From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stas Sergeev Subject: Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas within sighandler Date: Mon, 1 Feb 2016 22:01:49 +0300 Message-ID: <56AFAB9D.4070007@list.ru> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20160201185223.GA21136-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Oleg Nesterov Cc: Linux kernel , linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.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 List-Id: linux-api@vger.kernel.org 01.02.2016 21:52, Oleg Nesterov =D0=BF=D0=B8=D1=88=D0=B5=D1=82: > Stas, I probably missed something, but I don't understand your concer= ns, > > On 02/01, Stas Sergeev wrote: >> 01.02.2016 21:04, Oleg Nesterov =D0=BF=D0=B8=D1=88=D0=B5=D1=82: >>> 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. >>> I won't argue, but to me it would be better to keep this EPERM if != force. >>> Just because we should avoid the incompatible changes if possible. >> Ok then. Lets implement SS_FORCE. >> What semantic should it have wrt uc_stack? >> >> sigaltstack(SS_DISABLE | SS_FORCE); >> swapcontext(); >> sigaltstack(set up new_sas); >> rt_sigreturn(); > 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 t= he 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. >> 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 bec= ause > 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. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753033AbcBATCK (ORCPT ); Mon, 1 Feb 2016 14:02:10 -0500 Received: from smtp4.mail.ru ([94.100.179.57]:45930 "EHLO smtp4.mail.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752407AbcBATCI (ORCPT ); Mon, 1 Feb 2016 14:02:08 -0500 Subject: Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas within sighandler To: Oleg Nesterov 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> 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 From: Stas Sergeev Message-ID: <56AFAB9D.4070007@list.ru> Date: Mon, 1 Feb 2016 22:01:49 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0 MIME-Version: 1.0 In-Reply-To: <20160201185223.GA21136@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit X-Mras: Ok Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. >>> I won't argue, but to me it would be better to keep this EPERM if !force. >>> Just because we should avoid the incompatible changes if possible. >> Ok then. Lets implement SS_FORCE. >> What semantic should it have wrt uc_stack? >> >> sigaltstack(SS_DISABLE | SS_FORCE); >> swapcontext(); >> sigaltstack(set up new_sas); >> rt_sigreturn(); > 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. >> 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.