Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752387AbZGVLvv (ORCPT ); Wed, 22 Jul 2009 07:51:51 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752295AbZGVLvv (ORCPT ); Wed, 22 Jul 2009 07:51:51 -0400 Received: from bizon.gios.gov.pl ([195.187.34.71]:33857 "EHLO bizon.gios.gov.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751706AbZGVLvu (ORCPT ); Wed, 22 Jul 2009 07:51:50 -0400 Date: Wed, 22 Jul 2009 13:49:58 +0200 (CEST) From: Krzysztof Oledzki X-X-Sender: ole@bizon.gios.gov.pl To: Linus Torvalds cc: Troy Moure , Greg KH , Linux Kernel Mailing List , Andrew Morton , stable@kernel.org, lwn@lwn.net, Ian Lance Taylor Subject: Re: Linux 2.6.27.27 In-Reply-To: Message-ID: References: <20090720040655.GA11940@kroah.com> <4A645A45.9060509@ans.pl> <20090720151008.GC10015@suse.de> User-Agent: Alpine 1.10 (LNX 962 2008-03-14) MIME-Version: 1.0 Content-Type: MULTIPART/MIXED; BOUNDARY="-1144830174-1773904520-1248263398=:26357" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3891 Lines: 114 This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. ---1144830174-1773904520-1248263398=:26357 Content-Type: TEXT/PLAIN; charset=iso-8859-2; format=flowed Content-Transfer-Encoding: 8BIT On Tue, 21 Jul 2009, Linus Torvalds wrote: > > [ Added Ian Lance Taylor to the cc, he knows the background, and unlike me > is competent with gcc. ] > > On Tue, 21 Jul 2009, Troy Moure wrote: >> >> I think I've found something interesting. Look at the the code generated >> for edid_checksum() in driver/video/fbmon.c. This is what I see for the >> -fno-strict-overflow kernel: > > Ooh. > > Bingo. You're 100% right, and you definitely found it (of course, there > may be _other_ cases like this, but that's certainly _one_ of the > problems, and probably the only one). > > Just out of curiosity, how did you find it? Now that I know where to look, > it's very obvious in the assembler diffs, but I didn't notice it until you > pointed it out just because there is so _much_ of the diffs... > > And yes, that's very much a compiler bug. And I also bet it's very easily > fixed. > > The code in question is this loop: > > #define EDID_LENGTH 128 > > unsigned char i, ... > > for (i = 0; i < EDID_LENGTH; i++) { > csum += edid[i]; > all_null |= edid[i]; > } > > and gcc -fno-strict-overflow has apparently decided that that is an > infinite loop, even though it clearly is not. So then the stupid and buggy > compiler will compile that loop (and the whole rest of the function) to > the "optimized" version that is just > > loop: > jmp loop; > > I even bet I know why: it looks at "unsigned char", and sees that it is an > 8-bit variable, and then it looks at "i < EDID_LENGTH" and sees that it is > a _signed_ comparison (it's signed because the C type rules mean that > 'unsigned char' will be extended to 'int' in an expression), and then it > decides that in a signed comparison an 8-bit entry is always going to be > smaller than 128. > > Anyway, I bet we can work around the compiler bug by just changing the > type of "i" from "unsigned char" to be a plain "int". > > Krzysztof? Mind testing that? > > Ian? This is Linux 2.6.27.27 compiled with gcc-4.2.4. I'm not seeing the > bug in the gcc I have on my machine (gcc-4.4.0), but the bug is very clear > (once you _find_ it, which was the problem) in the binaries that Krzysztof > posted. They're still at: > > http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/vmlinux-fno-strict-overflow.bz2 (Hangs) > http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/vmlinux-fwrapv.bz2 (OK) > http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/vmlinux-fnone.bz2 (OK) > > and you can clearly see the 'edid_checksum' miscompilation in the objdump > disassembly. BTW: here is a simple testcase for this bug: --- fno-strict-overflow-fixed-bug.c --- #include int main() { unsigned char i; for (i = 0; i < 128; i++) printf("loop %u\n", i); return 0; } --- cut here --- The code should be compiled with: cc -o fno-strict-overflow-fixed-bug -Os -fno-strict-overflow fno-strict-overflow-fixed-bug.c or: cc -o fno-strict-overflow-fixed-bug -O2 -fno-strict-overflow fno-strict-overflow-fixed-bug.c This bug does not exist with -O1 or if the loop is controlled by "i < 127" or "i < 129". So, we should make sure there is no unsigned char i; (...) for (i = 0; i < 128; i++) somewhere inside the kernel. Best regards, Krzysztof Ol?dzki ---1144830174-1773904520-1248263398=:26357-- -- 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/