Re: [PATCH man v2] mmap.2: Document danger of mappings larger than PTRDIFF_MAX

[Jann Horn <jannh@google.com>]

On Thu, May 1, 2025 at 6:43 PM Alejandro Colomar <alx@kernel.org> wrote:
> On Tue, Apr 29, 2025 at 06:43:59PM +0200, Jann Horn wrote:
> > References:
> >  - C99 draft: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf
> >    section "6.5.6 Additive operators", paragraph 9
> >  - object size restriction in GCC:
> >    https://gcc.gnu.org/legacy-ml/gcc/2011-08/msg00221.html
> >  - glibc malloc restricts object size to <=PTRDIFF_MAX in
> >    checked_request2size() since glibc v2.30 (released in 2019, as pointed
> >    out by Jakub Wilk):
> >    https://sourceware.org/cgit/glibc/commit/?id=9bf8e29ca136094f
>
> Thanks!  I've applied the patch.  See some comments below.
> <https://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/commit/?h=contrib&id=7e5756fdeba1a4729f817079c64f0d87fdcdadfa>

Thanks!

> > +Unlike typical
> > +.BR malloc (3)
> > +implementations,
> > +.BR mmap ()
> > +does not prevent creating objects larger than
> > +.BR PTRDIFF_MAX .
> > +Objects that are larger than
> > +.B PTRDIFF_MAX
> > +only work in limited ways in standard C
>
> I've removed 'standard', since in any C it is problematic.
>
> Is it okay to you?  (We're still in time to amend if you prefer
> something else.)

Sounds good to me.