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

[Athira Rajeev]

> On 7 Jan 2025, at 2:55 AM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> 
> On Fri, Dec 27, 2024 at 04:18:32PM +0530, Athira Rajeev wrote:
>> 
>> 
>>> On 23 Dec 2024, at 7:28 PM, Athira Rajeev <atrajeev@xxxxxxxxxxxxxxxxxx> 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 <atrajeev@xxxxxxxxxxxxxxxxxx>
> 
> Reviewed-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> 
> Thanks,
> Namhyung
> 

Thanks Namhyung for reviewing the patch

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