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Date: Mon, 1 Feb 2016 18:27:33 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Stas Sergeev <stsp@list.ru>
Cc: Linux kernel <linux-kernel@vger.kernel.org>, linux-api@vger.kernel.org,
        Andy Lutomirski <luto@amacapital.net>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Amanieu d'Antras" <amanieu@gmail.com>,
        Richard Weinberger <richard@nod.at>, Tejun Heo <tj@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Jason Low <jason.low2@hp.com>,
        Heinrich Schuchardt <xypron.glpk@gmx.de>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Josh Triplett <josh@joshtriplett.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Aleksa Sarai <cyphar@cyphar.com>, Paul Moore <pmoore@redhat.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Vladimir Davydov <vdavydov@parallels.com>
Subject: Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas
 within sighandler
Message-ID: <20160201172733.GA20831@redhat.com>
References: <56AE3369.2090709@list.ru>
 <56AE3626.7080706@list.ru>
 <20160201160625.GA18276@redhat.com>
 <56AF8E89.5090400@list.ru>
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On 02/01, Stas Sergeev wrote:
>
> >So the sequence is
> >
> >	// running on alt stack
> >
> >	sigaltstack(SS_DISABLE);
> >
> >	temporary_run_on_another_stack();
> >
> >	sigaltstack(SS_ONSTACK);
> >
> >and SS_DISABLE saves us from another SA_ONSTACK signal, right?
> Yes.
> Note: there is a test-case in that patch serie from which
> you can see or copy/paste the sample code.

OK, I wasn't cc'ed

> >But afaics it can only help after we change the stack. Suppose that SA_ONSTACK signal
> >comess before temporary_run_on_another_stack(). get_sigframe() should be fine after
> >your changes (afaics), it won't pick the alt stack after SS_DISABLE.
> >
> >However, unless I missed something save_altstack_ex() will record SS_ONSTACK in
> >uc_stack->ss_flags, and after return from signal handler restore_altstack() will
> >enable alt stack again?
> I don't think so. Please see the following hunk:

Yes, see another email, I already noticed this change.

> So I understand this is very confusing, but I think the patch
> is correct.

Not sure, but I can hardly read this patch and I can't apply it.

> Do you think adding the SS_FORCE flag would be a better solution?

Yes, certainly. I see no point to remember that a thread actually has the alt stack
but it was disabled.

Oleg.