Re: [SC22WG14.34681] n3752, alx-0029r8 - Restore the traditional realloc(3) specification

[Alejandro Colomar <une+c@alejandro-colomar.es>]

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Hi Nevin,

On Wed, Jan 07, 2026 at 08:37:15PM -0600, Nevin Liber wrote:
> On Wed, Jan 7, 2026 at 8:31 AM David Svoboda <svoboda@cert.org> wrote:
> 
> > Here are some more thoughts on n3752
> > [...]
> > WRT this text:
> >
> >         Code written for platforms returning a null pointer can be
> > migrated to platforms returning non-null, without significant
> > issues.
> >
> > I am very skeptical that this is indeed true. But to be precise, this is
> > Glibc's problem rather than WG14's.  If they are willing to change glibc to
> > return non-null on realloc(0), then I am willing to agree to this change in
> > ISO C.
> >
> > Is there any evidence that changing this behavior breaks no code?  Any
> > test failures?  Any surveys?
> >
> 
> And if it breaks no code, is that because this is a corner case that
> doesn't occur in practice?

It's not because of a corner case.  It's because if it would go wrong,
the worst that can happen is a memory leak of 0 bytes plus metadata
(~16 B, usually).

And yes, it's a corner case, so it's not like you'll be leaking those
16 B regularly.

> That in itself doesn't mean we shouldn't change it.

The reason to change it is that with the current specification and
implementation you can get serious vulnerabilities: remote code
execution.

Also, that programming will be much easier if all implementations behave
in the most simple way.

> > This paper ignores Windows other than to mention that it would need to
> > change too.  I doubt MS will update MSVC to accommodate this paper.   So
> > accepting this paper makes MSVC noncompliant.
> >
> 
> This is the part that is troublesome to me.  What is the point of changing
> this behavior if two (or even just one) major implementations are going to
> ignore it?

Do you know anyone from MS in WG21?  It would be great to talk to them.


Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es>

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