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

From: Junio C Hamano <gitster@pobox.com>
Date: Sep 8, 2016, 17:03 UTC
Message-ID: <xmqq60q6jpc8.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <94ee698b2736929d37640012a1b1735b134dd3d6.1473321437.git.johannes.schindelin@gmx.de>

Johannes Schindelin <johannes.schindelin@gmx.de> writes:

Show 15 quoted lines

> 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() ;-)