perf: Fix perf_event_validate_size() lockdep splat

When lockdep is enabled, the for_each_sibling_event(sibling, event)
macro checks that event->ctx->mutex is held. When creating a new group
leader event, we call perf_event_validate_size() on a partially
initialized event where event->ctx is NULL, and so when
for_each_sibling_event() attempts to check event->ctx->mutex, we get a
splat, as reported by Lucas De Marchi:

  WARNING: CPU: 8 PID: 1471 at kernel/events/core.c:1950 __do_sys_perf_event_open+0xf37/0x1080

This only happens for a new event which is its own group_leader, and in
this case there cannot be any sibling events. Thus it's safe to skip the
check for siblings, which avoids having to make invasive and ugly
changes to for_each_sibling_event().

Avoid the splat by bailing out early when the new event is its own
group_leader.

Fixes: 382c27f4ed28f803 ("perf: Fix perf_event_validate_size()")
Closes: https://lore.kernel.org/lkml/20231214000620.3081018-1-lucas.demarchi@intel.com/
Closes: https://lore.kernel.org/lkml/ZXpm6gQ%2Fd59jGsuW@xpf.sh.intel.com/
Reported-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reported-by: Pengfei Xu <pengfei.xu@intel.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20231215112450.3972309-1-mark.rutland@arm.com

diff --git a/kernel/events/core.c b/kernel/events/core.c
index c9d123e13b57958bc9532ebdc45f17b13830327a..9efd0d7775e7ce400a1c966ba09af73ec3887f3d 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1947,6 +1947,16 @@ static bool perf_event_validate_size(struct perf_event *event)
                                   group_leader->nr_siblings + 1) > 16*1024)
                return false;
 
+       /*
+        * When creating a new group leader, group_leader->ctx is initialized
+        * after the size has been validated, but we cannot safely use
+        * for_each_sibling_event() until group_leader->ctx is set. A new group
+        * leader cannot have any siblings yet, so we can safely skip checking
+        * the non-existent siblings.
+        */
+       if (event == group_leader)
+               return true;
+
        for_each_sibling_event(sibling, group_leader) {
                if (__perf_event_read_size(sibling->attr.read_format,
                                           group_leader->nr_siblings + 1) > 16*1024)