Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754689Ab1BTSmU (ORCPT ); Sun, 20 Feb 2011 13:42:20 -0500 Received: from palinux.external.hp.com ([192.25.206.14]:48880 "EHLO mail.parisc-linux.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754477Ab1BTSmT (ORCPT ); Sun, 20 Feb 2011 13:42:19 -0500 Date: Sun, 20 Feb 2011 11:42:17 -0700 From: Matthew Wilcox To: Borislav Petkov , Borislav Petkov , Ingo Molnar , Dan Carpenter , "Herrmann3, Andreas" , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , "maintainer:X86 ARCHITECTURE..." , "open list:AMD MICROCODE UPD..." , open list , "kernel-janitors@vger.kernel.org" Subject: Re: [patch -next] x86, microcode, AMD: signedness bug in generic_load_microcode() Message-ID: <20110220184217.GB13726@parisc-linux.org> References: <20110218091716.GA4384@bicker> <20110220130214.GD25700@elte.hu> <20110220141452.GA12127@aftab> <20110220175011.GA13726@parisc-linux.org> <20110220180845.GA6713@liondog.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110220180845.GA6713@liondog.tnic> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1964 Lines: 44 On Sun, Feb 20, 2011 at 07:08:45PM +0100, Borislav Petkov wrote: > On Sun, Feb 20, 2011 at 10:50:11AM -0700, Matthew Wilcox wrote: > > No, that's not what's going on. GCC _is_ truncating to a byte, 0xa5, > > whether it's signed or not. Then at the time of the call to printf, > > the 0xa5 is cast to int. If the char is signed, 0xa5 is sign-extended; > > if unsigned, it's zero-extended. > > Yes, you're right, I missed the fact that printf does convert its > arguments based on the format string. I should've done > > printf("ret = 0x%hhx\n", ret); GCC's special treatment of the printf format string is only in the gneration of warnings. It doesn't promote differently based on the format string. You need to look at 6.5.2.2, parts 6 and 7. Part 7 says: The ellipsis notation in a function prototype declarator causes argument type conversion to stop after the last declared parameter. The default argument promotions are performed on trailing arguments. And part 6 describes the default argument promotions: If the expression that denotes the called function has a type that does not include a prototype, the integer promotions are performed on each argument, and arguments that have type float are promoted to double. These are called the default argument promotions. So passing a char to printf will cause it to be promoted to int, no matter what the format string says. All the format string will do is change how it's printed. Probably by casting it back to a char :-) -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step." -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/