bpftool: Add bpf_cookie to link output

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.

Implemented using the pid iterator BPF program to actually fetch
bpf_cookies, which allows constraining code changes only to bpftool.

$ bpftool link
1: type 7  prog 5
        bpf_cookie 123
        pids bootstrap(81)

Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220309163112.24141-1-9erthalion6@gmail.com

1 files changed, 22 insertions, 0 deletions

diff --git a/tools/bpf/bpftool/skeleton/pid_iter.bpf.c b/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
index f70702fcb224..eb05ea53afb1 100644
--- a/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
+++ b/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
@@ -38,6 +38,17 @@ static __always_inline __u32 get_obj_id(void *ent, enum bpf_obj_type type)
 	}
 }
 
+/* could be used only with BPF_LINK_TYPE_PERF_EVENT links */
+static __u64 get_bpf_cookie(struct bpf_link *link)
+{
+	struct bpf_perf_link *perf_link;
+	struct perf_event *event;
+
+	perf_link = container_of(link, struct bpf_perf_link, link);
+	event = BPF_CORE_READ(perf_link, perf_file, private_data);
+	return BPF_CORE_READ(event, bpf_cookie);
+}
+
 SEC("iter/task_file")
 int iter(struct bpf_iter__task_file *ctx)
 {
@@ -69,8 +80,19 @@ int iter(struct bpf_iter__task_file *ctx)
 	if (file->f_op != fops)
 		return 0;
 
+	__builtin_memset(&e, 0, sizeof(e));
 	e.pid = task->tgid;
 	e.id = get_obj_id(file->private_data, obj_type);
+
+	if (obj_type == BPF_OBJ_LINK) {
+		struct bpf_link *link = (struct bpf_link *) file->private_data;
+
+		if (BPF_CORE_READ(link, type) == BPF_LINK_TYPE_PERF_EVENT) {
+			e.has_bpf_cookie = true;
+			e.bpf_cookie = get_bpf_cookie(link);
+		}
+	}
+
 	bpf_probe_read_kernel_str(&e.comm, sizeof(e.comm),
 				  task->group_leader->comm);
 	bpf_seq_write(ctx->meta->seq, &e, sizeof(e));