Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935283AbcCJK02 (ORCPT ); Thu, 10 Mar 2016 05:26:28 -0500 Received: from mail-wm0-f54.google.com ([74.125.82.54]:38110 "EHLO mail-wm0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935129AbcCJK0W (ORCPT ); Thu, 10 Mar 2016 05:26:22 -0500 Date: Thu, 10 Mar 2016 11:26:18 +0100 From: Ingo Molnar To: Andy Lutomirski Cc: Stas Sergeev , "linux-kernel@vger.kernel.org" , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , X86 ML , Borislav Petkov , Brian Gerst , Oleg Nesterov , Richard Weinberger , Stas Sergeev Subject: Re: [PATCH] [Cleanup] x86: signal: unify the sigaltstack check with other arches Message-ID: <20160310102618.GB21593@gmail.com> References: <1456095685-23857-1-git-send-email-stsp@list.ru> <20160225082524.GA12294@gmail.com> <56CEF02C.7050906@list.ru> <20160308162046.GA30211@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3859 Lines: 94 * Andy Lutomirski wrote: > On Tue, Mar 8, 2016 at 8:20 AM, Ingo Molnar wrote: > > > > * Stas Sergeev wrote: > > > >> 25.02.2016 11:25, Ingo Molnar пишет: > >> > > >> > * Stas Sergeev wrote: > >> > > >> >> Currently x86's get_sigframe() checks for "current->sas_ss_size" > >> >> to determine whether there is a need to switch to sigaltstack. > >> >> The common practice used by all other arches is to check for > >> >> sas_ss_flags(sp) == 0 > >> >> > >> >> This patch makes the code consistent with other arches. > >> >> The slight complexity of the patch is added by the optimization on > >> >> !sigstack check that was requested by Andy Lutomirski: sas_ss_flags(sp)==0 > >> >> already implies that we are not on a sigstack, so the code is shuffled > >> >> to avoid the duplicate checking. > >> > > >> > So this changelog is missing an analysis about what effect this change will have > >> > on applications. Can any type of user-space code see a change in behavior? If yes, > >> > what will happen and is that effect desirable? > >> This is a clean-up, and as such, there is no visible effect. > >> If there is - it is a bug. > >> The purpose of this patch is only to unify the x86 code with > >> what all the other arches do. It was initially the part of the > >> rejected series, but now it is just a clean-up. > > > > Ok, so AFAICS the relevant change is: > > > > - if (current->sas_ss_size) > > - sp = current->sas_ss_sp + current->sas_ss_size; > > + if (sas_ss_flags(sp) == 0) > > + sp = current->sas_ss_sp + current->sas_ss_size; > > > > and since sas_ss_flags() is defined as: > > > > static inline int sas_ss_flags(unsigned long sp) > > { > > if (!current->sas_ss_size) > > return SS_DISABLE; > > > > return on_sig_stack(sp) ? SS_ONSTACK : 0; > > } > > > > sas_ss_flags() returns 0 iff current->sas_ss_size && !on_sig_stack(). > > > > But we already have on_sig_stack(sp) calculated. Why not write that as: > > > > + if (current->sas_ss_size && !onsigstack) > > + sp = current->sas_ss_sp + current->sas_ss_size; > > > > and since we check '!onsigstack' in both branches, we might as well factor it out > > into a single condition ... and arrive to the exact code that we began with. > > ISTM it's silly for us to be unconditionally computing onsigstack. > We're doing it because we need it later for this: > > /* > * If we are on the alternate signal stack and would overflow it, don't. > * Return an always-bogus address instead so we will die with SIGSEGV. > */ > if (onsigstack && !likely(on_sig_stack(sp))) > return (void __user *)-1L; > > This seems basically useless to me. Sure, it's nice to send SIGSEGV > if we overflow due to signal delivery. But we're almost as likely to > overflow in the signal handler as we are to overflow during delivery, > and we don't even try to catch that. Ok, I was just put off by the code size difference - but no strong opinion from me, I'd certainly be fine with (as a first step) harmonizing the implementation with other architectures. I withdraw my objection. > Anyway, I think we should make two changes to the sig_on_stack thing: > > 1. If SS_AUTODISARM, then we're not on the stack, regardless of what sp says. > > 2. If !user_64bit_mode(regs) && (regs->ss & 0x4), then we're not on > the signal stack. This will happen if we're running on an LDT stack > and we coincidentally have the ESP part of SS:ESP matching the signal > stack. > > In general, the existing design is crap and it should always have > worked the way that Stas is proposing with SS_AUTODISARM. Ok, no objections here either. Thanks, Ingo