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

From: Jeff King <peff@peff.net>
Date: Dec 1, 2020, 11:57 UTC
Message-ID: <X8YvlS29KKYje1Az@coredump.intra.peff.net>
In-Reply-To: <CAFQ2z_M3C=mCkiPxOLhBSrXR-PDQdb7GLPRWM2egV4cO4a-c0w@mail.gmail.com>

On Tue, Dec 01, 2020 at 12:10:14PM +0100, Han-Wen Nienhuys wrote:

Show 18 quoted lines

> > 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.

Sorry to be unclear. I meant that real code would not allocate a static zero-length array (though it might do so on the heap). I agree that making sure the function behaves in the 0-segment case is quite reasonable.

Passing NULL actually makes the test more rigorous IMHO (because it is a strict superset of failure modes, and is something an actual caller might do if it were dynamically growing an array that started at NULL).

-Peff