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

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Message-Id: <1247651305.14973.3.camel@johannes.local>
Date:	Wed, 15 Jul 2009 11:48:25 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Li Zefan <lizf@...fujitsu.com>
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

On Wed, 2009-07-15 at 09:08 +0800, Li Zefan wrote:

> 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. 

Well except for the corner case I pointed out, you can determine the
length of dynamic arrays by either
 - the next dynamic array's offset or
 - the length of the item.

So, afaict, the simplest solution would be to not embed the length of
the item in type_len if it's not divisible by four and contains dynamic
members, though the latter condition might be hard to check.

johannes

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