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

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Date:   Tue, 20 Dec 2016 20:31:53 +0100
From:   Markus Trippelsdorf <markus@...ppelsdorf.de>
To:     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 10:32 -0800, hpa@...or.com wrote:
> On December 20, 2016 3:51:09 AM PST, Markus Trippelsdorf <markus@...ppelsdorf.de> wrote:
> >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.
> 
> That seems, quite frankly, like a pretty idiotic lawyerism.
> Why would a pointer that by spec is never referenced not be able to be null?  

Thank you. Let me quote the standard for you:

7.1.4
»If an argument to a function has an invalid value (such as a value
outside the domain of the function, or a pointer outside the address
space of the program, or a null pointer, or a pointer to non-modifiable
storage when the corresponding parameter is not const-qualified) or a
type (after promotion) not expected by a function with variable number
of arguments, the behavior is undefined.«

7.24.1(2)
»Where an argument declared as size_t n specifies the length of the
array for a function, n can have the value zero […] pointer arguments on
such a call shall still have valid values, as described in 7.1.4.«

The same applies to memcpy, etc.

The compiler can assume that these pointers are not NULL and optimizes
accordingly.

-- 
Markus