[tip: perf/urgent] perf: Fix perf_event_validate_size() lockdep splat

[tip-bot2 for Mark Rutland]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     7e2c1e4b34f07d9aa8937fab88359d4a0fce468e
Gitweb:        https://git.kernel.org/tip/7e2c1e4b34f07d9aa8937fab88359d4a0fce468e
Author:        Mark Rutland <mark.rutland@xxxxxxx>
AuthorDate:    Fri, 15 Dec 2023 11:24:50 
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 15 Dec 2023 12:33:23 +01:00

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@xxxxxxxxx/
Closes: https://lore.kernel.org/lkml/ZXpm6gQ%2Fd59jGsuW@xxxxxxxxxxxxxxxx/
Reported-by: Lucas De Marchi <lucas.demarchi@xxxxxxxxx>
Reported-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20231215112450.3972309-1-mark.rutland@xxxxxxx
---
 kernel/events/core.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index c9d123e..9efd0d7 100644
--- 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)