Re: [PATCH v3 2/3] Introduce a function to run regexec() on non-NUL-terminated buffers

Johannes Schindelin [off-list ref] writes:

We just introduced a test that demonstrates that our sloppy use of
regexec() on a mmap()ed area can result in incorrect results or even
hard crashes.

So what we need to fix this is a function that calls regexec() on a
length-delimited, rather than a NUL-terminated, string.

Happily, there is an extension to regexec() introduced by the NetBSD
project and present in all major regex implementation including
Linux', MacOSX' and the one Git includes in compat/regex/: by using
the (non-POSIX) REG_STARTEND flag, it is possible to tell the
regexec() function that it should only look at the offsets between
pmatch[0].rm_so and pmatch[0].rm_eo.

That is exactly what we need.

Yes, that is good.

Since support for REG_STARTEND is so widespread by now, let's just
introduce a helper function that uses it, and fall back to allocating
and constructing a NUL-terminated when REG_STARTEND is not available.

I do not think this fallback is good; we do ship a compat/ fallback
that does support REG_STARTEND and you'd want to use that.  Not
having the copying fallback means you do not even have to worry
about the size+1 overflow and fix it with xmallocz() ;-)