linux-kernel - Re: [patch -next] ath5k: snprintf() returns largish values

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Message-ID: <AANLkTi==h70Cgj-SzRShVU1eJ1CGkL93-5XV55Ry6-eD@mail.gmail.com>
Date:	Fri, 23 Jul 2010 12:21:35 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Joe Perches <joe@...ches.com>
Cc:	Dan Carpenter <error27@...il.com>, Bruno Randolf <br1@...fach.org>,
	"Luis R. Rodriguez" <lrodriguez@...eros.com>,
	Nick Kossifidis <mickflemm@...il.com>,
	Jiri Slaby <jirislaby@...il.com>,
	Bob Copeland <me@...copeland.com>,
	"John W. Linville" <linville@...driver.com>,
	linux-wireless@...r.kernel.org, ath5k-devel@...ema.h4ckr.net,
	kernel-janitors <kernel-janitors@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch -next] ath5k: snprintf() returns largish values

On Fri, Jul 23, 2010 at 10:48 AM, Joe Perches <joe@...ches.com> wrote:
>
> There are also many repeated uses of snprintf in kernel sources
> that could similarly be a problem.
>
>        bar += snprintf(foo + bar, ...)
>        bar += snprintf(foo + bar, ...)
> or
>        foo += snprintf(foo, ...)
>        foo += snprintf(foo, ...)

As long as the number of bytes is updated correctly, this won't be a
security problem, although it can cause a (single) warning. The kernel
vsnprintf does

        if (WARN_ON_ONCE((int) size < 0))
                return 0;

so if somebody overflows a buffer with multiple snprintf calls, it
will all be ok as long as the buffer size thing is updated the natural
way (possibly using pointer arithmetic, eg "end - bar").

                Linus
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