linux-kernel - [PATCH] tools/perf: Fix segfault during perf record --off-cpu when debuginfo is not enabled

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Message-Id: <20241223135813.8175-1-atrajeev@linux.vnet.ibm.com>
Date: Mon, 23 Dec 2024 19:28:13 +0530
From: Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
To: acme@...nel.org, jolsa@...nel.org, adrian.hunter@...el.com,
        irogers@...gle.com, namhyung@...nel.org, hbathini@...ux.ibm.com
Cc: linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, maddy@...ux.ibm.com,
        atrajeev@...ux.vnet.ibm.com, kjain@...ux.ibm.com,
        disgoel@...ux.vnet.ibm.com
Subject: [PATCH] tools/perf: Fix segfault during perf record --off-cpu when debuginfo is not enabled

When kernel is built without debuginfo, running perf record with
--off-cpu results in segfault as below:

   ./perf record --off-cpu -e dummy sleep 1
   libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?
   libbpf: failed to find '.BTF' ELF section in /lib/modules/6.13.0-rc3+/build/vmlinux
   libbpf: failed to find valid kernel BTF
   Segmentation fault (core dumped)

The backtrace pointed to:

   #0  0x00000000100fb17c in btf.type_cnt ()
   #1  0x00000000100fc1a8 in btf_find_by_name_kind ()
   #2  0x00000000100fc38c in btf.find_by_name_kind ()
   #3  0x00000000102ee3ac in off_cpu_prepare ()
   #4  0x000000001002f78c in cmd_record ()
   #5  0x00000000100aee78 in run_builtin ()
   #6  0x00000000100af3e4 in handle_internal_command ()
   #7  0x000000001001004c in main ()

Code sequence is:
   static void check_sched_switch_args(void)
   {
        struct btf *btf = btf__load_vmlinux_btf();
        const struct btf_type *t1, *t2, *t3;
        u32 type_id;

        type_id = btf__find_by_name_kind(btf, "btf_trace_sched_switch",
                                         BTF_KIND_TYPEDEF);

btf__load_vmlinux_btf fails when CONFIG_DEBUG_INFO_BTF is not enabled.
Here bpf__find_by_name_kind calls btf__type_cnt with NULL btf
value and results in segfault. To fix this, add a check to see if
btf is not NULL before invoking bpf__find_by_name_kind

Signed-off-by: Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
---
 tools/perf/util/bpf_off_cpu.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/perf/util/bpf_off_cpu.c b/tools/perf/util/bpf_off_cpu.c
index a590a8ac1f9d..4269b41d1771 100644
--- a/tools/perf/util/bpf_off_cpu.c
+++ b/tools/perf/util/bpf_off_cpu.c
@@ -100,6 +100,11 @@ static void check_sched_switch_args(void)
 	const struct btf_type *t1, *t2, *t3;
 	u32 type_id;
 
+	if (!btf) {
+		pr_debug("Missing btf, check if CONFIG_DEBUG_INFO_BTF is enabled\n");
+		goto cleanup;
+	}
+
 	type_id = btf__find_by_name_kind(btf, "btf_trace_sched_switch",
 					 BTF_KIND_TYPEDEF);
 	if ((s32)type_id < 0)
-- 
2.43.5