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 <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Acked-by: Quentin Monnet <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/main.h

···
 
 struct obj_refs {
 	int ref_cnt;
+	bool has_bpf_cookie;
 	struct obj_ref *refs;
+	__u64 bpf_cookie;
 };
 
 struct btf;

tools/bpf/bpftool/pids.c

···
 	ref->pid = e->pid;
 	memcpy(ref->comm, e->comm, sizeof(ref->comm));
 	refs->ref_cnt = 1;
+	refs->has_bpf_cookie = e->has_bpf_cookie;
+	refs->bpf_cookie = e->bpf_cookie;
 
 	err = hashmap__append(map, u32_as_hash_field(e->id), refs);
 	if (err)
···
 		if (refs->ref_cnt == 0)
 			break;
 
+		if (refs->has_bpf_cookie)
+			jsonw_lluint_field(json_writer, "bpf_cookie", refs->bpf_cookie);
+
 		jsonw_name(json_writer, "pids");
 		jsonw_start_array(json_writer);
 		for (i = 0; i < refs->ref_cnt; i++) {
···
 
 		if (refs->ref_cnt == 0)
 			break;
+
+		if (refs->has_bpf_cookie)
+			printf("\n\tbpf_cookie %llu", (unsigned long long) refs->bpf_cookie);
 
 		printf("%s", prefix);
 		for (i = 0; i < refs->ref_cnt; i++) {

tools/bpf/bpftool/skeleton/pid_iter.bpf.c

···
 	}
 }
 
+/* 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)
 {
···
 	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));

tools/bpf/bpftool/skeleton/pid_iter.h

···
 struct pid_iter_entry {
 	__u32 id;
 	int pid;
+	__u64 bpf_cookie;
+	bool has_bpf_cookie;
 	char comm[16];
 };