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

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Date:	Mon, 11 Oct 2010 21:45:02 +0200
From:	Dan Carpenter <error27@...il.com>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Cc:	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 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.

regards,
dan carpenter


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