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Subject: Re: xor as a lazy comparison
From: Steven Rostedt <rostedt@goodmis.org>
To: Bernd Petrovitsch <bernd@firmix.at>
Cc: Andrew Morton <akpm@osdl.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Grant Coady <lkml@dodo.com.au>,
       Jan Engelhardt <jengelh@linux01.gwdg.de>,
       Puneet Vyas <vyas.puneet@gmail.com>
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Organization: Kihon Technologies
Date: Mon, 25 Jul 2005 13:55:50 -0400
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On Mon, 2005-07-25 at 10:57 +0200, Bernd Petrovitsch wrote:
> On Sun, 2005-07-24 at 18:15 -0400, Puneet Vyas wrote:
> [...]
> > I just compiled two identical program , one with "!=" and other with 
> > "^". The assembly output is identical.
> 
> Hmm, which compiler and which version?
> You might want to try much older and other compilers.
> 

Doesn't matter. The cycles saved for old compilers is not rational to
have obfuscated code.

Here's the patch to make the code more readable.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

--- linux-2.6.13-rc3/kernel/signal.c.orig	2005-07-25 13:50:20.000000000 -0400
+++ linux-2.6.13-rc3/kernel/signal.c	2005-07-25 13:50:51.000000000 -0400
@@ -665,8 +665,8 @@ static int check_kill_permission(int sig
 			(unsigned long)info != 2 && SI_FROMUSER(info)))
 	    && ((sig != SIGCONT) ||
 		(current->signal->session != t->signal->session))
-	    && (current->euid ^ t->suid) && (current->euid ^ t->uid)
-	    && (current->uid ^ t->suid) && (current->uid ^ t->uid)
+	    && (current->euid != t->suid) && (current->euid != t->uid)
+	    && (current->uid != t->suid) && (current->uid != t->uid)
 	    && !capable(CAP_KILL))
 		return error;