linux-kernel - Re: [PATCH] Repalce strncmp by memcmp

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Date:	Tue, 30 Nov 2010 11:51:06 +1300
From:	Ryan Mallon <ryan@...ewatersys.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	pavel@...linux.ru, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Repalce strncmp by memcmp

On 11/30/2010 11:26 AM, Steven Rostedt wrote:
> On Mon, 2010-11-29 at 17:18 -0500, Steven Rostedt wrote:
> 
>> Um, do you realize that the kernel does not always use the same memcmp
>> as gcc.
>>
> 
> Note, I'm not against the change, because in 99% of the cases, memcmp()
> can and will be faster, and we don't need to worry about these strange
> cases.

It can still break things in subtle ways. Lots of the replacements are
of the form:

  if (strncmp(string, "foo", 3) == 0)

Which can only be replaced with memcmp if the minimum length of string
is _always_ 3. This may be true for some callsites (with careful audit),
but in general I doubt it is and it will lead to subtle bugs.

I hardly think it is worth auditing a bunch of strncmp calls to ensure
that the minimum length of the checked string is always n in order to
remove a single instruction. Making such a change will also introduce
subtle bugs if the rules for the string ever change, eg. a change is
made to allow string = "".

~Ryan

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