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Date: Mon, 18 Nov 2013 01:33:49 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Joe Perches <joe@perches.com>
Cc: Erico Nunes <nunes.erico@gmail.com>, linux-sparse@vger.kernel.org,
        dwmw2 <dwmw2@infradead.org>, linux-mtd <linux-mtd@lists.infradead.org>,
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Subject: Re: [PATCH] jffs2: fix sparse errors: directive in argument list
Message-ID: <20131118013349.GM13318@ZenIV.linux.org.uk>
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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...