Re: [PATCH 2/4] avoid computing zero offsets from NULL pointer

From: Junio C Hamano <gitster@pobox.com>
To: Jeff King <peff@peff.net>
Cc: git@vger.kernel.org
Subject: Re: [PATCH 2/4] avoid computing zero offsets from NULL pointer
Date: Tue, 28 Jan 2020 21:16:39 -0800	[thread overview]
Message-ID: <xmqqblqm7sg8.fsf@gitster-ct.c.googlers.com> (raw)
In-Reply-To: <20200129023146.GA596379@coredump.intra.peff.net> (Jeff King's message of "Tue, 28 Jan 2020 21:31:46 -0500")

Jeff King <peff@peff.net> writes:

> Yep, but it's not much different than the hundreds of other function
> interfaces we have where sometimes you can pass NULL and sometimes not.
>
> So anyway. What do we want to do here? The fix I have? Something more
> elaborate and reusable? Or perhaps just switch it to:

My preference was to take the patch as-is, as it was clear enough,
before seeing this one ...

> diff --git a/xdiff-interface.c b/xdiff-interface.c
> index 3cd2ac2855..4d20069302 100644
> --- a/xdiff-interface.c
> +++ b/xdiff-interface.c
> @@ -84,8 +84,8 @@ static void trim_common_tail(mmfile_t *a, mmfile_t *b)
>  {
>  	const int blk = 1024;
>  	long trimmed = 0, recovered = 0;
> -	char *ap = a->ptr ? a->ptr + a->size : a->ptr;
> -	char *bp = b->ptr ? b->ptr + b->size : b->ptr;
> +	char *ap = a->size ? a->ptr + a->size : a->ptr;
> +	char *bp = b->size ? b->ptr + b->size : b->ptr;
>  	long smaller = (a->size < b->size) ? a->size : b->size;
>  
>  	while (blk + trimmed <= smaller && !memcmp(ap - blk, bp - blk, blk)) {
>
> By checking "size" instead of "ptr", then we know that the addition is a
> noop. And we'd continue to catch a NULL pointer mixed with a non-zero
> length (as a segfault). And a non-NULL pointer with a zero length does
> the right thing.

which makes quite a lot of sense.