[RFC PATCH bpf-next 7/8] bpf: Support ->fill_link_info for perf_event

[Yafang Shao <laoar.shao@xxxxxxxxx>]

By adding support for ->fill_link_info to the perf_event link, users will
be able to inspect it using `bpftool link show`. While users can currently
access this information via `bpftool perf show`, consolidating the link
information for all link types in one place would be more convenient.

Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
---
 include/uapi/linux/bpf.h       |  6 ++++++
 kernel/bpf/syscall.c           | 46 ++++++++++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h |  6 ++++++
 3 files changed, 58 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 6be9b1d..1f2be1d 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -6438,6 +6438,12 @@ struct bpf_link_info {
 			__aligned_u64 addrs;
 			__u32 count;
 		} kprobe_multi;
+		struct {
+			__aligned_u64 name;
+			__aligned_u64 addr;
+			__u32 name_len;
+			__u32 offset;
+		} perf_event;
 	};
 } __attribute__((aligned(8)));
 
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 33a72ec..b12707e 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3329,10 +3329,56 @@ static void bpf_perf_link_show_fdinfo(const struct bpf_link *link,
 	seq_printf(seq, "offset:\t%llu\n", probe_offset);
 }
 
+static int bpf_perf_link_fill_link_info(const struct bpf_link *link,
+					struct bpf_link_info *info)
+{
+	struct bpf_perf_link *perf_link = container_of(link, struct bpf_perf_link, link);
+	char __user *ubuf = u64_to_user_ptr(info->perf_event.name);
+	u32 ulen = info->perf_event.name_len;
+	const struct perf_event *event;
+	u64 probe_offset, probe_addr;
+	u32 prog_id, fd_type;
+	const char *buf;
+	size_t len;
+	int err;
+
+	if (!ulen ^ !ubuf)
+		return -EINVAL;
+	if (!ubuf)
+		return 0;
+
+	event = perf_get_event(perf_link->perf_file);
+	if (IS_ERR(event))
+		return PTR_ERR(event);
+
+	err = bpf_get_perf_event_info(event, &prog_id, &fd_type,
+				      &buf, &probe_offset,
+				      &probe_addr);
+	if (err)
+		return err;
+
+	len = strlen(buf);
+	info->perf_event.name_len = len + 1;
+	if (buf) {
+		err = bpf_copy_to_user(ubuf, buf, ulen, len);
+		if (err)
+			return err;
+	} else {
+		char zero = '\0';
+
+		if (put_user(zero, ubuf))
+			return -EFAULT;
+	}
+	info->perf_event.addr = probe_addr;
+	info->perf_event.offset = probe_offset;
+	return 0;
+}
+
 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,
+	.fill_link_info = bpf_perf_link_fill_link_info,
 };
 
 static int bpf_perf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 6be9b1d..1f2be1d 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -6438,6 +6438,12 @@ struct bpf_link_info {
 			__aligned_u64 addrs;
 			__u32 count;
 		} kprobe_multi;
+		struct {
+			__aligned_u64 name;
+			__aligned_u64 addr;
+			__u32 name_len;
+			__u32 offset;
+		} perf_event;
 	};
 } __attribute__((aligned(8)));
 
-- 
1.8.3.1