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From: Junio C Hamano <gitster@pobox.com>
To: Jeff King <peff@peff.net>
Cc: Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com>,
	git@vger.kernel.org,
	Johannes Schindelin <johannes.schindelin@gmx.de>
Subject: Re: [PATCH 1/1] untracked cache: fix off-by-one
Date: Fri, 12 Apr 2019 10:48:30 +0900	[thread overview]
Message-ID: <xmqq7ec00z9t.fsf@gitster-ct.c.googlers.com> (raw)
In-Reply-To: <20190410162029.GA30818@sigill.intra.peff.net> (Jeff King's message of "Wed, 10 Apr 2019 12:20:29 -0400")

Jeff King <peff@peff.net> writes:

> Yeah, every struct on a platform where FLEX_ARRAY is 1 ends up
> over-allocated by 1 byte. We could account for that in all of our flex
> allocations, but I don't it affects enough platforms to be worth the
> bother.

It was an explicit design decision to declare that the
overallocation was a non-problem back when we invented FLEX_ARRAY
macro.  We could probably have instead decided to pass number of
bytes to be stored in the flex-array (including NUL if that is a
character array) then subtract the value of FLEX_ARRAY if FLEX_ARRAY
were limited to only 0 or 1, but that was not workable as it is the
most natural to define FLEX_ARRAY to an empty string, i.e. making an
array field decl to "type field[];", for the more recent compilers.


>> As there is no obvious way to trigger this bug reliably, on all
>> platforms supported by Git, and as the bug is obvious enough, this patch
>> comes without a regression test.
>
> Makes sense. This code path should be well-covered by the existing tests
> anyway, so even if we could get those tools to trigger, I don't think
> there would be much point in adding a new test.

Yeah, it is already super that this obscure bug was spotted in the
first place.  It is true that this may regress without a test, but I
do not think it is reasonable to expect that it is possible to write
a reliable crash-detecting test for this one.

> The right thing is probably something like:
>
>   eos = memchr(data, '\0', end - data);
>   if (!eos)
> 	return error("malformed untracked cache extension");
>   len = eos - data;
>
> I wouldn't be at all surprised if other bits of the index code have the
> same issue, though. And at any rate, thinking about that should
> definitely not hold up your fix.

True, true.  I wonder if folks intereseted in libFuzzer can chime in
with something useful here, but that is totally independent.


  reply	other threads:[~2019-04-12  1:48 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-10 12:56 [PATCH 0/1] Fix an off-by-one bug in the untracked cache code Johannes Schindelin via GitGitGadget
2019-04-10 12:56 ` [PATCH 1/1] untracked cache: fix off-by-one Johannes Schindelin via GitGitGadget
2019-04-10 16:20   ` Jeff King
2019-04-12  1:48     ` Junio C Hamano [this message]
2019-04-18 21:14       ` [PATCH 0/3] untracked cache parsing fixups Jeff King
2019-04-18 21:17         ` [PATCH 1/3] untracked-cache: be defensive about missing NULs in index Jeff King
2019-04-19  5:29           ` Junio C Hamano
2019-04-18 21:17         ` [PATCH 2/3] untracked-cache: simplify parsing by dropping "next" Jeff King
2019-04-19  5:33           ` Junio C Hamano
2019-04-18 21:18         ` [PATCH 3/3] untracked-cache: simplify parsing by dropping "len" Jeff King
2019-04-18 21:24         ` [PATCH 4/3] untracked-cache: use FLEX_ALLOC to create internal structs Jeff King
2019-04-19  9:18           ` Duy Nguyen
2019-04-19 19:43             ` Jeff King

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