Re: [PATCH 2/2] fsck: detect and warn a commit with embedded NUL

From: Jeff King <peff@peff.net>
Date: Apr 14, 2016, 18:21 UTC
Message-ID: <20160414182102.GB22068@sigill.intra.peff.net>
In-Reply-To: <20160414180709.28968-2-gitster@pobox.com>

On Thu, Apr 14, 2016 at 11:07:09AM -0700, Junio C Hamano wrote:

Show 12 quoted lines

> Even though a Git commit object is designed to be capable of storing
> any binary data as its payload, in practice people use it to describe
> the changes in textual form, and tools like "git log" are designed to
> treat the payload as text.
> 
> Detect and warn when we see any commit object with a NUL byte in
> it.
> 
> Note that a NUL byte in the header part is already detected as a
> grave error.  This change is purely about the message part.
> 
> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Thanks, I was just reading over some of the old threads, and wondering if it was time to resurrect this idea.

Show 8 quoted lines

> @@ -610,6 +611,7 @@ static int fsck_commit_buffer(struct commit *commit, const char *buffer,
>  	struct commit_graft *graft;
>  	unsigned parent_count, parent_line_count = 0, author_count;
>  	int err;
> +	const char *buffer_begin = buffer;
>  
>  	if (verify_headers(buffer, size, &commit->object, options))
>  		return -1;

You need this "buffer_begin" because we move the "buffer" pointer forward as we parse. But perhaps whole-buffer checks should simply go at the top (next to verify_headers) before we start advancing the pointer. To me, that makes the function's flow more natural.

But alternatively...

Show 10 quoted lines

> @@ -671,6 +673,12 @@ static int fsck_commit_buffer(struct commit *commit, const char *buffer,
>  		if (err)
>  			return err;
>  	}
> +	if (memchr(buffer_begin, '\0', size)) {
> +		err = report(options, &commit->object, FSCK_MSG_NUL_IN_COMMIT,
> +			     "NUL byte in the commit object body");
> +		if (err)
> +			return err;
> +	}

Here we've parsed to the end of the headers we know about. We know there's no NUL there, because verify_headers() would have complained. And because the individual header parsers would have complained. So I actually think we could check from "buffer" (of course we do still need to record the beginning of the buffer to adjust "size" appropriately).

It's a little more efficient (we don't have to memchr over the same bytes again). But I'd worry a little that doing it that way would introduce coupling between this check and verify_headers(), though (so that if the latter ever changes, our check may start missing cases).

So yet another alternative would be to include this check in verify_headers(). It would parse to the end of the headers as now, and then from there additionally look for a NUL in the body.

Of the three approaches, I think I like that third one. It's the most efficient, and I think the flow is pretty clear. We'd probably want to rename verify_headers(), though. :)

-Peff