From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753398AbcBAT3o (ORCPT ); Mon, 1 Feb 2016 14:29:44 -0500 Received: from mx1.redhat.com ([209.132.183.28]:46957 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752938AbcBAT3n (ORCPT ); Mon, 1 Feb 2016 14:29:43 -0500 Date: Mon, 1 Feb 2016 20:29:36 +0100 From: Oleg Nesterov To: Stas Sergeev Cc: Linux kernel , linux-api@vger.kernel.org, Andy Lutomirski , Ingo Molnar , Peter Zijlstra , Andrew Morton , "Amanieu d'Antras" , Richard Weinberger , Tejun Heo , "Kirill A. Shutemov" , Jason Low , Heinrich Schuchardt , Andrea Arcangeli , Konstantin Khlebnikov , Josh Triplett , "Eric W. Biederman" , Aleksa Sarai , Paul Moore , Palmer Dabbelt , Vladimir Davydov Subject: Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas within sighandler Message-ID: <20160201192936.GA21214@redhat.com> References: <56AE3369.2090709@list.ru> <56AE3626.7080706@list.ru> <20160201160625.GA18276@redhat.com> <20160201170958.GA20735@redhat.com> <56AF955D.7060601@list.ru> <20160201180443.GA21064@redhat.com> <56AFA0E2.1030302@list.ru> <20160201185223.GA21136@redhat.com> <56AFAB9D.4070007@list.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <56AFAB9D.4070007@list.ru> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/01, Stas Sergeev wrote: > > 01.02.2016 21:52, Oleg Nesterov пишет: > >Stas, I probably missed something, but I don't understand your concerns, > > > >On 02/01, Stas Sergeev wrote: > >>01.02.2016 21:04, Oleg Nesterov пишет: > >>>Yes, and SS_FORCE means "I know what I do", looks very simple. > >>But to me its not because I don't know what to do with > >>uc_stack after SS_FORCE is applied. > >Nothing? restore_sigaltstack() should work as expected? > That's likely the reason for EPERM: restore_sigaltstack() > does the job, so manual modifications are disallowed. > Allowing them will bring in the surprises where the changes > done by the user are ignored. Unlikely. Suppose you do sigalstack() and then a non SA_ONSTACK signal handler runs and calls sigaltstack() again. This won't fail, but restore_sigaltstack() will restore the old alt stack after return. I too do not know why uc_stack exists, in fact I do not know about it until today when I read your patch ;) But it is here, and I do not think SS_FORCE can add more confusion than we already have. > >Yes, or > > > > sigaltstack({ DISABLE | FORCE}, &old_ss); > > swapcontext(); > > sigaltstack(&old_ss, NULL); > > rt_sigreturn(); > > > >and if you are going to return from sighandler you do not even need the 2nd > >sigaltstack(), you can rely on sigreturn. > Yes, that's what I do in my app already. > But its only there when SA_SIGINFO is used. Hmm. how this connects to SA_SIGINFO ? > >>What's at the end? Do we want a surprise for the user > >>that he's new_sas got ignored? > >Can't understand.... do you mean "set up new_sas" will be ignored because > >rt_sigreturn() does restore_sigaltstack() ? I see no problem here... > Allowing the modifications that were previously EPERMed > but will now be silently ignored, may be seen as a problem. > But if it isn't - fine, lets code that. Still can't understand. The 2nd sigaltstack() is no longer EPERMed because application used SS_FORCED before that and disabled altstack. And it is not ignored, it actually changes alt stack. Until we return from handler. Oleg.