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

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Date:	Mon, 1 Feb 2016 10:28:39 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Linux kernel <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	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 Mon, Feb 1, 2016 at 10:16 AM, Stas Sergeev <stsp@...t.ru> 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.
>
>> 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();
>
> What's at the end? Do we want a surprise for the user
> that he's new_sas got ignored?

More detail please.  What context are you returning to with
rt_sigreturn?  What's in uc_stack?

Presumably we should continue to honor uc_stack in rt_sigreturn.  I'm
less clear on whether we should have an implicit SS_FORCE when
restoring uc_stack.  I'm also not clear on why uc_stack exists in the
first place.

If I were designing this from scratch, I'd have signal delivery for an
SA_ONSTACK signal save away the altstack information and clear it so
that nested signals work without checking sp during signal delivery.
But I'm not designing it from scratch, and I haven't spotted uc_stack
or similar mentioned in POSIX or the man page, so I'm not really clear
on what it's for.

--Andy