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

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Date:   Tue, 20 Dec 2016 12:51:09 +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 03:10 -0800, H. Peter Anvin wrote:
> 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. 
> 
> > 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

NULL does not point to any object, therefore it is UB.

-- 
Markus