Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934167AbbGVANf (ORCPT ); Tue, 21 Jul 2015 20:13:35 -0400 Received: from mail-la0-f46.google.com ([209.85.215.46]:36209 "EHLO mail-la0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934008AbbGVANd (ORCPT ); Tue, 21 Jul 2015 20:13:33 -0400 MIME-Version: 1.0 In-Reply-To: <20150719041235.GA3683@nazgul.tnic> References: <0d8517b7ab757e00a13b3abe2b677d9eb23362be.1437150175.git.jpoimboe@redhat.com> <20150717171726.GA21568@nazgul.tnic> <20150717173220.GA12761@treble.redhat.com> <20150718050536.GA23478@nazgul.tnic> <20150718134415.GA24587@treble.redhat.com> <20150718145629.GB30713@nazgul.tnic> <20150718155714.GB19968@treble.redhat.com> <20150719041235.GA3683@nazgul.tnic> From: Andy Lutomirski Date: Tue, 21 Jul 2015 17:13:12 -0700 Message-ID: Subject: Re: [RFC PATCH 04/21] x86/hweight: Add stack frame dependency for __arch_hweight*() To: Borislav Petkov Cc: Michal Marek , Thomas Gleixner , Ingo Molnar , Josh Poimboeuf , "linux-kernel@vger.kernel.org" , Andi Kleen , Pedro Alves , live-patching@vger.kernel.org, X86 ML , "H. Peter Anvin" , Linus Torvalds , Peter Zijlstra Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1785 Lines: 40 On Jul 18, 2015 9:13 PM, "Borislav Petkov" wrote: > > On Sat, Jul 18, 2015 at 10:57:14AM -0500, Josh Poimboeuf wrote: > > Currently, when stackvalidate sees an ALTERNATIVE, it assumes that > > either code path is possible, so it follows both paths in parallel. > > > > If I understand right, you're proposing that stackvalidate should only > > follow the POPCNT path and never follow the !POPCNT path? > > Actually, you don't even need to follow the POPCNT case either because > it is a single instruction - no stack operations there. > > So yeah, either that or special-case the case where the original insn is > CALL and the replacement is a POPCNT and ignore those CALL locations. > > The advantage is that the burden is put on the tool and not by adding > markers to kernel code paths. > > > In general, I agree, and I like the original patch much better. IMO, it > > achieved the goal of keeping the kernel code clean, while fixing the > > frame pointer bug. > > And I think that in that case, adding that rSP dependency is too much > because even though it fixes the "bug", it is very very unlikely any > stack trace will have __sw_hweight* in it for reasons pointed out > earlier and also because those functions can't fail and they get > integral types as args which can't fail when deref-fing either. And even > if they do, they don't call any other functions so rIP pointing to them > is already enough. Enough for oopses, perhaps, but maybe not enough for perf. It sounds like you want CFI unwinding :) --Andy -- 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/