Re: [PATCH v2 2/2] read-cache: check range before dereferencing an array element

From: Junio C Hamano <gitster@pobox.com>
Date: Apr 11, 2025, 14:50 UTC
Message-ID: <xmqq7c3qoipu.fsf@gitster.g>
In-Reply-To: <20250328054953.GA3590654@coredump.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 23 quoted lines

> On Thu, Mar 27, 2025 at 11:05:57AM +0000, Johannes Schindelin via GitGitGadget wrote:
>
>> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>> 
>> Before accessing an array element at a given index, we should make sure
>> that the index is within the desired bounds, otherwise it makes little
>> sense to access the array element in the first place.
>> 
>> In this instance, testing whether `ce->name[common]` is the trailing NUL
>> byte is technically different from testing whether `common` is within
>> the bounds of `previous_name`. It is also redundant, as the range-check
>> guarantees that `previous_name->buf[common]` cannot be NUL and therefore
>> the condition `ce->name[common] == previous_name->buf[common]` would not
>> be met if `ce->name[common]` evaluated to NUL.
>> 
>> However, in the interest of reducing the cognitive load to reason about
>> the correctness of this loop (so that I can focus on interesting
>> projects again), I'll simply move the range-check to the beginning of
>> the loop condition and keep the redundant NUL check.
>
> Thanks, I think this explanation works, and the patch looks fine. (I
> didn't dig deeply into patch 1, but I agree with Junio's analysis that
> it is also a false positive).

I think (1) working around a rare false positive to help us use an otherwise mostly useful tool is a worthy thing to do, and (2) when we need to make a workaround for a false positive, we should mark a change to do so as such. And I agree with you that this step in the updated form, both the change in the patch and the proposed log message do their job.

Thanks, both.  Will mark this one for 'next'.

Note that I think [1/2] needs similar updates relative to the initial iteration, but since these two patches do not depend on each other, I'll fast track this step without waiting updates to the other one.