Subject: Re: [PATCH 1/2] sigaltstack: implement SS_AUTODISARM flag
To: Andy Lutomirski <luto@amacapital.net>
References: <1456781345-8243-1-git-send-email-stsp@list.ru>
 <1456781345-8243-2-git-send-email-stsp@list.ru>
 <CALCETrWMJ-2xovNnPgsKZtmBWZpqBERCTzne2L81x0tcm0BhbQ@mail.gmail.com>
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>
From: Stas Sergeev <stsp@list.ru>
Message-ID: <56DA8D25.20600@list.ru>
Date: Sat, 5 Mar 2016 10:39:17 +0300
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05.03.2016 01:22, Andy Lutomirski пишет:
> 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?
Compile-time detection:
#ifdef SS_AUTODISARM
   # I have this feature
   ...
#endif

Run-time detection:
int err = sigaltstack(SS_ONSTACK | SS_AUTODISARM);
if (err == EINVAL) {
   i_dont_have_this_feature = 1;
   err = sigaltstack(SS_ONSTACK);
}

Note: if you want to keep such detection for the future
additions, the mask can be enlarged to, say, ((1 << 24) - 1),
and whenever someone adds a new flag, he can lower the
mask by one bit.
But I think this would be an overkill in that particular case.