On Fri, Dec 27, 2024 at 04:18:32PM +0530, Athira Rajeev wrote:

quoted

On 23 Dec 2024, at 7:28 PM, Athira Rajeev [off-list ref] wrote:

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 <redacted>

Reviewed-by: Namhyung Kim <namhyung@kernel.org>

Thanks,
Namhyung

quoted

---
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;
+ }
+

Hi,

Looking for review comments on this

Athira

quoted

type_id = btf__find_by_name_kind(btf, "btf_trace_sched_switch",
BTF_KIND_TYPEDEF);
if ((s32)type_id < 0)
-- 
2.43.5