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

[Namhyung Kim]

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

> > ---
> > 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
> > 
>