Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932934AbcCHQU6 (ORCPT ); Tue, 8 Mar 2016 11:20:58 -0500 Received: from mail-wm0-f46.google.com ([74.125.82.46]:34395 "EHLO mail-wm0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932361AbcCHQUv (ORCPT ); Tue, 8 Mar 2016 11:20:51 -0500 Date: Tue, 8 Mar 2016 17:20:46 +0100 From: Ingo Molnar To: Stas Sergeev Cc: linux-kernel@vger.kernel.org, Andy Lutomirski , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, 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: <20160308162046.GA30211@gmail.com> References: <1456095685-23857-1-git-send-email-stsp@list.ru> <20160225082524.GA12294@gmail.com> <56CEF02C.7050906@list.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <56CEF02C.7050906@list.ru> 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: 2709 Lines: 72 * 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. So what happened is that every other arch has a non-optimal version of this function. And if you look at the x86-32 defconfig build size difference: text data bss dec hex filename 4155 0 0 4155 103b signal.o.before 4299 0 0 4299 10cb signal.o.after i.e. your patch increases the generated code size. So I don't see the upside. If this is really duplicated across architectures then we should perhaps try to factor out this check into kernel/signal.c or so, and share it between architectures more seriously? Thanks, Ingo