Re: [PATCH v2 4/7] xdiff: NUL-terminate buffers read by read_mmfile()

[Patrick Steinhardt <ps@pks.im>]

On Wed, Sep 30, 2026 at 07:44:13PM -0400, Jeff King wrote:
> Since an mmfile_t is a ptr/len pair, our read_mmfile() allocates exactly
> the number of bytes we claim to store. But in many other places in Git,
> we add an extra NUL "just in case", which can help avoid read overruns
> due to off-by-ones or the use of string functions.
> 
> I don't know of any path that would benefit from this, but I noticed it
> while converting ll_ext_merge() to use read_mmfile(), since its original
> code did add a NUL byte (even though I cannot find any case where it
> would have mattered). Let's add the same defensive NUL in read_mmfile()
> by using xmallocz() instead of xmalloc().

Nit: I guess this is an artifact from the reorder, but this sounds as if
`ll_ext_merge()` wouldn't append the NUL byte anymore. But at this step
it still does, as the change to `read_mmfile()` now happens before the
change to `ll_ext_merge()`.

Patrick