linux-kernel - Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas within sighandler

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Date:	Mon, 1 Feb 2016 18:09:58 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	Linux kernel <linux-kernel@...r.kernel.org>,
	linux-api@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Amanieu d'Antras" <amanieu@...il.com>,
	Richard Weinberger <richard@....at>, Tejun Heo <tj@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Jason Low <jason.low2@...com>,
	Heinrich Schuchardt <xypron.glpk@....de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Josh Triplett <josh@...htriplett.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Aleksa Sarai <cyphar@...har.com>,
	Paul Moore <pmoore@...hat.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Vladimir Davydov <vdavydov@...allels.com>
Subject: Re: [PATCH 4/4] sigaltstack: allow disabling and re-enabling sas
 within sighandler

On 02/01, Oleg Nesterov wrote:
>
> > +        onsigstack = on_sig_stack(sp);
> > +        if (ss_size == 0) {
> > +            switch (ss_flags) {
> > +            case 0:
> > +                error = -EPERM;
> > +                if (onsigstack)
> > +                    goto out;
> > +                current->sas_ss_sp = 0;
> > +                current->sas_ss_size = 0;
> > +                current->sas_ss_flags = SS_DISABLE;
> > +                break;
> > +            case SS_ONSTACK:
> > +                /* re-enable previously disabled sas */
> > +                error = -EINVAL;
> > +                if (current->sas_ss_size == 0)
> > +                    goto out;
> > +                break;
> > +            default:
> > +                break;
> > +            }
>
> and iiuc the "default" case allows you to write SS_DISABLE into ->sas_ss_flags
> even if on_sig_stack().
>
> 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?
>
> 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?

OK, I didn't notice you modified save_altstack_ex() to use ->sas_ss_flags instead
of sas_ss_flags()... still doesn't look right, in this case restore_altstack() will
not restore sas_ss_size/sas_ss_sp and they can be changed by signal handler.

Anyway, whatever I missed I agree with Andy, SS_FORCE looks simpler and better to me.

Oleg.