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

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Date:   Tue, 20 Dec 2016 11:00:56 +0100
From:   Markus Trippelsdorf <markus@...ppelsdorf.de>
To:     "H. Peter Anvin" <hpa@...or.com>
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 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. 

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.

--
Markus