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

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Message-ID: <20101012095605.GC30933@rakim.wolfsonmicro.main>
Date:	Tue, 12 Oct 2010 10:56:05 +0100
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Takashi Iwai <tiwai@...e.de>
Cc:	Dan Carpenter <error27@...il.com>,
	Liam Girdwood <lrg@...mlogic.co.uk>,
	Jaroslav Kysela <perex@...ex.cz>,
	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 Tue, Oct 12, 2010 at 11:49:27AM +0200, Takashi Iwai wrote:
> Mark Brown wrote:

> Argh, yes, I'm (again) confused by that behavior.
> The problem is the potential buffer overflow, indeed.  snprintf()
> returns the size that would be printed.  Thus a safe code would be
> like:
> 
> 	list_for_each_entry(dai, &dai_list, list) {
>  		int len = snprintf(buf + ret, PAGE_SIZE - ret, "%s\n", dai->name);
>  		if (len < 0)
>  			continue;
>  		ret += len;
> 		if (ret >= PAGE_SIZE) {
> 			ret = PAGE_SIZE;
> 			break;
> 		}
> 	}

Yes, this form is better for that variant of the loop - that is safe and
legible without relying on current implementation details of snprintf().
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