linux-kernel - [PATCH] perf: fix segfault in `lock contention -b`

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Message-ID: <20260109164417.396138-1-tycho@kernel.org>
Date: Fri,  9 Jan 2026 09:44:17 -0700
From: Tycho Andersen <tycho@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	James Clark <james.clark@...aro.org>,
	Chun-Tse Shao <ctshao@...gle.com>,
	linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Ravi Bangoria <ravi.bangoria@....com>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: [PATCH] perf: fix segfault in `lock contention -b`

From: "Tycho Andersen (AMD)" <tycho@...nel.org>

When run on a kernel without BTF info, I get:

    libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?
    libbpf: failed to find valid kernel BTF

    Program received signal SIGSEGV, Segmentation fault.
    0x00005555556915b7 in btf.type_cnt ()
    (gdb) bt
    #0  0x00005555556915b7 in btf.type_cnt ()
    #1  0x0000555555691fbc in btf_find_by_name_kind ()
    #2  0x00005555556920d0 in btf.find_by_name_kind ()
    #3  0x00005555558a1b7c in init_numa_data (con=0x7fffffffd0a0) at util/bpf_lock_contention.c:125
    #4  0x00005555558a264b in lock_contention_prepare (con=0x7fffffffd0a0) at util/bpf_lock_contention.c:313
    #5  0x0000555555620702 in __cmd_contention (argc=0, argv=0x7fffffffea10) at builtin-lock.c:2084
    #6  0x0000555555622c8d in cmd_lock (argc=0, argv=0x7fffffffea10) at builtin-lock.c:2755
    #7  0x0000555555651451 in run_builtin (p=0x555556104f00 <commands+576>, argc=3, argv=0x7fffffffea10)
        at perf.c:349
    #8  0x00005555556516ed in handle_internal_command (argc=3, argv=0x7fffffffea10) at perf.c:401
    #9  0x000055555565184e in run_argv (argcp=0x7fffffffe7fc, argv=0x7fffffffe7f0) at perf.c:445
    #10 0x0000555555651b9f in main (argc=3, argv=0x7fffffffea10) at perf.c:553

If we really are running -b without BTF info, the error is fatal, so let's
propagate it and exit accordingly.

Signed-off-by: Tycho Andersen (AMD) <tycho@...nel.org>
CC: Ravi Bangoria <ravi.bangoria@....com>
CC: K Prateek Nayak <kprateek.nayak@....com>
---
 tools/perf/util/bpf_lock_contention.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
index 7b5671f13c53..0f207e577037 100644
--- a/tools/perf/util/bpf_lock_contention.c
+++ b/tools/perf/util/bpf_lock_contention.c
@@ -34,7 +34,7 @@ static bool slab_cache_equal(long key1, long key2, void *ctx __maybe_unused)
 	return key1 == key2;
 }
 
-static void check_slab_cache_iter(struct lock_contention *con)
+static int check_slab_cache_iter(struct lock_contention *con)
 {
 	s32 ret;
 
@@ -43,19 +43,20 @@ static void check_slab_cache_iter(struct lock_contention *con)
 	con->btf = btf__load_vmlinux_btf();
 	if (con->btf == NULL) {
 		pr_debug("BTF loading failed: %s\n", strerror(errno));
-		return;
+		return -errno;
 	}
 
 	ret = btf__find_by_name_kind(con->btf, "bpf_iter__kmem_cache", BTF_KIND_STRUCT);
 	if (ret < 0) {
 		bpf_program__set_autoload(skel->progs.slab_cache_iter, false);
 		pr_debug("slab cache iterator is not available: %d\n", ret);
-		return;
+		return ret;
 	}
 
 	has_slab_iter = true;
 
 	bpf_map__set_max_entries(skel->maps.slab_caches, con->map_nr_entries);
+	return 0;
 }
 
 static void run_slab_cache_iter(void)
@@ -301,7 +302,8 @@ int lock_contention_prepare(struct lock_contention *con)
 			skel->rodata->use_cgroup_v2 = 1;
 	}
 
-	check_slab_cache_iter(con);
+	if (check_slab_cache_iter(con) < 0)
+		return -1;
 
 	if (con->filters->nr_slabs && has_slab_iter) {
 		skel->rodata->has_slab = 1;

base-commit: 5572ad8fddecd4a0db19801262072ff5916b7589
-- 
2.52.0