Re: [PATCH v4 3/3] upload-archive: use start_command instead of fork

[Jeff King <peff@peff.net>]

On Tue, Nov 15, 2011 at 01:11:46PM +0100, Thomas Rast wrote:

> But after a closer look I think this patch just prodded it enough to
> unearth long-existing undefined behaviour: prepare_argv() summarizes
> to something like
> 
> static void prepare_argv(const char **sent_argv, const char **argv)
> {
> 	char *p, buf[4096];
> 
> 	for (p = buf;;) {
> 		len = packet_read_line(0, p, (buf + sizeof buf) - p);
> 		/* ... p always points into buf ... */
> 		sent_argv[sent_argc++] = p;
> 		p += len;
> 		*p++ = 0;
> 	}
> 	sent_argv[sent_argc] = NULL;
> }
> 
> The code appears to have looked like this ever since the addition of
> that file back in 39345a2 (Add git-upload-archive, 2006-09-07).  So
> the elements of sent_argv have apparently always pointed into the
> stack-allocated 'buf'.

Oh, yikes. That is definitely the problem, but it does come from
c09cd77e. The prepare_argv function used to be "run_upload_archive", and
it would prepare argv on the stack, call into write_archive with it, and
then return; nobody else cares about the value afterwards.

Erik's patch converts it into a function that writes the new argv into a
parameter and returns, and the now-invalid stack-allocated memory is
used by the calling function.

> A quick band-aid would be to heap-allocate it instead:

That works.  An even shorter band-aid is to mark it as "static".

I think the code would be more readable if it just used the new
argv_array.

Junio, this bug is in 1.7.8-rc*. Do you want the one-liner fix for the
release, or the nicer fix?

-Peff