libbpf: Support kfunc detection in light skeleton.

Teach gen_loader to find {btf_id, btf_obj_fd} of kernel variables and kfuncs
and populate corresponding ld_imm64 and bpf_call insns.

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/bpf_gen_internal.h

···
 	int insn_idx;
 	bool is_weak;
 	bool is_typeless;
+	bool is_ld64;
 };
 
 struct ksym_desc {
···
 		bool typeless;
 	};
 	int insn;
+	bool is_ld64;
 };
 
 struct bpf_gen {
···
 void bpf_gen__map_freeze(struct bpf_gen *gen, int map_idx);
 void bpf_gen__record_attach_target(struct bpf_gen *gen, const char *name, enum bpf_attach_type type);
 void bpf_gen__record_extern(struct bpf_gen *gen, const char *name, bool is_weak,
-			    bool is_typeless, int kind, int insn_idx);
+			    bool is_typeless, bool is_ld64, int kind, int insn_idx);
 void bpf_gen__record_relo_core(struct bpf_gen *gen, const struct bpf_core_relo *core_relo);
 void bpf_gen__populate_outer_map(struct bpf_gen *gen, int outer_map_idx, int key, int inner_map_idx);
 

tools/lib/bpf/gen_loader.c

···
 }
 
 void bpf_gen__record_extern(struct bpf_gen *gen, const char *name, bool is_weak,
-			    bool is_typeless, int kind, int insn_idx)
+			    bool is_typeless, bool is_ld64, int kind, int insn_idx)
 {
 	struct ksym_relo_desc *relo;
 
···
 	relo->name = name;
 	relo->is_weak = is_weak;
 	relo->is_typeless = is_typeless;
+	relo->is_ld64 = is_ld64;
 	relo->kind = kind;
 	relo->insn_idx = insn_idx;
 	gen->relo_cnt++;
···
 	int i;
 
 	for (i = 0; i < gen->nr_ksyms; i++) {
-		if (!strcmp(gen->ksyms[i].name, relo->name)) {
-			gen->ksyms[i].ref++;
-			return &gen->ksyms[i];
+		kdesc = &gen->ksyms[i];
+		if (kdesc->kind == relo->kind && kdesc->is_ld64 == relo->is_ld64 &&
+		    !strcmp(kdesc->name, relo->name)) {
+			kdesc->ref++;
+			return kdesc;
 		}
 	}
 	kdesc = libbpf_reallocarray(gen->ksyms, gen->nr_ksyms + 1, sizeof(*kdesc));
···
 	kdesc->ref = 1;
 	kdesc->off = 0;
 	kdesc->insn = 0;
+	kdesc->is_ld64 = relo->is_ld64;
 	return kdesc;
 }
 
···
 {
 	int insn;
 
-	pr_debug("gen: emit_relo (%d): %s at %d\n", relo->kind, relo->name, relo->insn_idx);
+	pr_debug("gen: emit_relo (%d): %s at %d %s\n",
+		 relo->kind, relo->name, relo->insn_idx, relo->is_ld64 ? "ld64" : "call");
 	insn = insns + sizeof(struct bpf_insn) * relo->insn_idx;
 	emit2(gen, BPF_LD_IMM64_RAW_FULL(BPF_REG_8, BPF_PSEUDO_MAP_IDX_VALUE, 0, 0, 0, insn));
-	switch (relo->kind) {
-	case BTF_KIND_VAR:
+	if (relo->is_ld64) {
 		if (relo->is_typeless)
 			emit_relo_ksym_typeless(gen, relo, insn);
 		else
 			emit_relo_ksym_btf(gen, relo, insn);
-		break;
-	case BTF_KIND_FUNC:
+	} else {
 		emit_relo_kfunc_btf(gen, relo, insn);
-		break;
-	default:
-		pr_warn("Unknown relocation kind '%d'\n", relo->kind);
-		gen->error = -EDOM;
-		return;
 	}
 }
 
···
 
 static void cleanup_relos(struct bpf_gen *gen, int insns)
 {
+	struct ksym_desc *kdesc;
 	int i, insn;
 
 	for (i = 0; i < gen->nr_ksyms; i++) {
+		kdesc = &gen->ksyms[i];
 		/* only close fds for typed ksyms and kfuncs */
-		if (gen->ksyms[i].kind == BTF_KIND_VAR && !gen->ksyms[i].typeless) {
+		if (kdesc->is_ld64 && !kdesc->typeless) {
 			/* close fd recorded in insn[insn_idx + 1].imm */
-			insn = gen->ksyms[i].insn;
+			insn = kdesc->insn;
 			insn += sizeof(struct bpf_insn) + offsetof(struct bpf_insn, imm);
 			emit_sys_close_blob(gen, insn);
-		} else if (gen->ksyms[i].kind == BTF_KIND_FUNC) {
-			emit_sys_close_blob(gen, blob_fd_array_off(gen, gen->ksyms[i].off));
-			if (gen->ksyms[i].off < MAX_FD_ARRAY_SZ)
+		} else if (!kdesc->is_ld64) {
+			emit_sys_close_blob(gen, blob_fd_array_off(gen, kdesc->off));
+			if (kdesc->off < MAX_FD_ARRAY_SZ)
 				gen->nr_fd_array--;
 		}
 	}

tools/lib/bpf/libbpf.c

···
 	for (i = 0; i < prog->nr_reloc; i++) {
 		struct reloc_desc *relo = &prog->reloc_desc[i];
 		struct extern_desc *ext = &obj->externs[relo->sym_off];
+		int kind;
 
 		switch (relo->type) {
 		case RELO_EXTERN_LD64:
 			if (ext->type != EXT_KSYM)
 				continue;
+			kind = btf_is_var(btf__type_by_id(obj->btf, ext->btf_id)) ?
+				BTF_KIND_VAR : BTF_KIND_FUNC;
 			bpf_gen__record_extern(obj->gen_loader, ext->name,
 					       ext->is_weak, !ext->ksym.type_id,
-					       BTF_KIND_VAR, relo->insn_idx);
+					       true, kind, relo->insn_idx);
 			break;
 		case RELO_EXTERN_CALL:
 			bpf_gen__record_extern(obj->gen_loader, ext->name,
-					       ext->is_weak, false, BTF_KIND_FUNC,
+					       ext->is_weak, false, false, BTF_KIND_FUNC,
 					       relo->insn_idx);
 			break;
 		case RELO_CORE: {