From: Jeff King <peff@peff.net>
To: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Cc: git@vger.kernel.org, "Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>
Subject: Re: [PATCH 2/4] shallow.c: avoid theoretical pointer wrap-around
Date: Sat, 3 Dec 2016 00:17:21 -0500 [thread overview]
Message-ID: <20161203051720.z5elgoaapulqxlw5@sigill.intra.peff.net> (raw)
In-Reply-To: <1480710664-26290-2-git-send-email-rv@rasmusvillemoes.dk>
On Fri, Dec 02, 2016 at 09:31:02PM +0100, Rasmus Villemoes wrote:
> The expression info->free+size is technically undefined behaviour in
> exactly the case we want to test for. Moreover, the compiler is likely
> to translate the expression to
>
> (unsigned long)info->free + size > (unsigned long)info->end
>
> where there's at least a theoretical chance that the LHS could wrap
> around 0, giving a false negative.
>
> This might as well be written using pointer subtraction avoiding these
> issues.
> [...]
>
> - if (!info->slab_count || info->free + size > info->end) {
> + if (!info->slab_count || size > info->end - info->free) {
Yeah, I agree the correct way to write this is to compare the sizes
directly. That is how overflow checks _must_ be written. This one is
less likely to overflow, but even computing the value more than one past
the end of the array is technically undefined.
-Peff
next prev parent reply other threads:[~2016-12-03 5:17 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-02 20:31 [PATCH 1/4] shallow.c: make paint_alloc slightly more robust Rasmus Villemoes
2016-12-02 20:31 ` [PATCH 2/4] shallow.c: avoid theoretical pointer wrap-around Rasmus Villemoes
2016-12-03 5:17 ` Jeff King [this message]
2016-12-02 20:31 ` [PATCH 3/4] shallow.c: bit manipulation tweaks Rasmus Villemoes
2016-12-03 5:21 ` Jeff King
2016-12-02 20:31 ` [PATCH 4/4] shallow.c: remove useless test Rasmus Villemoes
2016-12-03 5:24 ` Jeff King
2016-12-05 12:00 ` Duy Nguyen
2016-12-03 5:14 ` [PATCH 1/4] shallow.c: make paint_alloc slightly more robust Jeff King
2016-12-05 10:02 ` Duy Nguyen
2016-12-06 12:53 ` [PATCH v2 0/6] shallow.c improvements Nguyễn Thái Ngọc Duy
2016-12-06 12:53 ` [PATCH v2 1/6] shallow.c: rename fields in paint_info to better express their purposes Nguyễn Thái Ngọc Duy
2016-12-06 12:53 ` [PATCH v2 2/6] shallow.c: stop abusing COMMIT_SLAB_SIZE for paint_info's memory pools Nguyễn Thái Ngọc Duy
2016-12-06 12:53 ` [PATCH v2 3/6] shallow.c: make paint_alloc slightly more robust Nguyễn Thái Ngọc Duy
2016-12-06 12:53 ` [PATCH v2 4/6] shallow.c: avoid theoretical pointer wrap-around Nguyễn Thái Ngọc Duy
2016-12-06 12:53 ` [PATCH v2 5/6] shallow.c: bit manipulation tweaks Nguyễn Thái Ngọc Duy
2016-12-06 12:53 ` [PATCH v2 6/6] shallow.c: remove useless code Nguyễn Thái Ngọc Duy
2016-12-06 13:42 ` [PATCH v2 0/6] shallow.c improvements Jeff King
2016-12-06 13:47 ` Duy Nguyen
2016-12-07 23:42 ` Junio C Hamano
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