libbpf: Print hint when PERF_EVENT_IOC_SET_BPF returns -EPROTO

The kernel prevents potential unwinder warnings and crashes by blocking
BPF program with bpf_get_[stack|stackid] on perf_event without
PERF_SAMPLE_CALLCHAIN, or with exclude_callchain_[kernel|user]. Print a
hint message in libbpf to help the user debug such issues.

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200723180648.1429892-4-songliubraving@fb.com

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index a05aa7e..e51479d 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -7833,6 +7833,9 @@ struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
                pr_warn("program '%s': failed to attach to pfd %d: %s\n",
                        bpf_program__title(prog, false), pfd,
                           libbpf_strerror_r(err, errmsg, sizeof(errmsg)));
+               if (err == -EPROTO)
+                       pr_warn("program '%s': try add PERF_SAMPLE_CALLCHAIN to or remove exclude_callchain_[kernel|user] from pfd %d\n",
+                               bpf_program__title(prog, false), pfd);
                return ERR_PTR(err);
        }
        if (ioctl(pfd, PERF_EVENT_IOC_ENABLE, 0) < 0) {