[PATCH] free: preserve errno [BZ#17924]

[H.J. Lu]

On Mon, Dec 28, 2020 at 11:24 AM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
>
>
> On 23/12/2020 22:03, Paul Eggert wrote:
> > Thanks for the comments about the patch's test case. I modified the test case to reflect nearly all the comments, resulting in the attached revised patch. I'm replying below only to the comments that didn't result in a change to the patch.
> >
> > On 12/23/20 11:19 AM, Adhemerval Zanella wrote:
> >
> >> Fixing in a more fine grained would require a lot of more work to check if
> >> the shared routines that calls mmap, madvise or brk won't interfere with other
> >> symbols; so maybe it should be ok to use this large hammer for now.
> >
> > Yes, that was my thought as well.
>
> I will try to revise this for 2.33.
>
> >
> >>> +The @code{free} function preserves the value of @code{errno}, so that
> >>> +cleanup code need not worry about saving and restoring @code{errno}
> >>> +around a call to @code{free}.  Although neither @w{ISO C} nor
> >>> +POSIX.1-2017 requires @code{free} to preserve @code{errno}, a future
> >>> +version of POSIX is planned to require it.
> > ...
> >> Not sure if this is worth to add, since we will need to update the manual
> >> once the POSIX does require it.
> >
> > I'll volunteer to update the manual. :-)
> >
> > It's worth mentioning that preserving errno is not something that portable C or POSIX code should assume for 'free'. If there's a better way for the manual to warn its readers about this, that'd be fine of course.
> >
> > I did consider changing "Although neither @w{ISO C} nor POSIX.1-2017 requires @code{free} to preserve @code{errno}, a future version of POSIX is planned to require it" to "Neither @w{ISO C} nor POSIX requires @code{free} to preserve @code{errno}", but that wording would be less informative and would still need updating once POSIX does require 'free' to preserve errno.
>
> Thanks.
>
> > From afbf4ff042cf3a5c8f983d5aa3bd0de3fb696dd3 Mon Sep 17 00:00:00 2001
> > From: Paul Eggert <eggert@cs.ucla.edu>
> > Date: Wed, 23 Dec 2020 11:27:25 -0800
> > Subject: [PATCH] free: preserve errno [BZ#17924]
> >
> > In the next release of POSIX, free must preserve errno
> > <https://www.austingroupbugs.net/view.php?id=385>.
> > Modify __libc_free to save and restore errno, so that
> > any internal munmap etc. syscalls do not disturb the caller's errno.
> > Add a test malloc/tst-free-errno.c (almost all by Bruno Haible),
> > and document that free preserves errno.
>
> LGTM with the small test change below.
>
> Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
>

On a machine with 192 GB RAM, I got

[hjl@gnu-clx-1 build-x86_64-linux]$ cat malloc/tst-free-errno-mcheck.out
error: tst-free-errno.c:124: not true: get_errno () == 1789
error: 1 test failures
[hjl@gnu-clx-1 build-x86_64-linux]$

-- 
H.J.