Commit f73e22ab authored Sep 09, 2015 by Peter Zijlstra Committed by Ingo Molnar Sep 18, 2015

perf: Fix races in computing the header sizes



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().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent a723968c

kernel/events/core.c

+9 −6

Original line number	Diff line number	Diff line
		@@ -8399,6 +8399,15 @@ 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);

		@@ -8414,12 +8423,6 @@ SYSCALL_DEFINE5(perf_event_open,
		list_add_tail(&event->owner_entry, &current->perf_event_list);
		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

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