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

[Athira Rajeev <atrajeev@linux.vnet.ibm.com>]

> On 23 Dec 2024, at 7:28 PM, Athira Rajeev <atrajeev@linux.vnet.ibm.com> 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@linux.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;
> + }
> +

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
>