linux-kernel - Re: event tracing, ringbuffer and RB_MAX_SMALL

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Date:	Wed, 15 Jul 2009 09:08:06 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Johannes Berg <johannes@...solutions.net>
CC:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: event tracing, ringbuffer and RB_MAX_SMALL_DATA

Johannes Berg wrote:
> Hi,
> 
> By code inspection, I think there's a bug if your event contains only
> 
> 	__dynamic_array(u8, buf, buflen)
> 
> and your buflen is only, say, 3. Then the event size will be the common
> event size (12 bytes) plus this, so 15, which is well below
> RB_MAX_SMALL_DATA (4*28 = 112), so it'll be divided by four to be stored
> into type_len. At this point, however, you've lost the information that
> your dynamic array was only three bytes, and it'll be considered _four_
> bytes long by parsers as far as I can tell?
> 

Right, the length of a dynamic array is not recorded, and this
causes 2 problems:

- the event filter is not working properly for dynamic strings
- userspace parsers can't figure out the length of those arrays

I had an idea some time ago, and hopefully will send out a
patch today or tomorrow. 

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