From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joe Perches <joe@perches.com>
Subject: Re: [RFC PATCH] vsnprintf: Remove use of %n and convert existing
 uses
Date: Wed, 11 Sep 2013 16:43:03 -0700
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Cc: LKML <linux-kernel@vger.kernel.org>,
	KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Dan Carpenter <dan.carpenter@oracle.com>,
	devel@driverdev.osuosl.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Tushar Behera <tushar.behera@linaro.org>,
	Lidza Louina <lidza.louina@gmail.com>,
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	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	James Morris <jmorris@namei.org>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	Patrick McHardy <kaber@trash.net>,
	Remi Denis-Courmont <courmisch@gmail.com>,
	Vlad Yasevich <vyasevich@gmail.com>,
	Neil Horman <nhorman@tuxdriver.com>, netdev@vger.kernel.org,
	linux-sctp@vger.kernel.org
To: Kees Cook <keescook@chromium.org>
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On Wed, 2013-09-11 at 16:29 -0700, Kees Cook wrote:
> On Wed, Sep 11, 2013 at 4:22 PM, Joe Perches <joe@perches.com> wrote:
> > Using vsnprintf or its derivatives with %n can have security
> > vulnerability implications.
> >
> > Prior to commit fef20d9c1380
> > ("vsprintf: unify the format decoding layer for its 3 users"),
> > any use of %n was ignored.
> >
> > Reintroduce this feature and convert the existing uses of %n
> > to use the return length from vsnprintf or its derivatives.
> >
> > Signed-off-by: Joe Perches <joe@perches.com>
> > Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> (proc bits)
> > cc: Kees Cook <keescook@chromium.org>
> > cc: Frederic Weisbecker <fweisbec@gmail.com>
> 
> Yes, please. It might also be worth updating
> Documentation/printk-formats.txt to mention that %n has intentionally
> removed and will be ignored.

Fine with me if you want to update that file.

It doesn't currently try to be a complete man page
for vsnprintf though.

vsprintf.c does have kernel-doc documentation and
that already does show that %n is ignored.