Re: [PATCH 3/6] fixup! reftable: rest of library

From: Han-Wen Nienhuys <hanwen@google.com>
Date: Dec 1, 2020, 11:10 UTC
Message-ID: <CAFQ2z_M3C=mCkiPxOLhBSrXR-PDQdb7GLPRWM2egV4cO4a-c0w@mail.gmail.com>
In-Reply-To: <X8YaWySkyRU3DA87@coredump.intra.peff.net>

On Tue, Dec 1, 2020 at 11:26 AM Jeff King <peff@peff.net> wrote:

Show 30 quoted lines

>
> On Sat, Nov 28, 2020 at 06:44:35AM +0000, Johannes Schindelin via GitGitGadget wrote:
>
> > From: Johannes Schindelin <johannes.schindelin@gmx.de>
> >
> > 0-sized arrays are actually not portable.
>
> Definitely.
>
> >  static void test_sizes_to_segments_empty(void)
> >  {
> > -     uint64_t sizes[0];
> > +     uint64_t sizes[1];
> >
> >       int seglen = 0;
> >       struct segment *segs =
> > -             sizes_to_segments(&seglen, sizes, ARRAY_SIZE(sizes));
> > +             sizes_to_segments(&seglen, sizes, 0);
> >       EXPECT(seglen == 0);
> >       reftable_free(segs);
>
> I think passing:
>
>   sizes_to_segments(&seglen, NULL, 0);
>
> may make the code more obvious. Unlike system functions like memcpy(),
> we can be assured of whether our functions avoid looking at a
> zero-length array (and size_to_segments does follow that rule).
>
>   This function, of course, is nonsense that real code would not do, and

This test was added because 'real' code did this. In particular, if you initialize a stack of reftables, the stack has zero elements. The test was for the following bugfix

https://github.com/google/reftable/commit/b2e42ecb54e413e494c1fcc13c21e24422645007

Changing the array size to 1 here also prevents Valgrind to mark a regression as a OOB write.

-- 
Han-Wen Nienhuys - Google Munich
I work 80%. Don't expect answers from me on Fridays.
--
Google Germany GmbH, Erika-Mann-Strasse 33, 80636 Munich
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg
Geschäftsführer: Paul Manicle, Halimah DeLaine Prado