linux-kernel - Re: [PATCH] jffs2: fix sparse errors: directive in argument list

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Message-ID: <20131120235159.GA25026@jtriplet-mobl1>
Date:	Wed, 20 Nov 2013 15:51:59 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Joe Perches <joe@...ches.com>, Erico Nunes <nunes.erico@...il.com>,
	linux-sparse@...r.kernel.org, dwmw2 <dwmw2@...radead.org>,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] jffs2: fix sparse errors: directive in argument list

On Mon, Nov 18, 2013 at 01:33:49AM +0000, Al Viro wrote:
> On Sun, Nov 17, 2013 at 02:45:05PM -0800, Joe Perches wrote:
> > On Sun, 2013-11-17 at 20:34 -0200, Erico Nunes wrote:
> > > Do you mean it as an error in the sparse tool?
> > 
> > Yes.  I think it's a defect in how sparse
> > treats string concatenation.
> > 
> > That style:
> > 
> > 	printk("%s\n",
> > #ifdef FOO
> > 	"foo"
> > #endif
> > #ifdef BAR
> > 	"bar"
> > #endif
> > 	"string");
> > 
> > is pretty common in the kernel sources.
> 
> ... and it's perfectly fine, until somebody starts playing in nasal
> daemon country and do that in *macro* arguments.  And a nasal daemon
> country it is - it's an undefined behaviour.  See 6.10.3p11 in C99.
> And trying to define a semantics for that gets real ugly real fast.
> sparse matches gcc behaviour (I hope), but it warns about such abuses.
> It's a defect, all right - one being reported by sparse.
> 
> Folks, please, RTFStandard if you decide to play clever games with
> preprocessing.  Chapter 6.10 is not particulary long or complicated.
> C99 has improved the preprocessor semantics a whole lot compared to
> the earlier horrible mess (mostly by defining it in terms of token
> stream transformations rather then text ones), but it's still very
> easy to abuse...

And since pr_foo (and the underlying pr_fmt) are already macros, that's
exactly the problem.

- Josh Triplett
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