linux-kernel - Re: [patch] ASoC: soc: snprintf() doesn't return negative

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Message-ID: <20101011211138.GL5851@bicker>
Date:	Mon, 11 Oct 2010 23:11:38 +0200
From:	Dan Carpenter <error27@...il.com>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Liam Girdwood <lrg@...mlogic.co.uk>,
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.de>,
	Peter Ujfalusi <peter.ujfalusi@...ia.com>,
	Jassi Brar <jassi.brar@...sung.com>,
	alsa-devel@...a-project.org, kernel-janitors@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] ASoC: soc: snprintf() doesn't return negative

On Mon, Oct 11, 2010 at 09:45:02PM +0200, Dan Carpenter wrote:
> On Mon, Oct 11, 2010 at 07:51:48PM +0100, Mark Brown wrote:
> > In actual fact quite a few devices have enough registers to be
> > truncated, meaning that it's not only possible but likely we'll exercise
> > the cases that deal with the end of buffer.  If snprintf() is returning
> > values larger than buffer size it was given we're likely to have an
> > issue but it seems that there's something missing in your analysis since
> > we're never seeing WARN_ON()s and are instead seeing the behaviour the
> > code is intended to give, which is to truncate the output when we run
> > out of space.
> > 
> > Could you re-check your analysis, please?
> 
> That's odd.  I'm sorry, I can't explain why you wouldn't see a stack
> trace...  The code is straight forward:
> 
>         /* Reject out-of-range values early.  Large positive sizes are
>            used for unknown buffer sizes. */
>         if (WARN_ON_ONCE((int) size < 0))
>                 return 0;
> 
> It would still give you truncated output but after the NULL terminator
> there would be information leaked from the kernel.  If the reader
> program had allocated a large enough buffer to handle the extra
> information it wouldn't cause a problem.
> 

Actually it will never cause a problem with userspace because we pass
the size of the userspace buffer to the kernel.  The only issues are the
information leak if the user passes in a 8k buffer and the also the
WARN_ON_ONCE()


regards,
dan carpenter

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