linux-kernel - Re: [tip:x86/urgent] x86/tools: Fix gcc-7 warning in relocs.c

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Message-ID: <2966d559-2cea-fd95-76d4-df34192a9b88@zytor.com>
Date:   Tue, 20 Dec 2016 03:10:15 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, mingo@...nel.org,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86/tools: Fix gcc-7 warning in relocs.c

On 12/20/16 02:00, Markus Trippelsdorf wrote:
> On 2016.12.20 at 01:30 -0800, H. Peter Anvin wrote:
>> I'd strongly prefer a non-data-dependent solution, specifically adding
>> at the top of sort_relocs():
>>
>> if (!r->count)
>> 	return;
>>
>> However, by my reading of the C and POSIX standards, this is a gcc
>> error: qsort() should do nothing if the count is zero.
> 
> No, it is invoking undefined behavior. 

H

> Notice the nonnull attribute in /usr/include/stdlib.h:
> 
> 739 /* Sort NMEMB elements of BASE, of SIZE bytes each,
> 740    using COMPAR to perform the comparisons.  */
> 741 extern void qsort (void *__base, size_t __nmemb, size_t __size,
> 742                    __compar_fn_t __compar) __nonnull ((1, 4));
> 
> But feel free to revert my patch and add your solution.

Well, s/gcc/glibc/ then.

>        The  qsort()  function  shall sort an array of nel objects, the
>        initial element of which is pointed to by base.   The  size  of
>        each  object,  in bytes, is specified by the width argument. If
>        the nel argument has the value zero,  the  comparison  function
>        pointed  to  by compar shall not be called and no rearrangement
>        shall take place.

	-hpa