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

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

[Tycho Andersen]

From: "Tycho Andersen (AMD)" <tycho@kernel.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.

Signed-off-by: Tycho Andersen (AMD) <tycho@kernel.org>
CC: Ravi Bangoria <ravi.bangoria@amd.com>
CC: K Prateek Nayak <kprateek.nayak@amd.com>
---
 tools/perf/util/bpf_lock_contention.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
index 7b5671f13c53..0f207e577037 100644
--- a/tools/perf/util/bpf_lock_contention.c
+++ b/tools/perf/util/bpf_lock_contention.c
@@ -34,7 +34,7 @@ static bool slab_cache_equal(long key1, long key2, void *ctx __maybe_unused)
 	return key1 == key2;
 }
 
-static void check_slab_cache_iter(struct lock_contention *con)
+static int check_slab_cache_iter(struct lock_contention *con)
 {
 	s32 ret;
 
@@ -43,19 +43,20 @@ static void check_slab_cache_iter(struct lock_contention *con)
 	con->btf = btf__load_vmlinux_btf();
 	if (con->btf == NULL) {
 		pr_debug("BTF loading failed: %s\n", strerror(errno));
-		return;
+		return -errno;
 	}
 
 	ret = btf__find_by_name_kind(con->btf, "bpf_iter__kmem_cache", BTF_KIND_STRUCT);
 	if (ret < 0) {
 		bpf_program__set_autoload(skel->progs.slab_cache_iter, false);
 		pr_debug("slab cache iterator is not available: %d\n", ret);
-		return;
+		return ret;
 	}
 
 	has_slab_iter = true;
 
 	bpf_map__set_max_entries(skel->maps.slab_caches, con->map_nr_entries);
+	return 0;
 }
 
 static void run_slab_cache_iter(void)
@@ -301,7 +302,8 @@ int lock_contention_prepare(struct lock_contention *con)
 			skel->rodata->use_cgroup_v2 = 1;
 	}
 
-	check_slab_cache_iter(con);
+	if (check_slab_cache_iter(con) < 0)
+		return -1;
 
 	if (con->filters->nr_slabs && has_slab_iter) {
 		skel->rodata->has_slab = 1;

base-commit: 5572ad8fddecd4a0db19801262072ff5916b7589
-- 
2.52.0

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

[Namhyung Kim]

On Fri, Jan 09, 2026 at 09:44:17AM -0700, Tycho Andersen wrote:
> From: "Tycho Andersen (AMD)" <tycho@kernel.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?

Thanks,
Namhyung

> 
> Signed-off-by: Tycho Andersen (AMD) <tycho@kernel.org>
> CC: Ravi Bangoria <ravi.bangoria@amd.com>
> CC: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
>  tools/perf/util/bpf_lock_contention.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/perf/util/bpf_lock_contention.c b/tools/perf/util/bpf_lock_contention.c
> index 7b5671f13c53..0f207e577037 100644
> --- a/tools/perf/util/bpf_lock_contention.c
> +++ b/tools/perf/util/bpf_lock_contention.c
> @@ -34,7 +34,7 @@ static bool slab_cache_equal(long key1, long key2, void *ctx __maybe_unused)
>  	return key1 == key2;
>  }
>  
> -static void check_slab_cache_iter(struct lock_contention *con)
> +static int check_slab_cache_iter(struct lock_contention *con)
>  {
>  	s32 ret;
>  
> @@ -43,19 +43,20 @@ static void check_slab_cache_iter(struct lock_contention *con)
>  	con->btf = btf__load_vmlinux_btf();
>  	if (con->btf == NULL) {
>  		pr_debug("BTF loading failed: %s\n", strerror(errno));
> -		return;
> +		return -errno;
>  	}
>  
>  	ret = btf__find_by_name_kind(con->btf, "bpf_iter__kmem_cache", BTF_KIND_STRUCT);
>  	if (ret < 0) {
>  		bpf_program__set_autoload(skel->progs.slab_cache_iter, false);
>  		pr_debug("slab cache iterator is not available: %d\n", ret);
> -		return;
> +		return ret;
>  	}
>  
>  	has_slab_iter = true;
>  
>  	bpf_map__set_max_entries(skel->maps.slab_caches, con->map_nr_entries);
> +	return 0;
>  }
>  
>  static void run_slab_cache_iter(void)
> @@ -301,7 +302,8 @@ int lock_contention_prepare(struct lock_contention *con)
>  			skel->rodata->use_cgroup_v2 = 1;
>  	}
>  
> -	check_slab_cache_iter(con);
> +	if (check_slab_cache_iter(con) < 0)
> +		return -1;
>  
>  	if (con->filters->nr_slabs && has_slab_iter) {
>  		skel->rodata->has_slab = 1;
> 
> base-commit: 5572ad8fddecd4a0db19801262072ff5916b7589
> -- 
> 2.52.0
> 

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

[Tycho Andersen]

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@kernel.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

Tycho

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

[K Prateek Nayak]

Hello Tycho,

On 1/9/2026 10:14 PM, Tycho Andersen wrote:
> @@ -43,19 +43,20 @@ static void check_slab_cache_iter(struct lock_contention *con)
>  	con->btf = btf__load_vmlinux_btf();
>  	if (con->btf == NULL) {
>  		pr_debug("BTF loading failed: %s\n", strerror(errno));
> -		return;
> +		return -errno;
>  	}
>  
>  	ret = btf__find_by_name_kind(con->btf, "bpf_iter__kmem_cache", BTF_KIND_STRUCT);
>  	if (ret < 0) {
>  		bpf_program__set_autoload(skel->progs.slab_cache_iter, false);
>  		pr_debug("slab cache iterator is not available: %d\n", ret);
> -		return;
> +		return ret;
>  	}
>  
>  	has_slab_iter = true;
>  
>  	bpf_map__set_max_entries(skel->maps.slab_caches, con->map_nr_entries);
> +	return 0;
>  }
>  
>  static void run_slab_cache_iter(void)
> @@ -301,7 +302,8 @@ int lock_contention_prepare(struct lock_contention *con)
>  			skel->rodata->use_cgroup_v2 = 1;
>  	}
>  
> -	check_slab_cache_iter(con);
> +	if (check_slab_cache_iter(con) < 0)
> +		return -1;
>  
>  	if (con->filters->nr_slabs && has_slab_iter) {

nit.

Now that lock_contention_prepare() bails out when
check_slab_cache_iter() fails, we no longer need the "has_slab_iter"
global indicator.

Both this, and run_slab_cache_iter() that depend on "has_slab_iter"
will only run if check_slab_cache_iter() succeeded and the global
indicator will be true in both those cases.

Can be separate follow-up cleanup.

>  		skel->rodata->has_slab = 1;
> 
> base-commit: 5572ad8fddecd4a0db19801262072ff5916b7589

-- 
Thanks and Regards,
Prateek

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

[Tycho Andersen]

On Tue, Jan 13, 2026 at 08:41:24AM +0530, K Prateek Nayak wrote:
> Hello Tycho,
> 
> On 1/9/2026 10:14 PM, Tycho Andersen wrote:
> > @@ -43,19 +43,20 @@ static void check_slab_cache_iter(struct lock_contention *con)
> >  	con->btf = btf__load_vmlinux_btf();
> >  	if (con->btf == NULL) {
> >  		pr_debug("BTF loading failed: %s\n", strerror(errno));
> > -		return;
> > +		return -errno;
> >  	}
> >  
> >  	ret = btf__find_by_name_kind(con->btf, "bpf_iter__kmem_cache", BTF_KIND_STRUCT);
> >  	if (ret < 0) {
> >  		bpf_program__set_autoload(skel->progs.slab_cache_iter, false);
> >  		pr_debug("slab cache iterator is not available: %d\n", ret);
> > -		return;
> > +		return ret;
> >  	}
> >  
> >  	has_slab_iter = true;
> >  
> >  	bpf_map__set_max_entries(skel->maps.slab_caches, con->map_nr_entries);
> > +	return 0;
> >  }
> >  
> >  static void run_slab_cache_iter(void)
> > @@ -301,7 +302,8 @@ int lock_contention_prepare(struct lock_contention *con)
> >  			skel->rodata->use_cgroup_v2 = 1;
> >  	}
> >  
> > -	check_slab_cache_iter(con);
> > +	if (check_slab_cache_iter(con) < 0)
> > +		return -1;
> >  
> >  	if (con->filters->nr_slabs && has_slab_iter) {
> 
> nit.
> 
> Now that lock_contention_prepare() bails out when
> check_slab_cache_iter() fails, we no longer need the "has_slab_iter"
> global indicator.
> 
> Both this, and run_slab_cache_iter() that depend on "has_slab_iter"
> will only run if check_slab_cache_iter() succeeded and the global
> indicator will be true in both those cases.
> 
> Can be separate follow-up cleanup.

I guess this depends on whether it should work without BTF. Presumably
we'd want to keep this check if the goal is to make it work without
BTF.

I'm can send an addition cleanup patch if we decide not to go that
route.

Tycho

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

[Namhyung Kim]

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@kernel.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

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

[Tycho Andersen]

On Tue, Jan 13, 2026 at 01:18:52PM -0800, Namhyung Kim wrote:
> 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:
> > > 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.

Gentle ping, I was just reminded of this crash again this morning.

Tycho

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

[Namhyung Kim]

Hello,

On Wed, Feb 04, 2026 at 08:20:34AM -0700, Tycho Andersen wrote:
> On Tue, Jan 13, 2026 at 01:18:52PM -0800, Namhyung Kim wrote:
> > 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:
> > > > 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.
> 
> Gentle ping, I was just reminded of this crash again this morning.

Sorry for the delay.  Please send a fix with the NULL check in the
init_numa_data().  I think it'd be simpler and easier to improve.

Thanks,
Namhyung