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

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Date:	Mon, 1 Feb 2016 18:27:33 +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, 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.