bpftool: btf: Support dumping a specific types from file

Some projects, for example xdp-tools [0], prefer to check in a minimized
vmlinux.h rather than the complete file which can get rather large.

However, when you try to add a minimized version of a complex struct (eg
struct xfrm_state), things can get quite complex if you're trying to
manually untangle and deduplicate the dependencies.

This commit teaches bpftool to do a minimized dump of a specific types by
providing a optional root_id argument(s).

Example usage:

$ ./bpftool btf dump file ~/dev/linux/vmlinux | rg "STRUCT 'xfrm_state'"
[12643] STRUCT 'xfrm_state' size=912 vlen=58

$ ./bpftool btf dump file ~/dev/linux/vmlinux root_id 12643 format c
#ifndef __VMLINUX_H__
#define __VMLINUX_H__

[..]

struct xfrm_type_offload;

struct xfrm_sec_ctx;

struct xfrm_state {
possible_net_t xs_net;
union {
struct hlist_node gclist;
struct hlist_node bydst;
};
union {
struct hlist_node dev_gclist;
struct hlist_node bysrc;
};
struct hlist_node byspi;
[..]

[0]: https://github.com/xdp-project/xdp-tools/blob/master/headers/bpf/vmlinux.h

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

tools/bpf/bpftool/Documentation/bpftool-btf.rst

···
 =============
 
 | **bpftool** **btf** { **show** | **list** } [**id** *BTF_ID*]
-| **bpftool** **btf dump** *BTF_SRC* [**format** *FORMAT*]
+| **bpftool** **btf dump** *BTF_SRC* [**format** *FORMAT*] [**root_id** *ROOT_ID*]
 | **bpftool** **btf help**
 |
 | *BTF_SRC* := { **id** *BTF_ID* | **prog** *PROG* | **map** *MAP* [{**key** | **value** | **kv** | **all**}] | **file** *FILE* }
···
     that hold open file descriptors (FDs) against BTF objects. On such kernels
     bpftool will automatically emit this information as well.
 
-bpftool btf dump *BTF_SRC* [format *FORMAT*]
+bpftool btf dump *BTF_SRC* [format *FORMAT*] [root_id *ROOT_ID*]
     Dump BTF entries from a given *BTF_SRC*.
 
     When **id** is specified, BTF object with that ID will be loaded and all
···
     (**raw**) or C-syntax (**c**) output formats are supported. With C-style
     formatting, the output is sorted by default. Use the **unsorted** option
     to avoid sorting the output.
+
+    **root_id** option can be used to filter a dump to a single type and all
+    its dependent types. It cannot be used with any other types of filtering
+    (such as the "key", "value", or "kv" arguments when dumping BTF for a map).
+    It can be passed multiple times to dump multiple types.
 
 bpftool btf help
     Print short help message.

tools/bpf/bpftool/btf.c

···
 #define KFUNC_DECL_TAG		"bpf_kfunc"
 #define FASTCALL_DECL_TAG	"bpf_fastcall"
 
+#define MAX_ROOT_IDS		16
+
 static const char * const btf_kind_str[NR_BTF_KINDS] = {
 	[BTF_KIND_UNKN]		= "UNKNOWN",
 	[BTF_KIND_INT]		= "INT",
···
 {
 	bool dump_c = false, sort_dump_c = true;
 	struct btf *btf = NULL, *base = NULL;
-	__u32 root_type_ids[2];
+	__u32 root_type_ids[MAX_ROOT_IDS];
+	bool have_id_filtering;
 	int root_type_cnt = 0;
 	__u32 btf_id = -1;
 	const char *src;
···
 		goto done;
 	}
 
+	have_id_filtering = !!root_type_cnt;
+
 	while (argc) {
 		if (is_prefix(*argv, "format")) {
 			NEXT_ARG();
···
 				err = -EINVAL;
 				goto done;
 			}
+			NEXT_ARG();
+		} else if (is_prefix(*argv, "root_id")) {
+			__u32 root_id;
+			char *end;
+
+			if (have_id_filtering) {
+				p_err("cannot use root_id with other type filtering");
+				err = -EINVAL;
+				goto done;
+			} else if (root_type_cnt == MAX_ROOT_IDS) {
+				p_err("only %d root_id are supported", MAX_ROOT_IDS);
+				err = -E2BIG;
+				goto done;
+			}
+
+			NEXT_ARG();
+			root_id = strtoul(*argv, &end, 0);
+			if (*end) {
+				err = -1;
+				p_err("can't parse %s as root ID", *argv);
+				goto done;
+			}
+			for (i = 0; i < root_type_cnt; i++) {
+				if (root_type_ids[i] == root_id) {
+					err = -EINVAL;
+					p_err("duplicate root_id %d supplied", root_id);
+					goto done;
+				}
+			}
+			root_type_ids[root_type_cnt++] = root_id;
 			NEXT_ARG();
 		} else if (is_prefix(*argv, "unsorted")) {
 			sort_dump_c = false;
···
 
 	fprintf(stderr,
 		"Usage: %1$s %2$s { show | list } [id BTF_ID]\n"
-		"       %1$s %2$s dump BTF_SRC [format FORMAT]\n"
+		"       %1$s %2$s dump BTF_SRC [format FORMAT] [root_id ROOT_ID]\n"
 		"       %1$s %2$s help\n"
 		"\n"
 		"       BTF_SRC := { id BTF_ID | prog PROG | map MAP [{key | value | kv | all}] | file FILE }\n"