ring-buffer: Make sure event has enough room for extend and padding (7d75e683) · Commits · 戴 / test

Now that events only add time extends after it is committed, in case an event comes in before it can discard the allocated event, the time extend needs to be stored within the event. If the event is bigger than then size needed for the time extend, padding must be added. The minimum padding size is 8 bytes. Thus if the event is 12 bytes (size of time extend + 4), there will not be enough room to add both the time extend and padding. Make sure all events are either 8 bytes or 16 or more bytes. Signed-off-by:

Steven Rostedt <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

+15 −0

Original line number	Diff line number	Diff line
		@@ -2208,6 +2208,21 @@ static unsigned rb_calculate_event_length(unsigned length)
		length += RB_EVNT_HDR_SIZE;
		length = ALIGN(length, RB_ARCH_ALIGNMENT);

		/*
		* In case the time delta is larger than the 27 bits for it
		* in the header, we need to add a timestamp. If another
		* event comes in when trying to discard this one to increase
		* the length, then the timestamp will be added in the allocated
		* space of this event. If length is bigger than the size needed
		* for the TIME_EXTEND, then padding has to be used. The events
		* length must be either RB_LEN_TIME_EXTEND, or greater than or equal
		* to RB_LEN_TIME_EXTEND + 8, as 8 is the minimum size for padding.
		* As length is a multiple of 4, we only need to worry if it
		* is 12 (RB_LEN_TIME_EXTEND + 4).
		*/
		if (length == RB_LEN_TIME_EXTEND + RB_ALIGNMENT)
		length += RB_ALIGNMENT;

		return length;
		}

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