Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932357AbZJUTWX (ORCPT ); Wed, 21 Oct 2009 15:22:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932074AbZJUTWV (ORCPT ); Wed, 21 Oct 2009 15:22:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:8953 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932273AbZJUTWS (ORCPT ); Wed, 21 Oct 2009 15:22:18 -0400 Message-ID: <4ADF5F52.10508@redhat.com> Date: Wed, 21 Oct 2009 14:21:54 -0500 From: Eric Sandeen User-Agent: Thunderbird 2.0.0.21 (X11/20090320) MIME-Version: 1.0 To: Ingo Molnar CC: Arjan van de Ven , Dave Jones , Linux Kernel , Thomas Gleixner , esandeen@redhat.com, cebbert@redhat.com, "H. Peter Anvin" Subject: Re: XFS stack overhead References: <20091015183540.GA8098@redhat.com> <20091015190720.GA19467@elte.hu> <4ADF2DAA.9030604@redhat.com> <20091021110053.26ab9982@infradead.org> <4ADF59F8.7010205@redhat.com> <20091021191648.GA12259@elte.hu> In-Reply-To: <20091021191648.GA12259@elte.hu> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3481 Lines: 90 Ingo Molnar wrote: > * Eric Sandeen wrote: > >> Arjan van de Ven wrote: >>> On Wed, 21 Oct 2009 10:50:02 -0500 >>> Eric Sandeen wrote: >>> >>>> Ingo Molnar wrote: >>>>> (Cc:-ed Arjan too.) >>>>> >>>>> * Dave Jones wrote: >>>>> >>>>>> 113c5413cf9051cc50b88befdc42e3402bb92115 introduced a change that >>>>>> made CC_STACKPROTECTOR_ALL not-selectable if someone enables >>>>>> CC_STACKPROTECTOR. >>>>>> >>>>>> We've noticed in Fedora that this has introduced noticable >>>>>> overhead on some functions, including those which don't even have >>>>>> any on-stack variables. >>>>>> >>>>>> According to the gcc manpage, -fstack-protector will protect >>>>>> functions with as little as 8 bytes of stack usage. So we're >>>>>> introducing a huge amount of overhead, to close a small amount of >>>>>> vulnerability (the >0 && <8 case). >>>>>> >>>>>> The overhead as it stands right now means this whole option is >>>>>> unusable for a distro kernel without reverting the above commit. >>>>> Exactly what workload showed overhead, and how much? >>>>> >>>>> Ingo >>>> I had xfs blowing up pretty nicely; granted, xfs is not svelte but it >>>> was never this bad before. >>>> >>> do you have any indication that SP actually increases the stack >>> footprint by that much? it's only a few bytes.... >>> >>> >> Here's a sample of some of the largest xfs stack users, >> and the effect stack-protector had on them. This was just >> done with objdump -d xfs.ko | scripts/checkstack.pl; I don't >> know if there's extra runtime stack overhead w/ stackprotector? >> >> -Eric >> >> function nostack stackprot delta delta % >> xfs_bmapi 376 408 32 9% >> xfs_bulkstat 328 344 16 5% >> _xfs_trans_commit 296 312 16 5% >> xfs_iomap_write_delay 264 280 16 6% >> xfs_file_ioctl 248 312 64 26% >> xfs_symlink 248 264 16 6% >> xfs_bunmapi 232 280 48 21% >> xlog_do_recovery_pass 232 248 16 7% >> xfs_trans_unreserve_and_mod_sb 224 240 16 7% >> xfs_bmap_del_extent 216 248 32 15% >> xfs_cluster_write 216 232 16 7% >> xfs_file_compat_ioctl 216 296 80 37% >> xfs_attr_set_int 200 216 16 8% >> xfs_bmap_add_extent_delay_real 200 248 48 24% > > Note that those are very large stack frames to begin with. > > 3496 bytes - that's a _lot_ - can anyone even run XFS with 4K stacks on? The above isn't a callchain; those are just the biggest users. Yes, xfs works w/4k stacks but sometimes not over complex storage. > With stackprotector it's 3928 - a 12% increase - which certainly does > not help - but the basic problem is the large stack footprint to begin > with. I can find plenty of examples of > 300 bytes stack users in the core kernel write path too, I'm just using xfs as an example... > Also, the posting apparently mixes 'stack overhead' with 'runtime > overhead'. right, that's why I asked, I'm not sure if stackprotector has runtime overhead as well. -Eric > Ingo -- 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/