- * maintain low-level spec for target as well. Also keep updating target
- * bit offset.
- *
- * Searching is performed through recursive exhaustive enumeration of all
- * fields of a struct/union. If there are any anonymous (embedded)
- * structs/unions, they are recursively searched as well. If field with
- * desired name is found, check compatibility between local and target types,
- * before returning result.
- *
- * 1 is returned, if field is found.
- * 0 is returned if no compatible field is found.
- * <0 is returned on error.
- */
-static int bpf_core_match_member(const struct btf *local_btf,
-				 const struct bpf_core_accessor *local_acc,
-				 const struct btf *targ_btf,
-				 __u32 targ_id,
-				 struct bpf_core_spec *spec,
-				 __u32 *next_targ_id)
-{
-	const struct btf_type *local_type, *targ_type;
-	const struct btf_member *local_member, *m;
-	const char *local_name, *targ_name;
-	__u32 local_id;
-	int i, n, found;
-
-	targ_type = skip_mods_and_typedefs(targ_btf, targ_id, &targ_id);
-	if (!targ_type)
-		return -EINVAL;
-	if (!btf_is_composite(targ_type))
-		return 0;
-
-	local_id = local_acc->type_id;
-	local_type = btf__type_by_id(local_btf, local_id);
-	local_member = btf_members(local_type) + local_acc->idx;
-	local_name = btf__name_by_offset(local_btf, local_member->name_off);
-
-	n = btf_vlen(targ_type);
-	m = btf_members(targ_type);
-	for (i = 0; i < n; i++, m++) {
-		__u32 bit_offset;
-
-		bit_offset = btf_member_bit_offset(targ_type, i);
-
-		/* too deep struct/union/array nesting */
-		if (spec->raw_len == BPF_CORE_SPEC_MAX_LEN)
-			return -E2BIG;
-
-		/* speculate this member will be the good one */
-		spec->bit_offset += bit_offset;
-		spec->raw_spec[spec->raw_len++] = i;
-
-		targ_name = btf__name_by_offset(targ_btf, m->name_off);
-		if (str_is_empty(targ_name)) {
-			/* embedded struct/union, we need to go deeper */
-			found = bpf_core_match_member(local_btf, local_acc,
-						      targ_btf, m->type,
-						      spec, next_targ_id);
-			if (found) /* either found or error */
-				return found;
-		} else if (strcmp(local_name, targ_name) == 0) {
-			/* matching named field */
-			struct bpf_core_accessor *targ_acc;
-
-			targ_acc = &spec->spec[spec->len++];
-			targ_acc->type_id = targ_id;
-			targ_acc->idx = i;
-			targ_acc->name = targ_name;
-
-			*next_targ_id = m->type;
-			found = bpf_core_fields_are_compat(local_btf,
-							   local_member->type,
-							   targ_btf, m->type);
-			if (!found)
-				spec->len--; /* pop accessor */
-			return found;
-		}
-		/* member turned out not to be what we looked for */
-		spec->bit_offset -= bit_offset;
-		spec->raw_len--;
-	}
-
-	return 0;
-}
-
 /* Check local and target types for compatibility. This check is used for
  * type-based CO-RE relocations and follow slightly different rules than
  * field-based relocations. This function assumes that root types were already
···
  * These rules are not set in stone and probably will be adjusted as we get
  * more experience with using BPF CO-RE relocations.
  */
-static int bpf_core_types_are_compat(const struct btf *local_btf, __u32 local_id,
-				     const struct btf *targ_btf, __u32 targ_id)
+int bpf_core_types_are_compat(const struct btf *local_btf, __u32 local_id,
+			      const struct btf *targ_btf, __u32 targ_id)
 {
 	const struct btf_type *local_type, *targ_type;
 	int depth = 32; /* max recursion depth */
···
 	}
 }
 
-/*
- * Try to match local spec to a target type and, if successful, produce full
- * target spec (high-level, low-level + bit offset).
- */
-static int bpf_core_spec_match(struct bpf_core_spec *local_spec,
-			       const struct btf *targ_btf, __u32 targ_id,
-			       struct bpf_core_spec *targ_spec)
-{
-	const struct btf_type *targ_type;
-	const struct bpf_core_accessor *local_acc;
-	struct bpf_core_accessor *targ_acc;
-	int i, sz, matched;
-
-	memset(targ_spec, 0, sizeof(*targ_spec));
-	targ_spec->btf = targ_btf;
-	targ_spec->root_type_id = targ_id;
-	targ_spec->relo_kind = local_spec->relo_kind;
-
-	if (core_relo_is_type_based(local_spec->relo_kind)) {
-		return bpf_core_types_are_compat(local_spec->btf,
-						 local_spec->root_type_id,
-						 targ_btf, targ_id);
-	}
-
-	local_acc = &local_spec->spec[0];
-	targ_acc = &targ_spec->spec[0];
-
-	if (core_relo_is_enumval_based(local_spec->relo_kind)) {
-		size_t local_essent_len, targ_essent_len;
-		const struct btf_enum *e;
-		const char *targ_name;
-
-		/* has to resolve to an enum */
-		targ_type = skip_mods_and_typedefs(targ_spec->btf, targ_id, &targ_id);
-		if (!btf_is_enum(targ_type))
-			return 0;
-
-		local_essent_len = bpf_core_essential_name_len(local_acc->name);
-
-		for (i = 0, e = btf_enum(targ_type); i < btf_vlen(targ_type); i++, e++) {
-			targ_name = btf__name_by_offset(targ_spec->btf, e->name_off);
-			targ_essent_len = bpf_core_essential_name_len(targ_name);
-			if (targ_essent_len != local_essent_len)
-				continue;
-			if (strncmp(local_acc->name, targ_name, local_essent_len) == 0) {
-				targ_acc->type_id = targ_id;
-				targ_acc->idx = i;
-				targ_acc->name = targ_name;
-				targ_spec->len++;
-				targ_spec->raw_spec[targ_spec->raw_len] = targ_acc->idx;
-				targ_spec->raw_len++;
-				return 1;
-			}
-		}
-		return 0;
-	}
-
-	if (!core_relo_is_field_based(local_spec->relo_kind))
-		return -EINVAL;
-
-	for (i = 0; i < local_spec->len; i++, local_acc++, targ_acc++) {
-		targ_type = skip_mods_and_typedefs(targ_spec->btf, targ_id,
-						   &targ_id);
-		if (!targ_type)
-			return -EINVAL;
-
-		if (local_acc->name) {
-			matched = bpf_core_match_member(local_spec->btf,
-							local_acc,
-							targ_btf, targ_id,
-							targ_spec, &targ_id);
-			if (matched <= 0)
-				return matched;
-		} else {
-			/* for i=0, targ_id is already treated as array element
-			 * type (because it's the original struct), for others
-			 * we should find array element type first
-			 */
-			if (i > 0) {
-				const struct btf_array *a;
-				bool flex;
-
-				if (!btf_is_array(targ_type))
-					return 0;
-
-				a = btf_array(targ_type);
-				flex = is_flex_arr(targ_btf, targ_acc - 1, a);
-				if (!flex && local_acc->idx >= a->nelems)
-					return 0;
-				if (!skip_mods_and_typedefs(targ_btf, a->type,
-							    &targ_id))
-					return -EINVAL;
-			}
-
-			/* too deep struct/union/array nesting */
-			if (targ_spec->raw_len == BPF_CORE_SPEC_MAX_LEN)
-				return -E2BIG;
-
-			targ_acc->type_id = targ_id;
-			targ_acc->idx = local_acc->idx;
-			targ_acc->name = NULL;
-			targ_spec->len++;
-			targ_spec->raw_spec[targ_spec->raw_len] = targ_acc->idx;
-			targ_spec->raw_len++;
-
-			sz = btf__resolve_size(targ_btf, targ_id);
-			if (sz < 0)
-				return sz;
-			targ_spec->bit_offset += local_acc->idx * sz * 8;
-		}
-	}
-
-	return 1;
-}
-
-static int bpf_core_calc_field_relo(const char *prog_name,
-				    const struct bpf_core_relo *relo,
-				    const struct bpf_core_spec *spec,
-				    __u32 *val, __u32 *field_sz, __u32 *type_id,
-				    bool *validate)
-{
-	const struct bpf_core_accessor *acc;
-	const struct btf_type *t;
-	__u32 byte_off, byte_sz, bit_off, bit_sz, field_type_id;
-	const struct btf_member *m;
-	const struct btf_type *mt;
-	bool bitfield;
-	__s64 sz;
-
-	*field_sz = 0;
-
-	if (relo->kind == BPF_FIELD_EXISTS) {
-		*val = spec ? 1 : 0;
-		return 0;
-	}
-
-	if (!spec)
-		return -EUCLEAN; /* request instruction poisoning */
-
-	acc = &spec->spec[spec->len - 1];
-	t = btf__type_by_id(spec->btf, acc->type_id);
-
-	/* a[n] accessor needs special handling */
-	if (!acc->name) {
-		if (relo->kind == BPF_FIELD_BYTE_OFFSET) {
-			*val = spec->bit_offset / 8;
-			/* remember field size for load/store mem size */
-			sz = btf__resolve_size(spec->btf, acc->type_id);
-			if (sz < 0)
-				return -EINVAL;
-			*field_sz = sz;
-			*type_id = acc->type_id;
-		} else if (relo->kind == BPF_FIELD_BYTE_SIZE) {
-			sz = btf__resolve_size(spec->btf, acc->type_id);
-			if (sz < 0)
-				return -EINVAL;
-			*val = sz;
-		} else {
-			pr_warn("prog '%s': relo %d at insn #%d can't be applied to array access\n",
-				prog_name, relo->kind, relo->insn_off / 8);
-			return -EINVAL;
-		}
-		if (validate)
-			*validate = true;
-		return 0;
-	}
-
-	m = btf_members(t) + acc->idx;
-	mt = skip_mods_and_typedefs(spec->btf, m->type, &field_type_id);
-	bit_off = spec->bit_offset;
-	bit_sz = btf_member_bitfield_size(t, acc->idx);
-
-	bitfield = bit_sz > 0;
-	if (bitfield) {
-		byte_sz = mt->size;
-		byte_off = bit_off / 8 / byte_sz * byte_sz;
-		/* figure out smallest int size necessary for bitfield load */
-		while (bit_off + bit_sz - byte_off * 8 > byte_sz * 8) {
-			if (byte_sz >= 8) {
-				/* bitfield can't be read with 64-bit read */
-				pr_warn("prog '%s': relo %d at insn #%d can't be satisfied for bitfield\n",
-					prog_name, relo->kind, relo->insn_off / 8);
-				return -E2BIG;
-			}
-			byte_sz *= 2;
-			byte_off = bit_off / 8 / byte_sz * byte_sz;
-		}
-	} else {
-		sz = btf__resolve_size(spec->btf, field_type_id);
-		if (sz < 0)
-			return -EINVAL;
-		byte_sz = sz;
-		byte_off = spec->bit_offset / 8;
-		bit_sz = byte_sz * 8;
-	}
-
-	/* for bitfields, all the relocatable aspects are ambiguous and we
-	 * might disagree with compiler, so turn off validation of expected
-	 * value, except for signedness
-	 */
-	if (validate)
-		*validate = !bitfield;
-
-	switch (relo->kind) {
-	case BPF_FIELD_BYTE_OFFSET:
-		*val = byte_off;
-		if (!bitfield) {
-			*field_sz = byte_sz;
-			*type_id = field_type_id;
-		}
-		break;
-	case BPF_FIELD_BYTE_SIZE:
-		*val = byte_sz;
-		break;
-	case BPF_FIELD_SIGNED:
-		/* enums will be assumed unsigned */
-		*val = btf_is_enum(mt) ||
-		       (btf_int_encoding(mt) & BTF_INT_SIGNED);
-		if (validate)
-			*validate = true; /* signedness is never ambiguous */
-		break;
-	case BPF_FIELD_LSHIFT_U64:
-#if __BYTE_ORDER == __LITTLE_ENDIAN
-		*val = 64 - (bit_off + bit_sz - byte_off  * 8);
-#else
-		*val = (8 - byte_sz) * 8 + (bit_off - byte_off * 8);
-#endif
-		break;
-	case BPF_FIELD_RSHIFT_U64:
-		*val = 64 - bit_sz;
-		if (validate)
-			*validate = true; /* right shift is never ambiguous */
-		break;
-	case BPF_FIELD_EXISTS:
-	default:
-		return -EOPNOTSUPP;
-	}
-
-	return 0;
-}
-
-static int bpf_core_calc_type_relo(const struct bpf_core_relo *relo,
-				   const struct bpf_core_spec *spec,
-				   __u32 *val)
-{
-	__s64 sz;
-
-	/* type-based relos return zero when target type is not found */
-	if (!spec) {
-		*val = 0;
-		return 0;
-	}
-
-	switch (relo->kind) {
-	case BPF_TYPE_ID_TARGET:
-		*val = spec->root_type_id;
-		break;
-	case BPF_TYPE_EXISTS:
-		*val = 1;
-		break;
-	case BPF_TYPE_SIZE:
-		sz = btf__resolve_size(spec->btf, spec->root_type_id);
-		if (sz < 0)
-			return -EINVAL;
-		*val = sz;
-		break;
-	case BPF_TYPE_ID_LOCAL:
-	/* BPF_TYPE_ID_LOCAL is handled specially and shouldn't get here */
-	default:
-		return -EOPNOTSUPP;
-	}
-
-	return 0;
-}
-
-static int bpf_core_calc_enumval_relo(const struct bpf_core_relo *relo,
-				      const struct bpf_core_spec *spec,
-				      __u32 *val)
-{
-	const struct btf_type *t;
-	const struct btf_enum *e;
-
-	switch (relo->kind) {
-	case BPF_ENUMVAL_EXISTS:
-		*val = spec ? 1 : 0;
-		break;
-	case BPF_ENUMVAL_VALUE:
-		if (!spec)
-			return -EUCLEAN; /* request instruction poisoning */
-		t = btf__type_by_id(spec->btf, spec->spec[0].type_id);
-		e = btf_enum(t) + spec->spec[0].idx;
-		*val = e->val;
-		break;
-	default:
-		return -EOPNOTSUPP;
-	}
-
-	return 0;
-}
-
-struct bpf_core_relo_res
-{
-	/* expected value in the instruction, unless validate == false */
-	__u32 orig_val;
-	/* new value that needs to be patched up to */
-	__u32 new_val;
-	/* relocation unsuccessful, poison instruction, but don't fail load */
-	bool poison;
-	/* some relocations can't be validated against orig_val */
-	bool validate;
-	/* for field byte offset relocations or the forms:
-	 *     *(T *)(rX + <off>) = rY
-	 *     rX = *(T *)(rY + <off>),
-	 * we remember original and resolved field size to adjust direct
-	 * memory loads of pointers and integers; this is necessary for 32-bit
-	 * host kernel architectures, but also allows to automatically
-	 * relocate fields that were resized from, e.g., u32 to u64, etc.
-	 */
-	bool fail_memsz_adjust;
-	__u32 orig_sz;
-	__u32 orig_type_id;
-	__u32 new_sz;
-	__u32 new_type_id;
-};
-
-/* Calculate original and target relocation values, given local and target
- * specs and relocation kind. These values are calculated for each candidate.
- * If there are multiple candidates, resulting values should all be consistent
- * with each other. Otherwise, libbpf will refuse to proceed due to ambiguity.
- * If instruction has to be poisoned, *poison will be set to true.
- */
-static int bpf_core_calc_relo(const char *prog_name,
-			      const struct bpf_core_relo *relo,
-			      int relo_idx,
-			      const struct bpf_core_spec *local_spec,
-			      const struct bpf_core_spec *targ_spec,
-			      struct bpf_core_relo_res *res)
-{
-	int err = -EOPNOTSUPP;
-
-	res->orig_val = 0;
-	res->new_val = 0;
-	res->poison = false;
-	res->validate = true;
-	res->fail_memsz_adjust = false;
-	res->orig_sz = res->new_sz = 0;
-	res->orig_type_id = res->new_type_id = 0;
-
-	if (core_relo_is_field_based(relo->kind)) {
-		err = bpf_core_calc_field_relo(prog_name, relo, local_spec,
-					       &res->orig_val, &res->orig_sz,
-					       &res->orig_type_id, &res->validate);
-		err = err ?: bpf_core_calc_field_relo(prog_name, relo, targ_spec,
-						      &res->new_val, &res->new_sz,
-						      &res->new_type_id, NULL);
-		if (err)
-			goto done;
-		/* Validate if it's safe to adjust load/store memory size.
-		 * Adjustments are performed only if original and new memory
-		 * sizes differ.
-		 */
-		res->fail_memsz_adjust = false;
-		if (res->orig_sz != res->new_sz) {
-			const struct btf_type *orig_t, *new_t;
-
-			orig_t = btf__type_by_id(local_spec->btf, res->orig_type_id);
-			new_t = btf__type_by_id(targ_spec->btf, res->new_type_id);
-
-			/* There are two use cases in which it's safe to
-			 * adjust load/store's mem size:
-			 *   - reading a 32-bit kernel pointer, while on BPF
-			 *   size pointers are always 64-bit; in this case
-			 *   it's safe to "downsize" instruction size due to
-			 *   pointer being treated as unsigned integer with
-			 *   zero-extended upper 32-bits;
-			 *   - reading unsigned integers, again due to
-			 *   zero-extension is preserving the value correctly.
-			 *
-			 * In all other cases it's incorrect to attempt to
-			 * load/store field because read value will be
-			 * incorrect, so we poison relocated instruction.
-			 */
-			if (btf_is_ptr(orig_t) && btf_is_ptr(new_t))
-				goto done;
-			if (btf_is_int(orig_t) && btf_is_int(new_t) &&
-			    btf_int_encoding(orig_t) != BTF_INT_SIGNED &&
-			    btf_int_encoding(new_t) != BTF_INT_SIGNED)
-				goto done;
-
-			/* mark as invalid mem size adjustment, but this will
-			 * only be checked for LDX/STX/ST insns
-			 */
-			res->fail_memsz_adjust = true;
-		}
-	} else if (core_relo_is_type_based(relo->kind)) {
-		err = bpf_core_calc_type_relo(relo, local_spec, &res->orig_val);
-		err = err ?: bpf_core_calc_type_relo(relo, targ_spec, &res->new_val);
-	} else if (core_relo_is_enumval_based(relo->kind)) {
-		err = bpf_core_calc_enumval_relo(relo, local_spec, &res->orig_val);
-		err = err ?: bpf_core_calc_enumval_relo(relo, targ_spec, &res->new_val);
-	}
-
-done:
-	if (err == -EUCLEAN) {
-		/* EUCLEAN is used to signal instruction poisoning request */
-		res->poison = true;
-		err = 0;
-	} else if (err == -EOPNOTSUPP) {
-		/* EOPNOTSUPP means unknown/unsupported relocation */
-		pr_warn("prog '%s': relo #%d: unrecognized CO-RE relocation %s (%d) at insn #%d\n",
-			prog_name, relo_idx, core_relo_kind_str(relo->kind),
-			relo->kind, relo->insn_off / 8);
-	}
-
-	return err;
-}
-
-/*
- * Turn instruction for which CO_RE relocation failed into invalid one with
- * distinct signature.
- */
-static void bpf_core_poison_insn(const char *prog_name, int relo_idx,
-				 int insn_idx, struct bpf_insn *insn)
-{
-	pr_debug("prog '%s': relo #%d: substituting insn #%d w/ invalid insn\n",
-		 prog_name, relo_idx, insn_idx);
-	insn->code = BPF_JMP | BPF_CALL;
-	insn->dst_reg = 0;
-	insn->src_reg = 0;
-	insn->off = 0;
-	/* if this instruction is reachable (not a dead code),
-	 * verifier will complain with the following message:
-	 * invalid func unknown#195896080
-	 */
-	insn->imm = 195896080; /* => 0xbad2310 => "bad relo" */
-}
-
-static int insn_bpf_size_to_bytes(struct bpf_insn *insn)
-{
-	switch (BPF_SIZE(insn->code)) {
-	case BPF_DW: return 8;
-	case BPF_W: return 4;
-	case BPF_H: return 2;
-	case BPF_B: return 1;
-	default: return -1;
-	}
-}
-
-static int insn_bytes_to_bpf_size(__u32 sz)
-{
-	switch (sz) {
-	case 8: return BPF_DW;
-	case 4: return BPF_W;
-	case 2: return BPF_H;
-	case 1: return BPF_B;
-	default: return -1;
-	}
-}
-
-/*
- * Patch relocatable BPF instruction.
- *
- * Patched value is determined by relocation kind and target specification.
- * For existence relocations target spec will be NULL if field/type is not found.
- * Expected insn->imm value is determined using relocation kind and local
- * spec, and is checked before patching instruction. If actual insn->imm value
- * is wrong, bail out with error.
- *
- * Currently supported classes of BPF instruction are:
- * 1. rX = <imm> (assignment with immediate operand);
- * 2. rX += <imm> (arithmetic operations with immediate operand);
- * 3. rX = <imm64> (load with 64-bit immediate value);
- * 4. rX = *(T *)(rY + <off>), where T is one of {u8, u16, u32, u64};
- * 5. *(T *)(rX + <off>) = rY, where T is one of {u8, u16, u32, u64};
- * 6. *(T *)(rX + <off>) = <imm>, where T is one of {u8, u16, u32, u64}.
- */
-static int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
-			       int insn_idx, const struct bpf_core_relo *relo,
-			       int relo_idx, const struct bpf_core_relo_res *res)
-{
-	__u32 orig_val, new_val;
-	__u8 class;
-
-	class = BPF_CLASS(insn->code);
-
-	if (res->poison) {
-poison:
-		/* poison second part of ldimm64 to avoid confusing error from
-		 * verifier about "unknown opcode 00"
-		 */
-		if (is_ldimm64_insn(insn))
-			bpf_core_poison_insn(prog_name, relo_idx, insn_idx + 1, insn + 1);
-		bpf_core_poison_insn(prog_name, relo_idx, insn_idx, insn);
-		return 0;
-	}
-
-	orig_val = res->orig_val;
-	new_val = res->new_val;
-
-	switch (class) {
-	case BPF_ALU:
-	case BPF_ALU64:
-		if (BPF_SRC(insn->code) != BPF_K)
-			return -EINVAL;
-		if (res->validate && insn->imm != orig_val) {
-			pr_warn("prog '%s': relo #%d: unexpected insn #%d (ALU/ALU64) value: got %u, exp %u -> %u\n",
-				prog_name, relo_idx,
-				insn_idx, insn->imm, orig_val, new_val);
-			return -EINVAL;
-		}
-		orig_val = insn->imm;
-		insn->imm = new_val;
-		pr_debug("prog '%s': relo #%d: patched insn #%d (ALU/ALU64) imm %u -> %u\n",
-			 prog_name, relo_idx, insn_idx,
-			 orig_val, new_val);
-		break;
-	case BPF_LDX:
-	case BPF_ST:
-	case BPF_STX:
-		if (res->validate && insn->off != orig_val) {
-			pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDX/ST/STX) value: got %u, exp %u -> %u\n",
-				prog_name, relo_idx, insn_idx, insn->off, orig_val, new_val);
-			return -EINVAL;
-		}
-		if (new_val > SHRT_MAX) {
-			pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) value too big: %u\n",
-				prog_name, relo_idx, insn_idx, new_val);
-			return -ERANGE;
-		}
-		if (res->fail_memsz_adjust) {
-			pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) accesses field incorrectly. "
-				"Make sure you are accessing pointers, unsigned integers, or fields of matching type and size.\n",
-				prog_name, relo_idx, insn_idx);
-			goto poison;
-		}
-
-		orig_val = insn->off;
-		insn->off = new_val;
-		pr_debug("prog '%s': relo #%d: patched insn #%d (LDX/ST/STX) off %u -> %u\n",
-			 prog_name, relo_idx, insn_idx, orig_val, new_val);
-
-		if (res->new_sz != res->orig_sz) {
-			int insn_bytes_sz, insn_bpf_sz;
-
-			insn_bytes_sz = insn_bpf_size_to_bytes(insn);
-			if (insn_bytes_sz != res->orig_sz) {
-				pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) unexpected mem size: got %d, exp %u\n",
-					prog_name, relo_idx, insn_idx, insn_bytes_sz, res->orig_sz);
-				return -EINVAL;
-			}
-
-			insn_bpf_sz = insn_bytes_to_bpf_size(res->new_sz);
-			if (insn_bpf_sz < 0) {
-				pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) invalid new mem size: %u\n",
-					prog_name, relo_idx, insn_idx, res->new_sz);
-				return -EINVAL;
-			}
-
-			insn->code = BPF_MODE(insn->code) | insn_bpf_sz | BPF_CLASS(insn->code);
-			pr_debug("prog '%s': relo #%d: patched insn #%d (LDX/ST/STX) mem_sz %u -> %u\n",
-				 prog_name, relo_idx, insn_idx, res->orig_sz, res->new_sz);
-		}
-		break;
-	case BPF_LD: {
-		__u64 imm;
-
-		if (!is_ldimm64_insn(insn) ||
-		    insn[0].src_reg != 0 || insn[0].off != 0 ||
-		    insn[1].code != 0 || insn[1].dst_reg != 0 ||
-		    insn[1].src_reg != 0 || insn[1].off != 0) {
-			pr_warn("prog '%s': relo #%d: insn #%d (LDIMM64) has unexpected form\n",
-				prog_name, relo_idx, insn_idx);
-			return -EINVAL;
-		}
-
-		imm = insn[0].imm + ((__u64)insn[1].imm << 32);
-		if (res->validate && imm != orig_val) {
-			pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDIMM64) value: got %llu, exp %u -> %u\n",
-				prog_name, relo_idx,
-				insn_idx, (unsigned long long)imm,
-				orig_val, new_val);
-			return -EINVAL;
-		}
-
-		insn[0].imm = new_val;
-		insn[1].imm = 0; /* currently only 32-bit values are supported */
-		pr_debug("prog '%s': relo #%d: patched insn #%d (LDIMM64) imm64 %llu -> %u\n",
-			 prog_name, relo_idx, insn_idx,
-			 (unsigned long long)imm, new_val);
-		break;
-	}
-	default:
-		pr_warn("prog '%s': relo #%d: trying to relocate unrecognized insn #%d, code:0x%x, src:0x%x, dst:0x%x, off:0x%x, imm:0x%x\n",
-			prog_name, relo_idx, insn_idx, insn->code,
-			insn->src_reg, insn->dst_reg, insn->off, insn->imm);
-		return -EINVAL;
-	}
-
-	return 0;
-}
-
-/* Output spec definition in the format:
- * [<type-id>] (<type-name>) + <raw-spec> => <offset>@<spec>,
- * where <spec> is a C-syntax view of recorded field access, e.g.: x.a[3].b
- */
-static void bpf_core_dump_spec(int level, const struct bpf_core_spec *spec)
-{
-	const struct btf_type *t;
-	const struct btf_enum *e;
-	const char *s;
-	__u32 type_id;
-	int i;
-
-	type_id = spec->root_type_id;
-	t = btf__type_by_id(spec->btf, type_id);
-	s = btf__name_by_offset(spec->btf, t->name_off);
-
-	libbpf_print(level, "[%u] %s %s", type_id, btf_kind_str(t), str_is_empty(s) ? "<anon>" : s);
-
-	if (core_relo_is_type_based(spec->relo_kind))
-		return;
-
-	if (core_relo_is_enumval_based(spec->relo_kind)) {
-		t = skip_mods_and_typedefs(spec->btf, type_id, NULL);
-		e = btf_enum(t) + spec->raw_spec[0];
-		s = btf__name_by_offset(spec->btf, e->name_off);
-
-		libbpf_print(level, "::%s = %u", s, e->val);
-		return;
-	}
-
-	if (core_relo_is_field_based(spec->relo_kind)) {
-		for (i = 0; i < spec->len; i++) {
-			if (spec->spec[i].name)
-				libbpf_print(level, ".%s", spec->spec[i].name);
-			else if (i > 0 || spec->spec[i].idx > 0)
-				libbpf_print(level, "[%u]", spec->spec[i].idx);
-		}
-
-		libbpf_print(level, " (");
-		for (i = 0; i < spec->raw_len; i++)
-			libbpf_print(level, "%s%d", i == 0 ? "" : ":", spec->raw_spec[i]);
-
-		if (spec->bit_offset % 8)
-			libbpf_print(level, " @ offset %u.%u)",
-				     spec->bit_offset / 8, spec->bit_offset % 8);
-		else
-			libbpf_print(level, " @ offset %u)", spec->bit_offset / 8);
-		return;
-	}
-}
-
 static size_t bpf_core_hash_fn(const void *key, void *ctx)
 {
 	return (size_t)key;
···
 static void *u32_as_hash_key(__u32 x)
 {
 	return (void *)(uintptr_t)x;
-}
-
-/*
- * CO-RE relocate single instruction.
- *
- * The outline and important points of the algorithm:
- * 1. For given local type, find corresponding candidate target types.
- *    Candidate type is a type with the same "essential" name, ignoring
- *    everything after last triple underscore (___). E.g., `sample`,
- *    `sample___flavor_one`, `sample___flavor_another_one`, are all candidates
- *    for each other. Names with triple underscore are referred to as
- *    "flavors" and are useful, among other things, to allow to
- *    specify/support incompatible variations of the same kernel struct, which
- *    might differ between different kernel versions and/or build
- *    configurations.
- *
- *    N.B. Struct "flavors" could be generated by bpftool's BTF-to-C
- *    converter, when deduplicated BTF of a kernel still contains more than
- *    one different types with the same name. In that case, ___2, ___3, etc
- *    are appended starting from second name conflict. But start flavors are
- *    also useful to be defined "locally", in BPF program, to extract same
- *    data from incompatible changes between different kernel
- *    versions/configurations. For instance, to handle field renames between
- *    kernel versions, one can use two flavors of the struct name with the
- *    same common name and use conditional relocations to extract that field,
- *    depending on target kernel version.
- * 2. For each candidate type, try to match local specification to this
- *    candidate target type. Matching involves finding corresponding
- *    high-level spec accessors, meaning that all named fields should match,
- *    as well as all array accesses should be within the actual bounds. Also,
- *    types should be compatible (see bpf_core_fields_are_compat for details).
- * 3. It is supported and expected that there might be multiple flavors
- *    matching the spec. As long as all the specs resolve to the same set of
- *    offsets across all candidates, there is no error. If there is any
- *    ambiguity, CO-RE relocation will fail. This is necessary to accomodate
- *    imprefection of BTF deduplication, which can cause slight duplication of
- *    the same BTF type, if some directly or indirectly referenced (by
- *    pointer) type gets resolved to different actual types in different
- *    object files. If such situation occurs, deduplicated BTF will end up
- *    with two (or more) structurally identical types, which differ only in
- *    types they refer to through pointer. This should be OK in most cases and
- *    is not an error.
- * 4. Candidate types search is performed by linearly scanning through all
- *    types in target BTF. It is anticipated that this is overall more
- *    efficient memory-wise and not significantly worse (if not better)
- *    CPU-wise compared to prebuilding a map from all local type names to
- *    a list of candidate type names. It's also sped up by caching resolved
- *    list of matching candidates per each local "root" type ID, that has at
- *    least one bpf_core_relo associated with it. This list is shared
- *    between multiple relocations for the same type ID and is updated as some
- *    of the candidates are pruned due to structural incompatibility.
- */
-static int bpf_core_apply_relo_insn(const char *prog_name, struct bpf_insn *insn,
-				    int insn_idx,
-				    const struct bpf_core_relo *relo,
-				    int relo_idx,
-				    const struct btf *local_btf,
-				    struct bpf_core_cand_list *cands)
-{
-	struct bpf_core_spec local_spec, cand_spec, targ_spec = {};
-	struct bpf_core_relo_res cand_res, targ_res;
-	const struct btf_type *local_type;
-	const char *local_name;
-	__u32 local_id;
-	const char *spec_str;
-	int i, j, err;
-
-	local_id = relo->type_id;
-	local_type = btf__type_by_id(local_btf, local_id);
-	if (!local_type)
-		return -EINVAL;
-
-	local_name = btf__name_by_offset(local_btf, local_type->name_off);
-	if (!local_name)
-		return -EINVAL;
-
-	spec_str = btf__name_by_offset(local_btf, relo->access_str_off);
-	if (str_is_empty(spec_str))
-		return -EINVAL;
-
-	err = bpf_core_parse_spec(local_btf, local_id, spec_str, relo->kind, &local_spec);
-	if (err) {
-		pr_warn("prog '%s': relo #%d: parsing [%d] %s %s + %s failed: %d\n",
-			prog_name, relo_idx, local_id, btf_kind_str(local_type),
-			str_is_empty(local_name) ? "<anon>" : local_name,
-			spec_str, err);
-		return -EINVAL;
-	}
-
-	pr_debug("prog '%s': relo #%d: kind <%s> (%d), spec is ", prog_name,
-		 relo_idx, core_relo_kind_str(relo->kind), relo->kind);
-	bpf_core_dump_spec(LIBBPF_DEBUG, &local_spec);
-	libbpf_print(LIBBPF_DEBUG, "\n");
-
-	/* TYPE_ID_LOCAL relo is special and doesn't need candidate search */
-	if (relo->kind == BPF_TYPE_ID_LOCAL) {
-		targ_res.validate = true;
-		targ_res.poison = false;
-		targ_res.orig_val = local_spec.root_type_id;
-		targ_res.new_val = local_spec.root_type_id;
-		goto patch_insn;
-	}
-
-	/* libbpf doesn't support candidate search for anonymous types */
-	if (str_is_empty(spec_str)) {
-		pr_warn("prog '%s': relo #%d: <%s> (%d) relocation doesn't support anonymous types\n",
-			prog_name, relo_idx, core_relo_kind_str(relo->kind), relo->kind);
-		return -EOPNOTSUPP;
-	}
-
-
-	for (i = 0, j = 0; i < cands->len; i++) {
-		err = bpf_core_spec_match(&local_spec, cands->cands[i].btf,
-					  cands->cands[i].id, &cand_spec);
-		if (err < 0) {
-			pr_warn("prog '%s': relo #%d: error matching candidate #%d ",
-				prog_name, relo_idx, i);
-			bpf_core_dump_spec(LIBBPF_WARN, &cand_spec);
-			libbpf_print(LIBBPF_WARN, ": %d\n", err);
-			return err;
-		}
-
-		pr_debug("prog '%s': relo #%d: %s candidate #%d ", prog_name,
-			 relo_idx, err == 0 ? "non-matching" : "matching", i);
-		bpf_core_dump_spec(LIBBPF_DEBUG, &cand_spec);
-		libbpf_print(LIBBPF_DEBUG, "\n");
-
-		if (err == 0)
-			continue;
-
-		err = bpf_core_calc_relo(prog_name, relo, relo_idx, &local_spec, &cand_spec, &cand_res);
-		if (err)
-			return err;
-
-		if (j == 0) {
-			targ_res = cand_res;
-			targ_spec = cand_spec;
-		} else if (cand_spec.bit_offset != targ_spec.bit_offset) {
-			/* if there are many field relo candidates, they
-			 * should all resolve to the same bit offset
-			 */
-			pr_warn("prog '%s': relo #%d: field offset ambiguity: %u != %u\n",
-				prog_name, relo_idx, cand_spec.bit_offset,
-				targ_spec.bit_offset);
-			return -EINVAL;
-		} else if (cand_res.poison != targ_res.poison || cand_res.new_val != targ_res.new_val) {
-			/* all candidates should result in the same relocation
-			 * decision and value, otherwise it's dangerous to
-			 * proceed due to ambiguity
-			 */
-			pr_warn("prog '%s': relo #%d: relocation decision ambiguity: %s %u != %s %u\n",
-				prog_name, relo_idx,
-				cand_res.poison ? "failure" : "success", cand_res.new_val,
-				targ_res.poison ? "failure" : "success", targ_res.new_val);
-			return -EINVAL;
-		}
-
-		cands->cands[j++] = cands->cands[i];
-	}
-
-	/*
-	 * For BPF_FIELD_EXISTS relo or when used BPF program has field
-	 * existence checks or kernel version/config checks, it's expected
-	 * that we might not find any candidates. In this case, if field
-	 * wasn't found in any candidate, the list of candidates shouldn't
-	 * change at all, we'll just handle relocating appropriately,
-	 * depending on relo's kind.
-	 */
-	if (j > 0)
-		cands->len = j;
-
-	/*
-	 * If no candidates were found, it might be both a programmer error,
-	 * as well as expected case, depending whether instruction w/
-	 * relocation is guarded in some way that makes it unreachable (dead
-	 * code) if relocation can't be resolved. This is handled in
-	 * bpf_core_patch_insn() uniformly by replacing that instruction with
-	 * BPF helper call insn (using invalid helper ID). If that instruction
-	 * is indeed unreachable, then it will be ignored and eliminated by
-	 * verifier. If it was an error, then verifier will complain and point
-	 * to a specific instruction number in its log.
-	 */
-	if (j == 0) {
-		pr_debug("prog '%s': relo #%d: no matching targets found\n",
-			 prog_name, relo_idx);
-
-		/* calculate single target relo result explicitly */
-		err = bpf_core_calc_relo(prog_name, relo, relo_idx, &local_spec, NULL, &targ_res);
-		if (err)
-			return err;
-	}
-
-patch_insn:
-	/* bpf_core_patch_insn() should know how to handle missing targ_spec */
-	err = bpf_core_patch_insn(prog_name, insn, insn_idx, relo, relo_idx, &targ_res);
-	if (err) {
-		pr_warn("prog '%s': relo #%d: failed to patch insn #%zu: %d\n",
-			prog_name, relo_idx, relo->insn_off / BPF_INSN_SZ, err);
-		return -EINVAL;
-	}
-
-	return 0;
 }
 
 static int bpf_core_apply_relo(struct bpf_program *prog,

tools/lib/bpf/libbpf_internal.h

···
 	return ret;
 }
 
+static inline bool str_is_empty(const char *s)
+{
+	return !s || !s[0];
+}
+
+static inline bool is_ldimm64_insn(struct bpf_insn *insn)
+{
+	return insn->code == (BPF_LD | BPF_IMM | BPF_DW);
+}
+
 #endif /* __LIBBPF_LIBBPF_INTERNAL_H */

tools/lib/bpf/relo_core.c

···
+// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+/* Copyright (c) 2019 Facebook */
+
+#include <stdio.h>
+#include <string.h>
+#include <errno.h>
+#include <ctype.h>
+#include <linux/err.h>
+
+#include "libbpf.h"
+#include "bpf.h"
+#include "btf.h"
+#include "str_error.h"
+#include "libbpf_internal.h"
+
+#define BPF_CORE_SPEC_MAX_LEN 64
+
+/* represents BPF CO-RE field or array element accessor */
+struct bpf_core_accessor {
+	__u32 type_id;		/* struct/union type or array element type */
+	__u32 idx;		/* field index or array index */
+	const char *name;	/* field name or NULL for array accessor */
+};
+
+struct bpf_core_spec {
+	const struct btf *btf;
+	/* high-level spec: named fields and array indices only */
+	struct bpf_core_accessor spec[BPF_CORE_SPEC_MAX_LEN];
+	/* original unresolved (no skip_mods_or_typedefs) root type ID */
+	__u32 root_type_id;
+	/* CO-RE relocation kind */
+	enum bpf_core_relo_kind relo_kind;
+	/* high-level spec length */
+	int len;
+	/* raw, low-level spec: 1-to-1 with accessor spec string */
+	int raw_spec[BPF_CORE_SPEC_MAX_LEN];
+	/* raw spec length */
+	int raw_len;
+	/* field bit offset represented by spec */
+	__u32 bit_offset;
+};
+
+static bool is_flex_arr(const struct btf *btf,
+			const struct bpf_core_accessor *acc,
+			const struct btf_array *arr)
+{
+	const struct btf_type *t;
+
+	/* not a flexible array, if not inside a struct or has non-zero size */
+	if (!acc->name || arr->nelems > 0)
+		return false;
+
+	/* has to be the last member of enclosing struct */
+	t = btf__type_by_id(btf, acc->type_id);
+	return acc->idx == btf_vlen(t) - 1;
+}
+
+static const char *core_relo_kind_str(enum bpf_core_relo_kind kind)
+{
+	switch (kind) {
+	case BPF_FIELD_BYTE_OFFSET: return "byte_off";
+	case BPF_FIELD_BYTE_SIZE: return "byte_sz";
+	case BPF_FIELD_EXISTS: return "field_exists";
+	case BPF_FIELD_SIGNED: return "signed";
+	case BPF_FIELD_LSHIFT_U64: return "lshift_u64";
+	case BPF_FIELD_RSHIFT_U64: return "rshift_u64";
+	case BPF_TYPE_ID_LOCAL: return "local_type_id";
+	case BPF_TYPE_ID_TARGET: return "target_type_id";
+	case BPF_TYPE_EXISTS: return "type_exists";
+	case BPF_TYPE_SIZE: return "type_size";
+	case BPF_ENUMVAL_EXISTS: return "enumval_exists";
+	case BPF_ENUMVAL_VALUE: return "enumval_value";
+	default: return "unknown";
+	}
+}
+
+static bool core_relo_is_field_based(enum bpf_core_relo_kind kind)
+{
+	switch (kind) {
+	case BPF_FIELD_BYTE_OFFSET:
+	case BPF_FIELD_BYTE_SIZE:
+	case BPF_FIELD_EXISTS:
+	case BPF_FIELD_SIGNED:
+	case BPF_FIELD_LSHIFT_U64:
+	case BPF_FIELD_RSHIFT_U64:
+		return true;
+	default:
+		return false;
+	}
+}
+
+static bool core_relo_is_type_based(enum bpf_core_relo_kind kind)
+{
+	switch (kind) {
+	case BPF_TYPE_ID_LOCAL:
+	case BPF_TYPE_ID_TARGET:
+	case BPF_TYPE_EXISTS:
+	case BPF_TYPE_SIZE:
+		return true;
+	default:
+		return false;
+	}
+}
+
+static bool core_relo_is_enumval_based(enum bpf_core_relo_kind kind)
+{
+	switch (kind) {
+	case BPF_ENUMVAL_EXISTS:
+	case BPF_ENUMVAL_VALUE:
+		return true;
+	default:
+		return false;
+	}
+}
+
+/*
+ * Turn bpf_core_relo into a low- and high-level spec representation,
+ * validating correctness along the way, as well as calculating resulting
+ * field bit offset, specified by accessor string. Low-level spec captures
+ * every single level of nestedness, including traversing anonymous
+ * struct/union members. High-level one only captures semantically meaningful
+ * "turning points": named fields and array indicies.
+ * E.g., for this case:
+ *
+ *   struct sample {
+ *       int __unimportant;
+ *       struct {
+ *           int __1;
+ *           int __2;
+ *           int a[7];
+ *       };
+ *   };
+ *
+ *   struct sample *s = ...;
+ *
+ *   int x = &s->a[3]; // access string = '0:1:2:3'
+ *
+ * Low-level spec has 1:1 mapping with each element of access string (it's
+ * just a parsed access string representation): [0, 1, 2, 3].
+ *
+ * High-level spec will capture only 3 points:
+ *   - intial zero-index access by pointer (&s->... is the same as &s[0]...);
+ *   - field 'a' access (corresponds to '2' in low-level spec);
+ *   - array element #3 access (corresponds to '3' in low-level spec).
+ *
+ * Type-based relocations (TYPE_EXISTS/TYPE_SIZE,
+ * TYPE_ID_LOCAL/TYPE_ID_TARGET) don't capture any field information. Their
+ * spec and raw_spec are kept empty.
+ *
+ * Enum value-based relocations (ENUMVAL_EXISTS/ENUMVAL_VALUE) use access
+ * string to specify enumerator's value index that need to be relocated.
+ */
+static int bpf_core_parse_spec(const struct btf *btf,
+			       __u32 type_id,
+			       const char *spec_str,
+			       enum bpf_core_relo_kind relo_kind,
+			       struct bpf_core_spec *spec)
+{
+	int access_idx, parsed_len, i;
+	struct bpf_core_accessor *acc;
+	const struct btf_type *t;
+	const char *name;
+	__u32 id;
+	__s64 sz;
+
+	if (str_is_empty(spec_str) || *spec_str == ':')
+		return -EINVAL;
+
+	memset(spec, 0, sizeof(*spec));
+	spec->btf = btf;
+	spec->root_type_id = type_id;
+	spec->relo_kind = relo_kind;
+
+	/* type-based relocations don't have a field access string */
+	if (core_relo_is_type_based(relo_kind)) {
+		if (strcmp(spec_str, "0"))
+			return -EINVAL;
+		return 0;
+	}
+
+	/* parse spec_str="0:1:2:3:4" into array raw_spec=[0, 1, 2, 3, 4] */
+	while (*spec_str) {
+		if (*spec_str == ':')
+			++spec_str;
+		if (sscanf(spec_str, "%d%n", &access_idx, &parsed_len) != 1)
+			return -EINVAL;
+		if (spec->raw_len == BPF_CORE_SPEC_MAX_LEN)
+			return -E2BIG;
+		spec_str += parsed_len;
+		spec->raw_spec[spec->raw_len++] = access_idx;
+	}
+
+	if (spec->raw_len == 0)
+		return -EINVAL;
+
+	t = skip_mods_and_typedefs(btf, type_id, &id);
+	if (!t)
+		return -EINVAL;
+
+	access_idx = spec->raw_spec[0];
+	acc = &spec->spec[0];
+	acc->type_id = id;
+	acc->idx = access_idx;
+	spec->len++;
+
+	if (core_relo_is_enumval_based(relo_kind)) {
+		if (!btf_is_enum(t) || spec->raw_len > 1 || access_idx >= btf_vlen(t))
+			return -EINVAL;
+
+		/* record enumerator name in a first accessor */
+		acc->name = btf__name_by_offset(btf, btf_enum(t)[access_idx].name_off);
+		return 0;
+	}
+
+	if (!core_relo_is_field_based(relo_kind))
+		return -EINVAL;
+
+	sz = btf__resolve_size(btf, id);
+	if (sz < 0)
+		return sz;
+	spec->bit_offset = access_idx * sz * 8;
+
+	for (i = 1; i < spec->raw_len; i++) {
+		t = skip_mods_and_typedefs(btf, id, &id);
+		if (!t)
+			return -EINVAL;
+
+		access_idx = spec->raw_spec[i];
+		acc = &spec->spec[spec->len];
+
+		if (btf_is_composite(t)) {
+			const struct btf_member *m;
+			__u32 bit_offset;
+
+			if (access_idx >= btf_vlen(t))
+				return -EINVAL;
+
+			bit_offset = btf_member_bit_offset(t, access_idx);
+			spec->bit_offset += bit_offset;
+
+			m = btf_members(t) + access_idx;
+			if (m->name_off) {
+				name = btf__name_by_offset(btf, m->name_off);
+				if (str_is_empty(name))
+					return -EINVAL;
+
+				acc->type_id = id;
+				acc->idx = access_idx;
+				acc->name = name;
+				spec->len++;
+			}
+
+			id = m->type;
+		} else if (btf_is_array(t)) {
+			const struct btf_array *a = btf_array(t);
+			bool flex;
+
+			t = skip_mods_and_typedefs(btf, a->type, &id);
+			if (!t)
+				return -EINVAL;
+
+			flex = is_flex_arr(btf, acc - 1, a);
+			if (!flex && access_idx >= a->nelems)
+				return -EINVAL;
+
+			spec->spec[spec->len].type_id = id;
+			spec->spec[spec->len].idx = access_idx;
+			spec->len++;
+
+			sz = btf__resolve_size(btf, id);
+			if (sz < 0)
+				return sz;
+			spec->bit_offset += access_idx * sz * 8;
+		} else {
+			pr_warn("relo for [%u] %s (at idx %d) captures type [%d] of unexpected kind %s\n",
+				type_id, spec_str, i, id, btf_kind_str(t));
+			return -EINVAL;
+		}
+	}
+
+	return 0;
+}
+
+/* Check two types for compatibility for the purpose of field access
+ * relocation. const/volatile/restrict and typedefs are skipped to ensure we
+ * are relocating semantically compatible entities:
+ *   - any two STRUCTs/UNIONs are compatible and can be mixed;
+ *   - any two FWDs are compatible, if their names match (modulo flavor suffix);
+ *   - any two PTRs are always compatible;
+ *   - for ENUMs, names should be the same (ignoring flavor suffix) or at
+ *     least one of enums should be anonymous;
+ *   - for ENUMs, check sizes, names are ignored;
+ *   - for INT, size and signedness are ignored;
+ *   - any two FLOATs are always compatible;
+ *   - for ARRAY, dimensionality is ignored, element types are checked for
+ *     compatibility recursively;
+ *   - everything else shouldn't be ever a target of relocation.
+ * These rules are not set in stone and probably will be adjusted as we get
+ * more experience with using BPF CO-RE relocations.
+ */
+static int bpf_core_fields_are_compat(const struct btf *local_btf,
+				      __u32 local_id,
+				      const struct btf *targ_btf,
+				      __u32 targ_id)
+{
+	const struct btf_type *local_type, *targ_type;
+
+recur:
+	local_type = skip_mods_and_typedefs(local_btf, local_id, &local_id);
+	targ_type = skip_mods_and_typedefs(targ_btf, targ_id, &targ_id);
+	if (!local_type || !targ_type)
+		return -EINVAL;
+
+	if (btf_is_composite(local_type) && btf_is_composite(targ_type))
+		return 1;
+	if (btf_kind(local_type) != btf_kind(targ_type))
+		return 0;
+
+	switch (btf_kind(local_type)) {
+	case BTF_KIND_PTR:
+	case BTF_KIND_FLOAT:
+		return 1;
+	case BTF_KIND_FWD:
+	case BTF_KIND_ENUM: {
+		const char *local_name, *targ_name;
+		size_t local_len, targ_len;
+
+		local_name = btf__name_by_offset(local_btf,
+						 local_type->name_off);
+		targ_name = btf__name_by_offset(targ_btf, targ_type->name_off);
+		local_len = bpf_core_essential_name_len(local_name);
+		targ_len = bpf_core_essential_name_len(targ_name);
+		/* one of them is anonymous or both w/ same flavor-less names */
+		return local_len == 0 || targ_len == 0 ||
+		       (local_len == targ_len &&
+			strncmp(local_name, targ_name, local_len) == 0);
+	}
+	case BTF_KIND_INT:
+		/* just reject deprecated bitfield-like integers; all other
+		 * integers are by default compatible between each other
+		 */
+		return btf_int_offset(local_type) == 0 &&
+		       btf_int_offset(targ_type) == 0;
+	case BTF_KIND_ARRAY:
+		local_id = btf_array(local_type)->type;
+		targ_id = btf_array(targ_type)->type;
+		goto recur;
+	default:
+		pr_warn("unexpected kind %d relocated, local [%d], target [%d]\n",
+			btf_kind(local_type), local_id, targ_id);
+		return 0;
+	}
+}
+
+/*
+ * Given single high-level named field accessor in local type, find
+ * corresponding high-level accessor for a target type. Along the way,
+ * maintain low-level spec for target as well. Also keep updating target
+ * bit offset.
+ *
+ * Searching is performed through recursive exhaustive enumeration of all
+ * fields of a struct/union. If there are any anonymous (embedded)
+ * structs/unions, they are recursively searched as well. If field with
+ * desired name is found, check compatibility between local and target types,
+ * before returning result.
+ *
+ * 1 is returned, if field is found.
+ * 0 is returned if no compatible field is found.
+ * <0 is returned on error.
+ */
+static int bpf_core_match_member(const struct btf *local_btf,
+				 const struct bpf_core_accessor *local_acc,
+				 const struct btf *targ_btf,
+				 __u32 targ_id,
+				 struct bpf_core_spec *spec,
+				 __u32 *next_targ_id)
+{
+	const struct btf_type *local_type, *targ_type;
+	const struct btf_member *local_member, *m;
+	const char *local_name, *targ_name;
+	__u32 local_id;
+	int i, n, found;
+
+	targ_type = skip_mods_and_typedefs(targ_btf, targ_id, &targ_id);
+	if (!targ_type)
+		return -EINVAL;
+	if (!btf_is_composite(targ_type))
+		return 0;
+
+	local_id = local_acc->type_id;
+	local_type = btf__type_by_id(local_btf, local_id);
+	local_member = btf_members(local_type) + local_acc->idx;
+	local_name = btf__name_by_offset(local_btf, local_member->name_off);
+
+	n = btf_vlen(targ_type);
+	m = btf_members(targ_type);
+	for (i = 0; i < n; i++, m++) {
+		__u32 bit_offset;
+
+		bit_offset = btf_member_bit_offset(targ_type, i);
+
+		/* too deep struct/union/array nesting */
+		if (spec->raw_len == BPF_CORE_SPEC_MAX_LEN)
+			return -E2BIG;
+
+		/* speculate this member will be the good one */
+		spec->bit_offset += bit_offset;
+		spec->raw_spec[spec->raw_len++] = i;
+
+		targ_name = btf__name_by_offset(targ_btf, m->name_off);
+		if (str_is_empty(targ_name)) {
+			/* embedded struct/union, we need to go deeper */
+			found = bpf_core_match_member(local_btf, local_acc,
+						      targ_btf, m->type,
+						      spec, next_targ_id);
+			if (found) /* either found or error */
+				return found;
+		} else if (strcmp(local_name, targ_name) == 0) {
+			/* matching named field */
+			struct bpf_core_accessor *targ_acc;
+
+			targ_acc = &spec->spec[spec->len++];
+			targ_acc->type_id = targ_id;
+			targ_acc->idx = i;
+			targ_acc->name = targ_name;
+
+			*next_targ_id = m->type;
+			found = bpf_core_fields_are_compat(local_btf,
+							   local_member->type,
+							   targ_btf, m->type);
+			if (!found)
+				spec->len--; /* pop accessor */
+			return found;
+		}
+		/* member turned out not to be what we looked for */
+		spec->bit_offset -= bit_offset;
+		spec->raw_len--;
+	}
+
+	return 0;
+}
+
+/*
+ * Try to match local spec to a target type and, if successful, produce full
+ * target spec (high-level, low-level + bit offset).
+ */
+static int bpf_core_spec_match(struct bpf_core_spec *local_spec,
+			       const struct btf *targ_btf, __u32 targ_id,
+			       struct bpf_core_spec *targ_spec)
+{
+	const struct btf_type *targ_type;
+	const struct bpf_core_accessor *local_acc;
+	struct bpf_core_accessor *targ_acc;
+	int i, sz, matched;
+
+	memset(targ_spec, 0, sizeof(*targ_spec));
+	targ_spec->btf = targ_btf;
+	targ_spec->root_type_id = targ_id;
+	targ_spec->relo_kind = local_spec->relo_kind;
+
+	if (core_relo_is_type_based(local_spec->relo_kind)) {
+		return bpf_core_types_are_compat(local_spec->btf,
+						 local_spec->root_type_id,
+						 targ_btf, targ_id);
+	}
+
+	local_acc = &local_spec->spec[0];
+	targ_acc = &targ_spec->spec[0];
+
+	if (core_relo_is_enumval_based(local_spec->relo_kind)) {
+		size_t local_essent_len, targ_essent_len;
+		const struct btf_enum *e;
+		const char *targ_name;
+
+		/* has to resolve to an enum */
+		targ_type = skip_mods_and_typedefs(targ_spec->btf, targ_id, &targ_id);
+		if (!btf_is_enum(targ_type))
+			return 0;
+
+		local_essent_len = bpf_core_essential_name_len(local_acc->name);
+
+		for (i = 0, e = btf_enum(targ_type); i < btf_vlen(targ_type); i++, e++) {
+			targ_name = btf__name_by_offset(targ_spec->btf, e->name_off);
+			targ_essent_len = bpf_core_essential_name_len(targ_name);
+			if (targ_essent_len != local_essent_len)
+				continue;
+			if (strncmp(local_acc->name, targ_name, local_essent_len) == 0) {
+				targ_acc->type_id = targ_id;
+				targ_acc->idx = i;
+				targ_acc->name = targ_name;
+				targ_spec->len++;
+				targ_spec->raw_spec[targ_spec->raw_len] = targ_acc->idx;
+				targ_spec->raw_len++;
+				return 1;
+			}
+		}
+		return 0;
+	}
+
+	if (!core_relo_is_field_based(local_spec->relo_kind))
+		return -EINVAL;
+
+	for (i = 0; i < local_spec->len; i++, local_acc++, targ_acc++) {
+		targ_type = skip_mods_and_typedefs(targ_spec->btf, targ_id,
+						   &targ_id);
+		if (!targ_type)
+			return -EINVAL;
+
+		if (local_acc->name) {
+			matched = bpf_core_match_member(local_spec->btf,
+							local_acc,
+							targ_btf, targ_id,
+							targ_spec, &targ_id);
+			if (matched <= 0)
+				return matched;
+		} else {
+			/* for i=0, targ_id is already treated as array element
+			 * type (because it's the original struct), for others
+			 * we should find array element type first
+			 */
+			if (i > 0) {
+				const struct btf_array *a;
+				bool flex;
+
+				if (!btf_is_array(targ_type))
+					return 0;
+
+				a = btf_array(targ_type);
+				flex = is_flex_arr(targ_btf, targ_acc - 1, a);
+				if (!flex && local_acc->idx >= a->nelems)
+					return 0;
+				if (!skip_mods_and_typedefs(targ_btf, a->type,
+							    &targ_id))
+					return -EINVAL;
+			}
+
+			/* too deep struct/union/array nesting */
+			if (targ_spec->raw_len == BPF_CORE_SPEC_MAX_LEN)
+				return -E2BIG;
+
+			targ_acc->type_id = targ_id;
+			targ_acc->idx = local_acc->idx;
+			targ_acc->name = NULL;
+			targ_spec->len++;
+			targ_spec->raw_spec[targ_spec->raw_len] = targ_acc->idx;
+			targ_spec->raw_len++;
+
+			sz = btf__resolve_size(targ_btf, targ_id);
+			if (sz < 0)
+				return sz;
+			targ_spec->bit_offset += local_acc->idx * sz * 8;
+		}
+	}
+
+	return 1;
+}
+
+static int bpf_core_calc_field_relo(const char *prog_name,
+				    const struct bpf_core_relo *relo,
+				    const struct bpf_core_spec *spec,
+				    __u32 *val, __u32 *field_sz, __u32 *type_id,
+				    bool *validate)
+{
+	const struct bpf_core_accessor *acc;
+	const struct btf_type *t;
+	__u32 byte_off, byte_sz, bit_off, bit_sz, field_type_id;
+	const struct btf_member *m;
+	const struct btf_type *mt;
+	bool bitfield;
+	__s64 sz;
+
+	*field_sz = 0;
+
+	if (relo->kind == BPF_FIELD_EXISTS) {
+		*val = spec ? 1 : 0;
+		return 0;
+	}
+
+	if (!spec)
+		return -EUCLEAN; /* request instruction poisoning */
+
+	acc = &spec->spec[spec->len - 1];
+	t = btf__type_by_id(spec->btf, acc->type_id);
+
+	/* a[n] accessor needs special handling */
+	if (!acc->name) {
+		if (relo->kind == BPF_FIELD_BYTE_OFFSET) {
+			*val = spec->bit_offset / 8;
+			/* remember field size for load/store mem size */
+			sz = btf__resolve_size(spec->btf, acc->type_id);
+			if (sz < 0)
+				return -EINVAL;
+			*field_sz = sz;
+			*type_id = acc->type_id;
+		} else if (relo->kind == BPF_FIELD_BYTE_SIZE) {
+			sz = btf__resolve_size(spec->btf, acc->type_id);
+			if (sz < 0)
+				return -EINVAL;
+			*val = sz;
+		} else {
+			pr_warn("prog '%s': relo %d at insn #%d can't be applied to array access\n",
+				prog_name, relo->kind, relo->insn_off / 8);
+			return -EINVAL;
+		}
+		if (validate)
+			*validate = true;
+		return 0;
+	}
+
+	m = btf_members(t) + acc->idx;
+	mt = skip_mods_and_typedefs(spec->btf, m->type, &field_type_id);
+	bit_off = spec->bit_offset;
+	bit_sz = btf_member_bitfield_size(t, acc->idx);
+
+	bitfield = bit_sz > 0;
+	if (bitfield) {
+		byte_sz = mt->size;
+		byte_off = bit_off / 8 / byte_sz * byte_sz;
+		/* figure out smallest int size necessary for bitfield load */
+		while (bit_off + bit_sz - byte_off * 8 > byte_sz * 8) {
+			if (byte_sz >= 8) {
+				/* bitfield can't be read with 64-bit read */
+				pr_warn("prog '%s': relo %d at insn #%d can't be satisfied for bitfield\n",
+					prog_name, relo->kind, relo->insn_off / 8);
+				return -E2BIG;
+			}
+			byte_sz *= 2;
+			byte_off = bit_off / 8 / byte_sz * byte_sz;
+		}
+	} else {
+		sz = btf__resolve_size(spec->btf, field_type_id);
+		if (sz < 0)
+			return -EINVAL;
+		byte_sz = sz;
+		byte_off = spec->bit_offset / 8;
+		bit_sz = byte_sz * 8;
+	}
+
+	/* for bitfields, all the relocatable aspects are ambiguous and we
+	 * might disagree with compiler, so turn off validation of expected
+	 * value, except for signedness
+	 */
+	if (validate)
+		*validate = !bitfield;
+
+	switch (relo->kind) {
+	case BPF_FIELD_BYTE_OFFSET:
+		*val = byte_off;
+		if (!bitfield) {
+			*field_sz = byte_sz;
+			*type_id = field_type_id;
+		}
+		break;
+	case BPF_FIELD_BYTE_SIZE:
+		*val = byte_sz;
+		break;
+	case BPF_FIELD_SIGNED:
+		/* enums will be assumed unsigned */
+		*val = btf_is_enum(mt) ||
+		       (btf_int_encoding(mt) & BTF_INT_SIGNED);
+		if (validate)
+			*validate = true; /* signedness is never ambiguous */
+		break;
+	case BPF_FIELD_LSHIFT_U64:
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+		*val = 64 - (bit_off + bit_sz - byte_off  * 8);
+#else
+		*val = (8 - byte_sz) * 8 + (bit_off - byte_off * 8);
+#endif
+		break;
+	case BPF_FIELD_RSHIFT_U64:
+		*val = 64 - bit_sz;
+		if (validate)
+			*validate = true; /* right shift is never ambiguous */
+		break;
+	case BPF_FIELD_EXISTS:
+	default:
+		return -EOPNOTSUPP;
+	}
+
+	return 0;
+}
+
+static int bpf_core_calc_type_relo(const struct bpf_core_relo *relo,
+				   const struct bpf_core_spec *spec,
+				   __u32 *val)
+{
+	__s64 sz;
+
+	/* type-based relos return zero when target type is not found */
+	if (!spec) {
+		*val = 0;
+		return 0;
+	}
+
+	switch (relo->kind) {
+	case BPF_TYPE_ID_TARGET:
+		*val = spec->root_type_id;
+		break;
+	case BPF_TYPE_EXISTS:
+		*val = 1;
+		break;
+	case BPF_TYPE_SIZE:
+		sz = btf__resolve_size(spec->btf, spec->root_type_id);
+		if (sz < 0)
+			return -EINVAL;
+		*val = sz;
+		break;
+	case BPF_TYPE_ID_LOCAL:
+	/* BPF_TYPE_ID_LOCAL is handled specially and shouldn't get here */
+	default:
+		return -EOPNOTSUPP;
+	}
+
+	return 0;
+}
+
+static int bpf_core_calc_enumval_relo(const struct bpf_core_relo *relo,
+				      const struct bpf_core_spec *spec,
+				      __u32 *val)
+{
+	const struct btf_type *t;
+	const struct btf_enum *e;
+
+	switch (relo->kind) {
+	case BPF_ENUMVAL_EXISTS:
+		*val = spec ? 1 : 0;
+		break;
+	case BPF_ENUMVAL_VALUE:
+		if (!spec)
+			return -EUCLEAN; /* request instruction poisoning */
+		t = btf__type_by_id(spec->btf, spec->spec[0].type_id);
+		e = btf_enum(t) + spec->spec[0].idx;
+		*val = e->val;
+		break;
+	default:
+		return -EOPNOTSUPP;
+	}
+
+	return 0;
+}
+
+struct bpf_core_relo_res
+{
+	/* expected value in the instruction, unless validate == false */
+	__u32 orig_val;
+	/* new value that needs to be patched up to */
+	__u32 new_val;
+	/* relocation unsuccessful, poison instruction, but don't fail load */
+	bool poison;
+	/* some relocations can't be validated against orig_val */
+	bool validate;
+	/* for field byte offset relocations or the forms:
+	 *     *(T *)(rX + <off>) = rY
+	 *     rX = *(T *)(rY + <off>),
+	 * we remember original and resolved field size to adjust direct
+	 * memory loads of pointers and integers; this is necessary for 32-bit
+	 * host kernel architectures, but also allows to automatically
+	 * relocate fields that were resized from, e.g., u32 to u64, etc.
+	 */
+	bool fail_memsz_adjust;
+	__u32 orig_sz;
+	__u32 orig_type_id;
+	__u32 new_sz;
+	__u32 new_type_id;
+};
+
+/* Calculate original and target relocation values, given local and target
+ * specs and relocation kind. These values are calculated for each candidate.
+ * If there are multiple candidates, resulting values should all be consistent
+ * with each other. Otherwise, libbpf will refuse to proceed due to ambiguity.
+ * If instruction has to be poisoned, *poison will be set to true.
+ */
+static int bpf_core_calc_relo(const char *prog_name,
+			      const struct bpf_core_relo *relo,
+			      int relo_idx,
+			      const struct bpf_core_spec *local_spec,
+			      const struct bpf_core_spec *targ_spec,
+			      struct bpf_core_relo_res *res)
+{
+	int err = -EOPNOTSUPP;
+
+	res->orig_val = 0;
+	res->new_val = 0;
+	res->poison = false;
+	res->validate = true;
+	res->fail_memsz_adjust = false;
+	res->orig_sz = res->new_sz = 0;
+	res->orig_type_id = res->new_type_id = 0;
+
+	if (core_relo_is_field_based(relo->kind)) {
+		err = bpf_core_calc_field_relo(prog_name, relo, local_spec,
+					       &res->orig_val, &res->orig_sz,
+					       &res->orig_type_id, &res->validate);
+		err = err ?: bpf_core_calc_field_relo(prog_name, relo, targ_spec,
+						      &res->new_val, &res->new_sz,
+						      &res->new_type_id, NULL);
+		if (err)
+			goto done;
+		/* Validate if it's safe to adjust load/store memory size.
+		 * Adjustments are performed only if original and new memory
+		 * sizes differ.
+		 */
+		res->fail_memsz_adjust = false;
+		if (res->orig_sz != res->new_sz) {
+			const struct btf_type *orig_t, *new_t;
+
+			orig_t = btf__type_by_id(local_spec->btf, res->orig_type_id);
+			new_t = btf__type_by_id(targ_spec->btf, res->new_type_id);
+
+			/* There are two use cases in which it's safe to
+			 * adjust load/store's mem size:
+			 *   - reading a 32-bit kernel pointer, while on BPF
+			 *   size pointers are always 64-bit; in this case
+			 *   it's safe to "downsize" instruction size due to
+			 *   pointer being treated as unsigned integer with
+			 *   zero-extended upper 32-bits;
+			 *   - reading unsigned integers, again due to
+			 *   zero-extension is preserving the value correctly.
+			 *
+			 * In all other cases it's incorrect to attempt to
+			 * load/store field because read value will be
+			 * incorrect, so we poison relocated instruction.
+			 */
+			if (btf_is_ptr(orig_t) && btf_is_ptr(new_t))
+				goto done;
+			if (btf_is_int(orig_t) && btf_is_int(new_t) &&
+			    btf_int_encoding(orig_t) != BTF_INT_SIGNED &&
+			    btf_int_encoding(new_t) != BTF_INT_SIGNED)
+				goto done;
+
+			/* mark as invalid mem size adjustment, but this will
+			 * only be checked for LDX/STX/ST insns
+			 */
+			res->fail_memsz_adjust = true;
+		}
+	} else if (core_relo_is_type_based(relo->kind)) {
+		err = bpf_core_calc_type_relo(relo, local_spec, &res->orig_val);
+		err = err ?: bpf_core_calc_type_relo(relo, targ_spec, &res->new_val);
+	} else if (core_relo_is_enumval_based(relo->kind)) {
+		err = bpf_core_calc_enumval_relo(relo, local_spec, &res->orig_val);
+		err = err ?: bpf_core_calc_enumval_relo(relo, targ_spec, &res->new_val);
+	}
+
+done:
+	if (err == -EUCLEAN) {
+		/* EUCLEAN is used to signal instruction poisoning request */
+		res->poison = true;
+		err = 0;
+	} else if (err == -EOPNOTSUPP) {
+		/* EOPNOTSUPP means unknown/unsupported relocation */
+		pr_warn("prog '%s': relo #%d: unrecognized CO-RE relocation %s (%d) at insn #%d\n",
+			prog_name, relo_idx, core_relo_kind_str(relo->kind),
+			relo->kind, relo->insn_off / 8);
+	}
+
+	return err;
+}
+
+/*
+ * Turn instruction for which CO_RE relocation failed into invalid one with
+ * distinct signature.
+ */
+static void bpf_core_poison_insn(const char *prog_name, int relo_idx,
+				 int insn_idx, struct bpf_insn *insn)
+{
+	pr_debug("prog '%s': relo #%d: substituting insn #%d w/ invalid insn\n",
+		 prog_name, relo_idx, insn_idx);
+	insn->code = BPF_JMP | BPF_CALL;
+	insn->dst_reg = 0;
+	insn->src_reg = 0;
+	insn->off = 0;
+	/* if this instruction is reachable (not a dead code),
+	 * verifier will complain with the following message:
+	 * invalid func unknown#195896080
+	 */
+	insn->imm = 195896080; /* => 0xbad2310 => "bad relo" */
+}
+
+static int insn_bpf_size_to_bytes(struct bpf_insn *insn)
+{
+	switch (BPF_SIZE(insn->code)) {
+	case BPF_DW: return 8;
+	case BPF_W: return 4;
+	case BPF_H: return 2;
+	case BPF_B: return 1;
+	default: return -1;
+	}
+}
+
+static int insn_bytes_to_bpf_size(__u32 sz)
+{
+	switch (sz) {
+	case 8: return BPF_DW;
+	case 4: return BPF_W;
+	case 2: return BPF_H;
+	case 1: return BPF_B;
+	default: return -1;
+	}
+}
+
+/*
+ * Patch relocatable BPF instruction.
+ *
+ * Patched value is determined by relocation kind and target specification.
+ * For existence relocations target spec will be NULL if field/type is not found.
+ * Expected insn->imm value is determined using relocation kind and local
+ * spec, and is checked before patching instruction. If actual insn->imm value
+ * is wrong, bail out with error.
+ *
+ * Currently supported classes of BPF instruction are:
+ * 1. rX = <imm> (assignment with immediate operand);
+ * 2. rX += <imm> (arithmetic operations with immediate operand);
+ * 3. rX = <imm64> (load with 64-bit immediate value);
+ * 4. rX = *(T *)(rY + <off>), where T is one of {u8, u16, u32, u64};
+ * 5. *(T *)(rX + <off>) = rY, where T is one of {u8, u16, u32, u64};
+ * 6. *(T *)(rX + <off>) = <imm>, where T is one of {u8, u16, u32, u64}.
+ */
+static int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
+			       int insn_idx, const struct bpf_core_relo *relo,
+			       int relo_idx, const struct bpf_core_relo_res *res)
+{
+	__u32 orig_val, new_val;
+	__u8 class;
+
+	class = BPF_CLASS(insn->code);
+
+	if (res->poison) {
+poison:
+		/* poison second part of ldimm64 to avoid confusing error from
+		 * verifier about "unknown opcode 00"
+		 */
+		if (is_ldimm64_insn(insn))
+			bpf_core_poison_insn(prog_name, relo_idx, insn_idx + 1, insn + 1);
+		bpf_core_poison_insn(prog_name, relo_idx, insn_idx, insn);
+		return 0;
+	}
+
+	orig_val = res->orig_val;
+	new_val = res->new_val;
+
+	switch (class) {
+	case BPF_ALU:
+	case BPF_ALU64:
+		if (BPF_SRC(insn->code) != BPF_K)
+			return -EINVAL;
+		if (res->validate && insn->imm != orig_val) {
+			pr_warn("prog '%s': relo #%d: unexpected insn #%d (ALU/ALU64) value: got %u, exp %u -> %u\n",
+				prog_name, relo_idx,
+				insn_idx, insn->imm, orig_val, new_val);
+			return -EINVAL;
+		}
+		orig_val = insn->imm;
+		insn->imm = new_val;
+		pr_debug("prog '%s': relo #%d: patched insn #%d (ALU/ALU64) imm %u -> %u\n",
+			 prog_name, relo_idx, insn_idx,
+			 orig_val, new_val);
+		break;
+	case BPF_LDX:
+	case BPF_ST:
+	case BPF_STX:
+		if (res->validate && insn->off != orig_val) {
+			pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDX/ST/STX) value: got %u, exp %u -> %u\n",
+				prog_name, relo_idx, insn_idx, insn->off, orig_val, new_val);
+			return -EINVAL;
+		}
+		if (new_val > SHRT_MAX) {
+			pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) value too big: %u\n",
+				prog_name, relo_idx, insn_idx, new_val);
+			return -ERANGE;
+		}
+		if (res->fail_memsz_adjust) {
+			pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) accesses field incorrectly. "
+				"Make sure you are accessing pointers, unsigned integers, or fields of matching type and size.\n",
+				prog_name, relo_idx, insn_idx);
+			goto poison;
+		}
+
+		orig_val = insn->off;
+		insn->off = new_val;
+		pr_debug("prog '%s': relo #%d: patched insn #%d (LDX/ST/STX) off %u -> %u\n",
+			 prog_name, relo_idx, insn_idx, orig_val, new_val);
+
+		if (res->new_sz != res->orig_sz) {
+			int insn_bytes_sz, insn_bpf_sz;
+
+			insn_bytes_sz = insn_bpf_size_to_bytes(insn);
+			if (insn_bytes_sz != res->orig_sz) {
+				pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) unexpected mem size: got %d, exp %u\n",
+					prog_name, relo_idx, insn_idx, insn_bytes_sz, res->orig_sz);
+				return -EINVAL;
+			}
+
+			insn_bpf_sz = insn_bytes_to_bpf_size(res->new_sz);
+			if (insn_bpf_sz < 0) {
+				pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) invalid new mem size: %u\n",
+					prog_name, relo_idx, insn_idx, res->new_sz);
+				return -EINVAL;
+			}
+
+			insn->code = BPF_MODE(insn->code) | insn_bpf_sz | BPF_CLASS(insn->code);
+			pr_debug("prog '%s': relo #%d: patched insn #%d (LDX/ST/STX) mem_sz %u -> %u\n",
+				 prog_name, relo_idx, insn_idx, res->orig_sz, res->new_sz);
+		}
+		break;
+	case BPF_LD: {
+		__u64 imm;
+
+		if (!is_ldimm64_insn(insn) ||
+		    insn[0].src_reg != 0 || insn[0].off != 0 ||
+		    insn[1].code != 0 || insn[1].dst_reg != 0 ||
+		    insn[1].src_reg != 0 || insn[1].off != 0) {
+			pr_warn("prog '%s': relo #%d: insn #%d (LDIMM64) has unexpected form\n",
+				prog_name, relo_idx, insn_idx);
+			return -EINVAL;
+		}
+
+		imm = insn[0].imm + ((__u64)insn[1].imm << 32);
+		if (res->validate && imm != orig_val) {
+			pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDIMM64) value: got %llu, exp %u -> %u\n",
+				prog_name, relo_idx,
+				insn_idx, (unsigned long long)imm,
+				orig_val, new_val);
+			return -EINVAL;
+		}
+
+		insn[0].imm = new_val;
+		insn[1].imm = 0; /* currently only 32-bit values are supported */
+		pr_debug("prog '%s': relo #%d: patched insn #%d (LDIMM64) imm64 %llu -> %u\n",
+			 prog_name, relo_idx, insn_idx,
+			 (unsigned long long)imm, new_val);
+		break;
+	}
+	default:
+		pr_warn("prog '%s': relo #%d: trying to relocate unrecognized insn #%d, code:0x%x, src:0x%x, dst:0x%x, off:0x%x, imm:0x%x\n",
+			prog_name, relo_idx, insn_idx, insn->code,
+			insn->src_reg, insn->dst_reg, insn->off, insn->imm);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+/* Output spec definition in the format:
+ * [<type-id>] (<type-name>) + <raw-spec> => <offset>@<spec>,
+ * where <spec> is a C-syntax view of recorded field access, e.g.: x.a[3].b
+ */
+static void bpf_core_dump_spec(int level, const struct bpf_core_spec *spec)
+{
+	const struct btf_type *t;
+	const struct btf_enum *e;
+	const char *s;
+	__u32 type_id;
+	int i;
+
+	type_id = spec->root_type_id;
+	t = btf__type_by_id(spec->btf, type_id);
+	s = btf__name_by_offset(spec->btf, t->name_off);
+
+	libbpf_print(level, "[%u] %s %s", type_id, btf_kind_str(t), str_is_empty(s) ? "<anon>" : s);
+
+	if (core_relo_is_type_based(spec->relo_kind))
+		return;
+
+	if (core_relo_is_enumval_based(spec->relo_kind)) {
+		t = skip_mods_and_typedefs(spec->btf, type_id, NULL);
+		e = btf_enum(t) + spec->raw_spec[0];
+		s = btf__name_by_offset(spec->btf, e->name_off);
+
+		libbpf_print(level, "::%s = %u", s, e->val);
+		return;
+	}
+
+	if (core_relo_is_field_based(spec->relo_kind)) {
+		for (i = 0; i < spec->len; i++) {
+			if (spec->spec[i].name)
+				libbpf_print(level, ".%s", spec->spec[i].name);
+			else if (i > 0 || spec->spec[i].idx > 0)
+				libbpf_print(level, "[%u]", spec->spec[i].idx);
+		}
+
+		libbpf_print(level, " (");
+		for (i = 0; i < spec->raw_len; i++)
+			libbpf_print(level, "%s%d", i == 0 ? "" : ":", spec->raw_spec[i]);
+
+		if (spec->bit_offset % 8)
+			libbpf_print(level, " @ offset %u.%u)",
+				     spec->bit_offset / 8, spec->bit_offset % 8);
+		else
+			libbpf_print(level, " @ offset %u)", spec->bit_offset / 8);
+		return;
+	}
+}
+
+/*
+ * CO-RE relocate single instruction.
+ *
+ * The outline and important points of the algorithm:
+ * 1. For given local type, find corresponding candidate target types.
+ *    Candidate type is a type with the same "essential" name, ignoring
+ *    everything after last triple underscore (___). E.g., `sample`,
+ *    `sample___flavor_one`, `sample___flavor_another_one`, are all candidates
+ *    for each other. Names with triple underscore are referred to as
+ *    "flavors" and are useful, among other things, to allow to
+ *    specify/support incompatible variations of the same kernel struct, which
+ *    might differ between different kernel versions and/or build
+ *    configurations.
+ *
+ *    N.B. Struct "flavors" could be generated by bpftool's BTF-to-C
+ *    converter, when deduplicated BTF of a kernel still contains more than
+ *    one different types with the same name. In that case, ___2, ___3, etc
+ *    are appended starting from second name conflict. But start flavors are
+ *    also useful to be defined "locally", in BPF program, to extract same
+ *    data from incompatible changes between different kernel
+ *    versions/configurations. For instance, to handle field renames between
+ *    kernel versions, one can use two flavors of the struct name with the
+ *    same common name and use conditional relocations to extract that field,
+ *    depending on target kernel version.
+ * 2. For each candidate type, try to match local specification to this
+ *    candidate target type. Matching involves finding corresponding
+ *    high-level spec accessors, meaning that all named fields should match,
+ *    as well as all array accesses should be within the actual bounds. Also,
+ *    types should be compatible (see bpf_core_fields_are_compat for details).
+ * 3. It is supported and expected that there might be multiple flavors
+ *    matching the spec. As long as all the specs resolve to the same set of
+ *    offsets across all candidates, there is no error. If there is any
+ *    ambiguity, CO-RE relocation will fail. This is necessary to accomodate
+ *    imprefection of BTF deduplication, which can cause slight duplication of
+ *    the same BTF type, if some directly or indirectly referenced (by
+ *    pointer) type gets resolved to different actual types in different
+ *    object files. If such situation occurs, deduplicated BTF will end up
+ *    with two (or more) structurally identical types, which differ only in
+ *    types they refer to through pointer. This should be OK in most cases and
+ *    is not an error.
+ * 4. Candidate types search is performed by linearly scanning through all
+ *    types in target BTF. It is anticipated that this is overall more
+ *    efficient memory-wise and not significantly worse (if not better)
+ *    CPU-wise compared to prebuilding a map from all local type names to
+ *    a list of candidate type names. It's also sped up by caching resolved
+ *    list of matching candidates per each local "root" type ID, that has at
+ *    least one bpf_core_relo associated with it. This list is shared
+ *    between multiple relocations for the same type ID and is updated as some
+ *    of the candidates are pruned due to structural incompatibility.
+ */
+int bpf_core_apply_relo_insn(const char *prog_name, struct bpf_insn *insn,
+			     int insn_idx,
+			     const struct bpf_core_relo *relo,
+			     int relo_idx,
+			     const struct btf *local_btf,
+			     struct bpf_core_cand_list *cands)
+{
+	struct bpf_core_spec local_spec, cand_spec, targ_spec = {};
+	struct bpf_core_relo_res cand_res, targ_res;
+	const struct btf_type *local_type;
+	const char *local_name;
+	__u32 local_id;
+	const char *spec_str;
+	int i, j, err;
+
+	local_id = relo->type_id;
+	local_type = btf__type_by_id(local_btf, local_id);
+	if (!local_type)
+		return -EINVAL;
+
+	local_name = btf__name_by_offset(local_btf, local_type->name_off);
+	if (!local_name)
+		return -EINVAL;
+
+	spec_str = btf__name_by_offset(local_btf, relo->access_str_off);
+	if (str_is_empty(spec_str))
+		return -EINVAL;
+
+	err = bpf_core_parse_spec(local_btf, local_id, spec_str, relo->kind, &local_spec);
+	if (err) {
+		pr_warn("prog '%s': relo #%d: parsing [%d] %s %s + %s failed: %d\n",
+			prog_name, relo_idx, local_id, btf_kind_str(local_type),
+			str_is_empty(local_name) ? "<anon>" : local_name,
+			spec_str, err);
+		return -EINVAL;
+	}
+
+	pr_debug("prog '%s': relo #%d: kind <%s> (%d), spec is ", prog_name,
+		 relo_idx, core_relo_kind_str(relo->kind), relo->kind);
+	bpf_core_dump_spec(LIBBPF_DEBUG, &local_spec);
+	libbpf_print(LIBBPF_DEBUG, "\n");
+
+	/* TYPE_ID_LOCAL relo is special and doesn't need candidate search */
+	if (relo->kind == BPF_TYPE_ID_LOCAL) {
+		targ_res.validate = true;
+		targ_res.poison = false;
+		targ_res.orig_val = local_spec.root_type_id;
+		targ_res.new_val = local_spec.root_type_id;
+		goto patch_insn;
+	}
+
+	/* libbpf doesn't support candidate search for anonymous types */
+	if (str_is_empty(spec_str)) {
+		pr_warn("prog '%s': relo #%d: <%s> (%d) relocation doesn't support anonymous types\n",
+			prog_name, relo_idx, core_relo_kind_str(relo->kind), relo->kind);
+		return -EOPNOTSUPP;
+	}
+
+
+	for (i = 0, j = 0; i < cands->len; i++) {
+		err = bpf_core_spec_match(&local_spec, cands->cands[i].btf,
+					  cands->cands[i].id, &cand_spec);
+		if (err < 0) {
+			pr_warn("prog '%s': relo #%d: error matching candidate #%d ",
+				prog_name, relo_idx, i);
+			bpf_core_dump_spec(LIBBPF_WARN, &cand_spec);
+			libbpf_print(LIBBPF_WARN, ": %d\n", err);
+			return err;
+		}
+
+		pr_debug("prog '%s': relo #%d: %s candidate #%d ", prog_name,
+			 relo_idx, err == 0 ? "non-matching" : "matching", i);
+		bpf_core_dump_spec(LIBBPF_DEBUG, &cand_spec);
+		libbpf_print(LIBBPF_DEBUG, "\n");
+
+		if (err == 0)
+			continue;
+
+		err = bpf_core_calc_relo(prog_name, relo, relo_idx, &local_spec, &cand_spec, &cand_res);
+		if (err)
+			return err;
+
+		if (j == 0) {
+			targ_res = cand_res;
+			targ_spec = cand_spec;
+		} else if (cand_spec.bit_offset != targ_spec.bit_offset) {
+			/* if there are many field relo candidates, they
+			 * should all resolve to the same bit offset
+			 */
+			pr_warn("prog '%s': relo #%d: field offset ambiguity: %u != %u\n",
+				prog_name, relo_idx, cand_spec.bit_offset,
+				targ_spec.bit_offset);
+			return -EINVAL;
+		} else if (cand_res.poison != targ_res.poison || cand_res.new_val != targ_res.new_val) {
+			/* all candidates should result in the same relocation
+			 * decision and value, otherwise it's dangerous to
+			 * proceed due to ambiguity
+			 */
+			pr_warn("prog '%s': relo #%d: relocation decision ambiguity: %s %u != %s %u\n",
+				prog_name, relo_idx,
+				cand_res.poison ? "failure" : "success", cand_res.new_val,
+				targ_res.poison ? "failure" : "success", targ_res.new_val);
+			return -EINVAL;
+		}
+
+		cands->cands[j++] = cands->cands[i];
+	}
+
+	/*
+	 * For BPF_FIELD_EXISTS relo or when used BPF program has field
+	 * existence checks or kernel version/config checks, it's expected
+	 * that we might not find any candidates. In this case, if field
+	 * wasn't found in any candidate, the list of candidates shouldn't
+	 * change at all, we'll just handle relocating appropriately,
+	 * depending on relo's kind.
+	 */
+	if (j > 0)
+		cands->len = j;
+
+	/*
+	 * If no candidates were found, it might be both a programmer error,
+	 * as well as expected case, depending whether instruction w/
+	 * relocation is guarded in some way that makes it unreachable (dead
+	 * code) if relocation can't be resolved. This is handled in
+	 * bpf_core_patch_insn() uniformly by replacing that instruction with
+	 * BPF helper call insn (using invalid helper ID). If that instruction
+	 * is indeed unreachable, then it will be ignored and eliminated by
+	 * verifier. If it was an error, then verifier will complain and point
+	 * to a specific instruction number in its log.
+	 */
+	if (j == 0) {
+		pr_debug("prog '%s': relo #%d: no matching targets found\n",
+			 prog_name, relo_idx);
+
+		/* calculate single target relo result explicitly */
+		err = bpf_core_calc_relo(prog_name, relo, relo_idx, &local_spec, NULL, &targ_res);
+		if (err)
+			return err;
+	}
+
+patch_insn:
+	/* bpf_core_patch_insn() should know how to handle missing targ_spec */
+	err = bpf_core_patch_insn(prog_name, insn, insn_idx, relo, relo_idx, &targ_res);
+	if (err) {
+		pr_warn("prog '%s': relo #%d: failed to patch insn #%u: %d\n",
+			prog_name, relo_idx, relo->insn_off / 8, err);
+		return -EINVAL;
+	}
+
+	return 0;
+}

tools/lib/bpf/relo_core.h

···
 	enum bpf_core_relo_kind kind;
 };
 
-struct bpf_core_cand
-{
+struct bpf_core_cand {
 	const struct btf *btf;
 	const struct btf_type *t;
 	const char *name;
···
 	int len;
 };
 
+int bpf_core_apply_relo_insn(const char *prog_name,
+			     struct bpf_insn *insn, int insn_idx,
+			     const struct bpf_core_relo *relo, int relo_idx,
+			     const struct btf *local_btf,
+			     struct bpf_core_cand_list *cands);
+int bpf_core_types_are_compat(const struct btf *local_btf, __u32 local_id,
+			      const struct btf *targ_btf, __u32 targ_id);
+
+size_t bpf_core_essential_name_len(const char *name);
 #endif