linux-kernel - Re: [PATCH] perf: fix segfault in `lock contention -b`

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Message-ID: <aWa2vDy_0QGh_a20@google.com>
Date: Tue, 13 Jan 2026 13:18:52 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Tycho Andersen <tycho@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	James Clark <james.clark@...aro.org>,
	Chun-Tse Shao <ctshao@...gle.com>, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ravi Bangoria <ravi.bangoria@....com>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [PATCH] perf: fix segfault in `lock contention -b`

On Mon, Jan 12, 2026 at 09:15:53AM -0700, Tycho Andersen wrote:
> On Fri, Jan 09, 2026 at 04:49:01PM -0800, Namhyung Kim wrote:
> > On Fri, Jan 09, 2026 at 09:44:17AM -0700, Tycho Andersen wrote:
> > > From: "Tycho Andersen (AMD)" <tycho@...nel.org>
> > > 
> > > When run on a kernel without BTF info, I get:
> > > 
> > >     libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?
> > >     libbpf: failed to find valid kernel BTF
> > > 
> > >     Program received signal SIGSEGV, Segmentation fault.
> > >     0x00005555556915b7 in btf.type_cnt ()
> > >     (gdb) bt
> > >     #0  0x00005555556915b7 in btf.type_cnt ()
> > >     #1  0x0000555555691fbc in btf_find_by_name_kind ()
> > >     #2  0x00005555556920d0 in btf.find_by_name_kind ()
> > >     #3  0x00005555558a1b7c in init_numa_data (con=0x7fffffffd0a0) at util/bpf_lock_contention.c:125
> > >     #4  0x00005555558a264b in lock_contention_prepare (con=0x7fffffffd0a0) at util/bpf_lock_contention.c:313
> > >     #5  0x0000555555620702 in __cmd_contention (argc=0, argv=0x7fffffffea10) at builtin-lock.c:2084
> > >     #6  0x0000555555622c8d in cmd_lock (argc=0, argv=0x7fffffffea10) at builtin-lock.c:2755
> > >     #7  0x0000555555651451 in run_builtin (p=0x555556104f00 <commands+576>, argc=3, argv=0x7fffffffea10)
> > >         at perf.c:349
> > >     #8  0x00005555556516ed in handle_internal_command (argc=3, argv=0x7fffffffea10) at perf.c:401
> > >     #9  0x000055555565184e in run_argv (argcp=0x7fffffffe7fc, argv=0x7fffffffe7f0) at perf.c:445
> > >     #10 0x0000555555651b9f in main (argc=3, argv=0x7fffffffea10) at perf.c:553
> > > 
> > > If we really are running -b without BTF info, the error is fatal, so let's
> > > propagate it and exit accordingly.
> > 
> > Thanks for the patch.
> > 
> > I'm curious how the kernel will react if we want to load the BPF.
> > Probably the verifier will reject as it calls some helpers like
> > bpf_get_current_task_btf().
> > 
> > I think it used to work without BTF - the callstack (default) and task
> > output mode won't need it.  It'd be nice if we could run it when the
> > kernel doesn't have BTF.  But maybe it's out of scope of this patch.
> > 
> > That said, can you add a NULL check in the init_numa_data() instead
> > and see how far it can go?
> 
> Looks like not much further, it fails in lock_contention_bpf__load():
> 
> ./perf lock contention -b
> libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?
> libbpf: failed to find valid kernel BTF
> libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?
> libbpf: failed to find valid kernel BTF
> libbpf: Error loading vmlinux BTF: -ESRCH
> libbpf: failed to load BPF skeleton 'lock_contention_bpf': -ESRCH
> Failed to load lock-contention BPF skeleton
> lock contention BPF setup failed

Thanks for checking this.  I think it's ok to fail like this when BTF is
not enabled.  Let me see if I can make some changes to run without it
later.

Thanks,
Namhyung