linux-kernel - Re: kfree(0)

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 15 Aug 2007 09:01:42 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
CC:	Rene Herman <rene.herman@...il.com>,
	Jason Uhlenkott <jasonuhl@...onuhl.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Tim Bird <tim.bird@...sony.com>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: kfree(0) - ok?

Jan Engelhardt wrote:
> 0 is all-bits-zero.
> NULL is 0. ("It is.", above)
> 
> Transitively, this would make NULL all-bits-zero.
> I might have missed something, though, perhaps that the cast to void* makes it
> intransitive.

It does -- a cast from integer to pointer isn't required to be a bitwise
noop.  Machines on which NULL isn't bitwise zero do exist, and the C
standard allows them.  What is almost certainly more common than "all
bits zero is not a NULL pointer" is "any NULL pointer is not necessarily
all bits zero"; there are quite a few machines on which there are
nonzero bitpatterns that are still valid NULL pointers, but an all-zero
memset() will still produce NULL pointers.

However, the particular supersets of the C standard that gcc provides
and which the kernel are written in (the latter being a proper subset of
the former) does not.

	-hpa
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/