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

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Date:	Mon, 1 Feb 2016 21:40:52 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...capital.net>
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

01.02.2016 21:28, Andy Lutomirski пишет:
> 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?
Whatever was saved there by save_altstack_ex() I guess.
Which is the sas params on signal entry.
And I am returning to the interrupted user context.
I am using SA_SIGINFO with sigaction().
This is actually what I was asking you already yeaterday.
I don't think SS_FORCE can play nicely with uc_stack and
you haven't clarified that part, so lets try again.

> Presumably we should continue to honor uc_stack in rt_sigreturn.
In this case, the above sigaltstack(set up new_sas) just
gets ignored. Are we allright with that? If so, I can code
up the patch. Whatever. :)

>    I'm
> less clear on whether we should have an implicit SS_FORCE when
> restoring uc_stack.
This is obscure and is likely outside of the scope of the
problem at hands.

>    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.
How would you then evaluate oss->ss_flags?