Commit 40490221 authored Dec 15, 2023 by Mark Rutland Committed by Greg Kroah-Hartman Dec 20, 2023

perf: Fix perf_event_validate_size() lockdep splat

commit 7e2c1e4b34f07d9aa8937fab88359d4a0fce468e upstream.

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

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 4109d9a8

kernel/events/core.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -1835,6 +1835,16 @@ 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)