linux-kernel - Re: somebody dropped a (warning) bomb

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Date:	Mon, 19 Feb 2007 15:17:03 +0300
From:	Sergei Organov <osv@...ad.com>
To:	Bodo Eggert <7eggert@....de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	J.A. MagallÃÃÃÂ³n 
	<jamagallon@....com>, Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	Jeff Garzik <jeff@...zik.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: somebody dropped a (warning) bomb

Bodo Eggert <7eggert@....de> writes:
> On Fri, 16 Feb 2007, Sergei Organov wrote:

[...]

> I'll say it again: Either the code using unspecified chars is correct, or 
> it isn't. If it's correct, neither using with signed nor with unsigned 
> chars is a bug and you should not warn at all, and if it's not correct, 
> you should always warn. Instead, gcc warns on "code compiles for
> $arch".

Here is where we disagree. In my opinion, no matter what the sign of
char for given architecture is, it's incorrect to pass either "signed
char*" or "unsigned char*" argument to a function expecting "char*".

>> > Therefore it's either always wrong to call your char* function with char*,
>> > unsigned char* _and_ signed char unless you can guarantee not to overflow any
>> > of them, or it's always correct to call char* functions with any kind
>> > of these.
>> 
>> How are you sure those who wrote foo(char*) agrees with your opinion or
>> even understands all the involved issues?
>
> Let's asume we have this piece of buggy code. We compile it on an unsigned
> char architecture. No warning. *BOOM*

There should be warning, -- that's my point. "char" is different. "char"
is distinct from either "signed char" or "unsigned char". Always. At
least it's how C is defined.

> Let's asume there is correct code, and we use it as designed:
> Warning: Wrong arch
> Warning: Wrong arch
> Warning: Wrong arch
> Warning: real issue 
> Warning: Wrong arch
> Warning: Wrong arch
> Warning: Wrong arch
> Warning: Wrong arch
> <scroll off screen/>
> Warning: Wrong arch
> Warning: Wrong arch
> Warning: Wrong arch
> Warning: Wrong arch
> Warning: Wrong arch
>
> You don't see "real issue". *BOOM*
>
>
> What can you do about this warning? Let's asume we cast everywhere:

I already gave an answer in response to Linus:

static inline size_t ustrlen(const unsigned char *s)
{
    return strlen((const char *)s);
}

>
> struct foo * p;
> printf(strlen(char*)p); *BOOM*

printf(ustrlen(p)); *NO BOOM*
unsigned char* u;
printf(ustrlen(u)); *NO WARNING*

-- Sergei.
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