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Re: [PATCH 1/2] malloc/malloc.c: Validate SIZE passed to aligned_alloc.


On Fri, Nov 08, 2013 at 10:20:29AM +0000, Will Newton wrote:
> On 8 November 2013 04:20, Rich Felker <dalias@aerifal.cx> wrote:
> > On Thu, Nov 07, 2013 at 08:09:24PM +0000, Will Newton wrote:
> >> On 7 November 2013 17:48, Paul Eggert <eggert@cs.ucla.edu> wrote:
> >> > On 11/07/2013 09:41 AM, Will Newton wrote:
> >> >> The ISO C11 standard specifies that a SIZE passed to aligned_alloc
> >> >> must be a multiple of ALIGNMENT. Aliasing aligned_alloc to memalign
> >> >> does not enforce this restriction, so create a new function that
> >> >> does this validation.
> >> >
> >> > This doesn't look right.  See the NEWS file's entry for glibc 2.16, which says:
> >> >
> >> >   + aligned_alloc.  NB: The code is deliberately allows the size parameter
> >> >     to not be a multiple of the alignment.  This is a moronic requirement
> >> >     in the standard but it is only a requirement on the caller, not the
> >> >     implementation.
> >>
> >> I disagree with Drepper on this point. If we don't enforce the
> >> contract on callers then it becomes possible for callers to write
> >> non-portable code with glibc aligned_alloc. Admittedly the spec of
> >> aligned_alloc isn't amazingly rigid so writing non-portable code is
> >> possible anyway, but I still think it is worth glibc validating what
> >> is actually written in the spec. If we want to write a function that
> >> implements "almost aligned_alloc" it should really be called something
> >> else IMO.
> >
> > I'm against unnecessary and (mildly) expensive validation of a
> > condition that the implementation is not required to validate and for
> > which the check has no purpose except for intentionally breaking
> > non-portable code.
> 
> My initial interest in this came from documenting the aligned_alloc
> interface. So should we document this non-standard behaviour?

No. It's not non-standard behavior. It's undefined behavior on the
part of the caller.

Rich


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