[RFC PATCH v2] bpftool: Add bpf_cookie to link output

[Dmitrii Dolgov <9erthalion6@xxxxxxxxx>]

Commit 82e6b1eee6a8 ("bpf: Allow to specify user-provided bpf_cookie for
BPF perf links") introduced the concept of user specified bpf_cookie,
which could be accessed by BPF programs using bpf_get_attach_cookie().
For troubleshooting purposes it is convenient to expose bpf_cookie via
bpftool as well, so there is no need to meddle with the target BPF
program itself.

    $ bpftool link
    1: type 7  prog 5  bpf_cookie 123
        pids bootstrap(87)

Signed-off-by: Dmitrii Dolgov <9erthalion6@xxxxxxxxx>
---
Changes in v2:
    - Display bpf_cookie in bpftool link command instead perf

    Previous discussion: https://lore.kernel.org/bpf/20220127082649.12134-1-9erthalion6@xxxxxxxxx

 include/uapi/linux/bpf.h       |  3 +++
 kernel/bpf/syscall.c           | 13 +++++++++++++
 tools/bpf/bpftool/link.c       |  2 ++
 tools/include/uapi/linux/bpf.h |  3 +++
 4 files changed, 21 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index a7f0ddedac1f..600da4496404 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -5850,6 +5850,9 @@ struct bpf_link_info {
 			__u32 target_obj_id; /* prog_id for PROG_EXT, otherwise btf object id */
 			__u32 target_btf_id; /* BTF type id inside the object */
 		} tracing;
+		struct {
+			__u64 bpf_cookie;
+		} perf;
 		struct {
 			__u64 cgroup_id;
 			__u32 attach_type;
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 72ce1edde950..94b7fa777fc7 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2948,6 +2948,7 @@ static const struct bpf_link_ops bpf_raw_tp_link_lops = {
 struct bpf_perf_link {
 	struct bpf_link link;
 	struct file *perf_file;
+	u64 bpf_cookie;
 };
 
 static void bpf_perf_link_release(struct bpf_link *link)
@@ -2966,9 +2967,20 @@ static void bpf_perf_link_dealloc(struct bpf_link *link)
 	kfree(perf_link);
 }
 
+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);
+
+	info->perf.bpf_cookie = perf_link->bpf_cookie;
+	return 0;
+}
+
 static const struct bpf_link_ops bpf_perf_link_lops = {
 	.release = bpf_perf_link_release,
 	.dealloc = bpf_perf_link_dealloc,
+	.fill_link_info = bpf_perf_link_fill_link_info,
 };
 
 static int bpf_perf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
@@ -2993,6 +3005,7 @@ static int bpf_perf_link_attach(const union bpf_attr *attr, struct bpf_prog *pro
 	}
 	bpf_link_init(&link->link, BPF_LINK_TYPE_PERF_EVENT, &bpf_perf_link_lops, prog);
 	link->perf_file = perf_file;
+	link->bpf_cookie = attr->link_create.perf_event.bpf_cookie;
 
 	err = bpf_link_prime(&link->link, &link_primer);
 	if (err) {
diff --git a/tools/bpf/bpftool/link.c b/tools/bpf/bpftool/link.c
index 97dec81950e5..3ddeacb3593f 100644
--- a/tools/bpf/bpftool/link.c
+++ b/tools/bpf/bpftool/link.c
@@ -243,6 +243,8 @@ static int show_link_close_plain(int fd, struct bpf_link_info *info)
 		printf("\n\tnetns_ino %u  ", info->netns.netns_ino);
 		show_link_attach_type_plain(info->netns.attach_type);
 		break;
+	case BPF_LINK_TYPE_PERF_EVENT:
+		printf("\n\tbpf_cookie %llu  ", info->perf.bpf_cookie);
 	default:
 		break;
 	}
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index a7f0ddedac1f..600da4496404 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -5850,6 +5850,9 @@ struct bpf_link_info {
 			__u32 target_obj_id; /* prog_id for PROG_EXT, otherwise btf object id */
 			__u32 target_btf_id; /* BTF type id inside the object */
 		} tracing;
+		struct {
+			__u64 bpf_cookie;
+		} perf;
 		struct {
 			__u64 cgroup_id;
 			__u32 attach_type;
-- 
2.32.0