Subject: Re: [PATCH] Repalce strncmp by memcmp
From: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
To: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Pavel Vasilyev <pavel@pavlinux.ru>, Ming Lei <tom.leiming@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, yoshfuji@linux-ipv6.org
In-Reply-To: <20101129041359.GA6991@core.coreip.homeip.net>
References: <4CF30B46.4000203@pavlinux.ru>
	 <AANLkTikjDfyWY=Gy7VkdvEGNnYB9kb_iCqNDP7H9W5z4@mail.gmail.com>
	 <4CF319D9.9060102@pavlinux.ru>
	 <20101129041359.GA6991@core.coreip.homeip.net>
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Organization: USAGI Project
Date: Mon, 29 Nov 2010 14:26:02 +0900
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2010-11-28 (Sun) 20:13 -0800, Dmitry Torokhov wrote:
> On Mon, Nov 29, 2010 at 06:11:21AM +0300, Pavel Vasilyev wrote:
> > On 29.11.2010 05:29, Ming Lei wrote:
> > > Hi,
> > >
> > > 2010/11/29 Pavel Vasilyev <pavel@pavlinux.ru>:
> > >>    This patch replace all strncmp(a, b, c) by  memcmp(a, b, c).
> > >>
> > >> I test on x86_64 (AMD Opteron 285).
> > > In fact, memcmp doesn't handle case of tail of string, so
> > > it is not safe to replace strncmp with memcmp
> > >
> > #include <stdio.h>
> > #include <errno.h>
> > 
> > int main() {
> > 
> >    char *STR = "XXXX\0";
> >    char *XXX = "XXXX";
> 
> Try comparing:
> 
> "XXXX\0YYYY" and
> "XXXX\0ZZZZ"
> 
> and observe the difference.
> 

Yes, if both of the strings are NOT known to have enough length.

It is safe to replace strncmp(a,b,n) with memcmp(a,b,n)
if a or b is/are known to have enough length; strlen(a) >= n ||
strlen(b) >= n.

I think some of the replacements in the original patch are valid,
but for even those valid replacement, I think it is worth doing
that in hot code paths only.

--yoshfuji

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