Re: [RFC 2/2] Make misuse of get_pathname() buffers detectable by valgrind

From: Nguyen Thai Ngoc Duy <pclouds@gmail.com>
Date: Nov 16, 2011, 13:57 UTC
Message-ID: <CACsJy8CG0A1kUUyA=1AW+3cNTjrgb_iKjdfs7M8Sa6s0cuDANw@mail.gmail.com>
In-Reply-To: <1317097687-11098-3-git-send-email-mhagger@alum.mit.edu>

On Tue, Sep 27, 2011 at 11:28 AM, Michael Haggerty <mhagger@alum.mit.edu> wrote:

Show 20 quoted lines

> A temporary buffer produced by get_pathname() is recycled after a few
> subsequent calls of get_pathname().  The use of such a buffer after it
> has been recycled can result in the wrong file being accessed with
> very strange effects.  Moreover, such a bug can lie dormant until code
> elsewhere is changed to use a temporary buffer, causing very
> mysterious, nonlocal failures that are hard to analyze.
>
> Add a second implementation of get_pathname() (activated if the
> VALGRIND preprocessor macro is defined) that allocates and frees
> buffers instead of recycling statically-allocated buffers.  This does
> not make the problem less serious, but it turns the errors into
> access-after-free errors, making it possible to locate the guilty code
> using valgrind.
>
> Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
> ---
>
> I believe that it is frowned upon to use #ifdefs in git code, but no
> good alternative is obvious to me for this type of use.  Suggestions
> are welcome.

Enable the code based on an environment variable, e.g. GIT_DEBUG_FENCE, then enable it by default in test-lib.sh :-)

-- 
Duy