linux-kernel - Re: Nasal demons in preprocessor use (Re: [PATCH] test-suite: new preprocessor test case)

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Date:	Thu, 19 Mar 2009 21:20:51 +0100
From:	Vegard Nossum <vegard.nossum@...il.com>
To:	Ingo Molnar <mingo@...e.hu>,
	Stephen Rothwell <sfr@...b.auug.org.au>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Christopher Li <sparse@...isli.org>,
	linux-sparse@...r.kernel.org, Hannes Eder <hannes@...neseder.net>,
	linux-kernel@...r.kernel.org
Subject: Re: Nasal demons in preprocessor use (Re: [PATCH] test-suite: new 
	preprocessor test case)

2009/3/19 Ingo Molnar <mingo@...e.hu>:
>
> * Al Viro <viro@...IV.linux.org.uk> wrote:
>
>> On Thu, Mar 19, 2009 at 07:51:22PM +0100, Hannes Eder wrote:
>> > When currently running sparse agains the current linux-next tree, a
>> > lot of checks produce error messages like this:
>> >
>> > include/linux/skbuff.h:381:9: error: expected preprocessor identifier
>>
>> Cute.  If anything, this kmemcheck_define_bitfield stuff needs to be moved
>> inside the ifdefs.
>>
>> Folks, this is not a valid C, period.  And no, there's no promise
>> that gcc won't change its behaviour on such constructs whenever
>> they feel like that.
>>
>> Preprocessor directives do not belong in argument lists.  Not
>> #ifdef, not #define, not #include; this is undefined behaviour.
>
> Agreed.
>
> Vegard, it's this bit:
>
>        kmemcheck_define_bitfield(flags2, {
> #ifdef CONFIG_IPV6_NDISC_NODETYPE
>                __u8                    ndisc_nodetype:2;
> #endif
> #if defined(CONFIG_MAC80211) || defined(CONFIG_MAC80211_MODULE)
>                __u8                    do_not_encrypt:1;
>                __u8                    requeue:1;
> #endif
>        });
>
>        Ingo
>

Hm.

Is this really not valid C?

It worked with GCC, so I assumed it was. My mistake.

Okay, that puts us in a bit of a tight spot, with regards to kmemcheck, I mean.

Maybe I should just take up GCC development instead, and implement a
-fkmemcheck or something.

(To get rid of the bitfield false positives, I mean.)

I guess this means that kmemcheck branch should be withdrawn from
linux-next, at least temporarily, as I have no immediate
workarounds/alternatives. Stephen, can you drop it?


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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