From: Florian Weimer <fweimer@redhat.com>
To: Andreas Schwab <schwab@suse.de>
Cc: libc-alpha@sourceware.org
Subject: Re: [review] manual: Clarify strnlen, wcsnlen, strndup null termination behavior
Date: Wed, 30 Oct 2019 10:55:00 -0000 [thread overview]
Message-ID: <87a79i33kt.fsf@oldenburg2.str.redhat.com> (raw)
In-Reply-To: <mvmd0eeh5sb.fsf@suse.de> (Andreas Schwab's message of "Wed, 30 Oct 2019 11:44:04 +0100")
* Andreas Schwab:
> On Okt 30 2019, Florian Weimer (Code Review) wrote:
>
>> +Note that @var{s} must be an array of at least @var{maxlen} bytes. It
>> +is undefined to call @code{strnlen} on a shorter array, even if it is
>> +known that the shorter array contains a null terminator.
>
> This is not true. strnlen _always_ stops before the null byte.
This is not how it is specified in POSIX.
Our generic implementation of strnlen performs out-of-bounds pointer
arithmetic in that case, and it looks really iffy:
const char *char_ptr, *end_ptr = str + maxlen;
…
if (__glibc_unlikely (end_ptr < str))
end_ptr = (const char *) ~0UL;
GCC does the right thing on x86-64, I think, but that's far from
guaranteed.
And what about wcsnlen?
Thanks,
Florian
next prev parent reply other threads:[~2019-10-30 10:55 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-30 10:25 Florian Weimer (Code Review)
2019-10-30 10:44 ` Andreas Schwab
2019-10-30 10:55 ` Florian Weimer [this message]
2019-10-30 11:00 ` Andreas Schwab
2019-10-30 11:03 ` Florian Weimer
2019-10-30 11:10 ` Andreas Schwab
2019-10-30 12:01 ` Zack Weinberg
2019-10-30 16:20 ` Andreas Schwab
2019-10-30 16:31 ` Zack Weinberg
2019-10-30 16:47 ` Andreas Schwab
2019-10-30 16:58 ` Zack Weinberg
2019-10-30 17:26 ` Andreas Schwab
2019-10-30 18:12 ` Zack Weinberg
2019-10-30 18:36 ` Florian Weimer
2019-10-30 17:24 ` Joseph Myers
2019-11-28 9:43 ` Florian Weimer
2019-11-28 15:56 ` Carlos O'Donell
2019-11-28 15:58 ` Carlos O'Donell
2019-11-28 18:23 ` Rich Felker
2019-11-28 18:38 ` Szabolcs Nagy
2019-11-29 18:20 ` Martin Sebor
2019-11-27 19:08 ` Carlos O'Donell (Code Review)
2019-11-27 19:14 ` Florian Weimer (Code Review)
2019-11-27 22:11 ` Carlos O'Donell (Code Review)
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