There are two races with the current code:
- Another event can join the group and compute a larger header_size
concurrently, if the smaller store wins we'll have an incorrect
header_size set.
- We compute the header_size after the event becomes active,
therefore its possible to use the size before its computed.
Remedy the first by moving the computation inside the ctx::mutex lock,
and the second by placing it _before_ perf_install_in_context().
Cc: Arnaldo Carvalho de Melo <a...@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
kernel/events/core.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8349,6 +8349,15 @@ SYSCALL_DEFINE5(perf_event_open,
put_ctx(gctx);
}
+ /*
+ * Precalculate sample_data sizes; do while holding ctx::mutex such
+ * that we're serialized against further additions and before
+ * perf_install_in_context() which is the point the event is active and
+ * can use these values.
+ */
+ perf_event__header_size(event);
+ perf_event__id_header_size(event);
+
perf_install_in_context(ctx, event, event->cpu);
perf_unpin_context(ctx);
@@ -8365,12 +8374,6 @@ SYSCALL_DEFINE5(perf_event_open,
mutex_unlock(¤t->perf_event_mutex);
/*
- * Precalculate sample_data sizes
- */
- perf_event__header_size(event);
- perf_event__id_header_size(event);
-
- /*
* Drop the reference on the group_event after placing the
* new event on the sibling_list. This ensures destruction
* of the group leader will find the pointer to itself in
--
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