[RFC][PATCH 3/3] perf: Fix races in computing the header sizes

[Peter Zijlstra]

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 <acme@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 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(&current->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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