[PATCH bpf-next v2 3/3] bpf: Show target_{obj,btf}_id in tracing link info

[Yafang Shao <laoar.shao@gmail.com>]

The target_btf_id can help us understand which kernel function is
linked by a tracing prog. The target_btf_id and target_obj_id have
already been exposed to userspace, so we just need to show them.

The result as follows,

tools/bpf/bpftool/bpftool link show
2: tracing  prog 13
        prog_type tracing  attach_type trace_fentry
        target_obj_id 1  target_btf_id 13964
        pids trace(10673)

$ tools/bpf/bpftool/bpftool link show -j
[{"id":2,"type":"tracing","prog_id":13,"prog_type":"tracing","attach_type":"trace_fentry","target_obj_id":1,"target_btf_id":13964,"pids":[{"pid":10673,"comm":"trace"}]}]

$ cat /proc/10673/fdinfo/10
pos:    0
flags:  02000000
mnt_id: 15
ino:    2094
link_type:      tracing
link_id:        2
prog_tag:       a04f5eef06a7f555
prog_id:        13
attach_type:    24
target_obj_id:  1
target_btf_id:  13964

$ tail /proc/kallsyms
ffffffffc0400fa0 t bpf_prog_a04f5eef06a7f555_fentry_run [bpf]
ffffffffc062f000 t bpf_trampoline_6442464908    [bpf]

$ echo $((6442464908 & 0x7fffffff)) $((6442464908 >> 32))
13964 1

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 kernel/bpf/syscall.c     | 12 ++++++++++--
 tools/bpf/bpftool/link.c |  4 ++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 909c112..870395a 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2968,10 +2968,18 @@ static void bpf_tracing_link_show_fdinfo(const struct bpf_link *link,
 {
 	struct bpf_tracing_link *tr_link =
 		container_of(link, struct bpf_tracing_link, link.link);
+	u32 target_btf_id;
+	u32 target_obj_id;
 
+	bpf_trampoline_unpack_key(tr_link->trampoline->key,
+							  &target_obj_id, &target_btf_id);
 	seq_printf(seq,
-		   "attach_type:\t%d\n",
-		   tr_link->attach_type);
+		   "attach_type:\t%d\n"
+		   "target_obj_id:\t%u\n"
+		   "target_btf_id:\t%u\n",
+		   tr_link->attach_type,
+		   target_obj_id,
+		   target_btf_id);
 }
 
 static int bpf_tracing_link_fill_link_info(const struct bpf_link *link,
diff --git a/tools/bpf/bpftool/link.c b/tools/bpf/bpftool/link.c
index 243b74e..cfe896f 100644
--- a/tools/bpf/bpftool/link.c
+++ b/tools/bpf/bpftool/link.c
@@ -195,6 +195,8 @@ static int show_link_close_json(int fd, struct bpf_link_info *info)
 
 		show_link_attach_type_json(info->tracing.attach_type,
 					   json_wtr);
+		jsonw_uint_field(json_wtr, "target_obj_id", info->tracing.target_obj_id);
+		jsonw_uint_field(json_wtr, "target_btf_id", info->tracing.target_btf_id);
 		break;
 	case BPF_LINK_TYPE_CGROUP:
 		jsonw_lluint_field(json_wtr, "cgroup_id",
@@ -375,6 +377,8 @@ static int show_link_close_plain(int fd, struct bpf_link_info *info)
 			printf("\n\tprog_type %u  ", prog_info.type);
 
 		show_link_attach_type_plain(info->tracing.attach_type);
+		printf("\n\ttarget_obj_id %u  target_btf_id %u  ",
+			   info->tracing.target_obj_id, info->tracing.target_btf_id);
 		break;
 	case BPF_LINK_TYPE_CGROUP:
 		printf("\n\tcgroup_id %zu  ", (size_t)info->cgroup.cgroup_id);
-- 
1.8.3.1