[RFC PATCH bpf-next 5/8] bpf: Support ->show_fdinfo for perf_event

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

By adding support for ->show_fdinfo to the perf_event link, users will be
able to examine it through the task's fdinfo. The expected result is as
follows:

$ cat /proc/9637/fdinfo/11
pos:    0
flags:  02000000
mnt_id: 15
ino:    2094
link_type:      perf
link_id:        1
prog_tag:       a04f5eef06a7f555
prog_id:        5
func:   kernel_clone
addr:   ffffffff8d0bc310
offset: 0

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 kernel/bpf/syscall.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 92a57ef..e6b5127 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3297,9 +3297,36 @@ static void bpf_perf_link_dealloc(struct bpf_link *link)
 	kfree(perf_link);
 }
 
+static void bpf_perf_link_show_fdinfo(const struct bpf_link *link,
+				      struct seq_file *seq)
+{
+	struct bpf_perf_link *perf_link = container_of(link, struct bpf_perf_link, link);
+	const struct perf_event *event;
+	u64 probe_offset, probe_addr;
+	u32 prog_id, fd_type;
+	const char *buf;
+	int err;
+
+	event = perf_get_event(perf_link->perf_file);
+	if (IS_ERR(event))
+		return;
+
+	err = bpf_get_perf_event_info(event, &prog_id, &fd_type,
+				      &buf, &probe_offset,
+				      &probe_addr);
+	if (err)
+		return;
+
+	if (buf)
+		seq_printf(seq, "func:\t%s\n", buf);
+	seq_printf(seq, "addr:\t%llx\n", probe_addr);
+	seq_printf(seq, "offset:\t%llu\n", probe_offset);
+}
+
 static const struct bpf_link_ops bpf_perf_link_lops = {
 	.release = bpf_perf_link_release,
 	.dealloc = bpf_perf_link_dealloc,
+	.show_fdinfo = bpf_perf_link_show_fdinfo,
 };
 
 static int bpf_perf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
-- 
1.8.3.1