MIME-Version: 1.0
In-Reply-To: <1456781345-8243-2-git-send-email-stsp@list.ru>
References: <1456781345-8243-1-git-send-email-stsp@list.ru> <1456781345-8243-2-git-send-email-stsp@list.ru>
From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 4 Mar 2016 14:22:24 -0800
Message-ID: <CALCETrWMJ-2xovNnPgsKZtmBWZpqBERCTzne2L81x0tcm0BhbQ@mail.gmail.com>
Subject: Re: [PATCH 1/2] sigaltstack: implement SS_AUTODISARM flag
To: Stas Sergeev <stsp@list.ru>
Cc: Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Richard Weinberger <richard@nod.at>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Tejun Heo <tj@kernel.org>,
        Heinrich Schuchardt <xypron.glpk@gmx.de>,
        Jason Low <jason.low2@hp.com>, Andrea Arcangeli <aarcange@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Josh Triplett <josh@joshtriplett.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "Amanieu d'Antras" <amanieu@gmail.com>, Paul Moore <pmoore@redhat.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Vladimir Davydov <vdavydov@parallels.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Stas Sergeev <stsp@users.sourceforge.net>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 715
Lines: 15

On Mon, Feb 29, 2016 at 1:29 PM, Stas Sergeev <stsp@list.ru> wrote:
> This patch implements the SS_AUTODISARM flag that can be ORed with
> SS_ONSTACK when forming ss_flags.
> When this flag is set, sigaltstack will be disabled when entering
> the signal handler; more precisely, after saving sas to uc_stack.
> When leaving the signal handler, the sigaltstack is restored by
> uc_stack.
> When this flag is used, it is safe to switch from sighandler with
> swapcontext(). Without this flag, the subsequent signal will corrupt
> the state of the switched-away sighandler.
>

This looks reasonable to me with one exception: how does a user
program detect the presence of this feature?  Anyone else have any
thoughts?