From: Michael Haggerty <mhagger@alum.mit.edu>
To: Jonathan Nieder <jrnieder@gmail.com>, Junio C Hamano <gitster@pobox.com>
Cc: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>,
Alex Riesen <raa.lkml@gmail.com>,
git@vger.kernel.org
Subject: Re: [PATCH v2] for_each_string_list_item: avoid undefined behavior for empty list
Date: Wed, 20 Sep 2017 09:00:48 +0200 [thread overview]
Message-ID: <124b960d-a863-18ef-54a2-b170036dfca2@alum.mit.edu> (raw)
In-Reply-To: <20170920052705.GC126984@aiede.mtv.corp.google.com>
On 09/20/2017 07:27 AM, Jonathan Nieder wrote:
> From: Michael Haggerty <mhagger@alum.mit.edu>
>
> If you pass a newly initialized or newly cleared `string_list` to
> `for_each_string_list_item()`, then the latter does
>
> for (
> item = (list)->items; /* NULL */
> item < (list)->items + (list)->nr; /* NULL + 0 */
> ++item)
>
> Even though this probably works almost everywhere, it is undefined
> behavior, and it could plausibly cause highly-optimizing compilers to
> misbehave. C99 section 6.5.6 paragraph 8 explains:
>
> If both the pointer operand and the result point to elements
> of the same array object, or one past the last element of the
> array object, the evaluation shall not produce an overflow;
> otherwise, the behavior is undefined.
>
> and (6.3.2.3.3) a null pointer does not point to anything.
>
> Guard the loop with a NULL check to make the intent crystal clear to
> even the most pedantic compiler. A suitably clever compiler could let
> the NULL check only run in the first iteration, but regardless, this
> overhead is likely to be dwarfed by the work to be done on each item.
>
> This problem was noticed by Coverity.
>
> [jn: using a NULL check instead of a placeholder empty list;
> fleshed out the commit message based on mailing list discussion]
Thanks for taking this over. This version LGTM.
> [...]
Michael
next prev parent reply other threads:[~2017-09-20 7:00 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-15 16:00 [PATCH] for_each_string_list_item(): behave correctly for empty list Michael Haggerty
2017-09-15 18:43 ` Jonathan Nieder
2017-09-16 4:06 ` Michael Haggerty
2017-09-16 11:51 ` SZEDER Gábor
2017-09-17 10:19 ` Michael Haggerty
2017-09-19 14:38 ` Kaartic Sivaraam
2017-09-20 1:38 ` Junio C Hamano
2017-09-20 1:43 ` Jonathan Nieder
2017-09-20 5:14 ` Junio C Hamano
2017-09-20 2:30 ` Jonathan Nieder
2017-09-20 3:54 ` Junio C Hamano
2017-09-20 5:27 ` [PATCH v2] for_each_string_list_item: avoid undefined behavior " Jonathan Nieder
2017-09-20 5:40 ` Junio C Hamano
2017-09-20 7:00 ` Michael Haggerty [this message]
2017-09-20 7:40 ` Kaartic Sivaraam
2017-09-20 12:22 ` [PATCH v2] doc: camelCase the config variables to improve readability Kaartic Sivaraam
2017-09-20 16:28 ` [PATCH v2] for_each_string_list_item: avoid undefined behavior for empty list Andreas Schwab
2017-09-20 17:31 ` Jonathan Nieder
2017-09-20 21:51 ` Andreas Schwab
2017-09-21 1:12 ` Junio C Hamano
2017-09-21 15:39 ` Andreas Schwab
2017-09-20 7:35 ` [PATCH] for_each_string_list_item(): behave correctly " Kaartic Sivaraam
2017-09-17 0:59 ` Junio C Hamano
2017-09-17 10:24 ` Michael Haggerty
2017-09-18 0:37 ` Junio C Hamano
2017-09-19 0:08 ` Stefan Beller
2017-09-19 6:51 ` Michael Haggerty
2017-09-19 13:38 ` SZEDER Gábor
2017-09-19 13:45 ` SZEDER Gábor
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