linux-kernel - [PATCH v5 bpf-next 3/5] libbpf: print hint when PERF_EVENT_IOC_SET

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Date:   Thu, 23 Jul 2020 11:06:46 -0700
From:   Song Liu <songliubraving@...com>
To:     <linux-kernel@...r.kernel.org>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>
CC:     <ast@...nel.org>, <daniel@...earbox.net>, <kernel-team@...com>,
        <john.fastabend@...il.com>, <kpsingh@...omium.org>,
        <brouer@...hat.com>, <peterz@...radead.org>,
        Song Liu <songliubraving@...com>
Subject: [PATCH v5 bpf-next 3/5] 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@...com>
---
 tools/lib/bpf/libbpf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 846164c79df1c..484e50d49a4cf 100644
--- 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) {
-- 
2.24.1