linux-kernel - RE: [RFC PATCH] vsnprintf: Remove use of %n and convert existing uses

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B732A@saturn3.aculab.com>
Date:	Thu, 12 Sep 2013 09:06:04 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Al Viro" <viro@...IV.linux.org.uk>,
	"Joe Perches" <joe@...ches.com>
Cc:	"Tetsuo Handa" <penguin-kernel@...ove.SAKURA.ne.jp>,
	<linux-kernel@...r.kernel.org>, <kosaki.motohiro@...fujitsu.com>,
	<keescook@...omium.org>, <fweisbec@...il.com>,
	<dan.carpenter@...cle.com>, <devel@...verdev.osuosl.org>,
	<gregkh@...uxfoundation.org>, <tushar.behera@...aro.org>,
	<lidza.louina@...il.com>, <davem@...emloft.net>,
	<kuznet@....inr.ac.ru>, <jmorris@...ei.org>,
	<yoshfuji@...ux-ipv6.org>, <kaber@...sh.net>,
	<courmisch@...il.com>, <vyasevich@...il.com>,
	<nhorman@...driver.com>, <netdev@...r.kernel.org>,
	<linux-sctp@...r.kernel.org>
Subject: RE: [RFC PATCH] vsnprintf: Remove use of %n and convert existing uses

> On Wed, Sep 11, 2013 at 05:04:17PM -0700, Joe Perches wrote:
> > On Thu, 2013-09-12 at 08:40 +0900, Tetsuo Handa wrote:
> > > Joe Perches wrote:
> > > > -	seq_printf(m, "%s%d%n", con->name, con->index, &len);
> > > > +	len = seq_printf(m, "%s%d", con->name, con->index);
> > >
> > > Isn't len always 0 or -1 ?
> >
> > Right.  Well you're no fun...
> >
> > These uses would seem broken anyway because the
> > seq_printf isn't itself tested for correctness.
> >
> > Hmm.
> >
> > Also, there's a large amount of code that appears
> > to do calculations with pos or len like:
> >
> > 	pos += seq_printf(handle, fmt. ...)
> 
> ... and most of that code proceeds to ignore pos completely.
> Note that ->show() is *NOT* supposed to return the number of
> characters it has/would like to have produced.  Just return
> 0 and be done with that; overflows are dealt with just fine.
> The large amount, BTW, is below 100 lines, AFAICS, in rather
> few files.
> 
> > There are very few that seem to use it correctly
> > like netfilter.
> 
> > Suggestions?

Change the return type of seq_printf() to void and require that
code use access functions/macros to find the length and error
status. Possibly save the length of the last call separately.

	David



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