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

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Date:	Mon, 1 Feb 2016 19:04:43 +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:
>
> 01.02.2016 20:09, Oleg Nesterov пишет:
> >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.
> How?
> Trying to change them in a sighandler with sigaltstack()
> will get EPERM.

Only if on_sig_stack() and this is not true if we change the stack.

> >Anyway, whatever I missed I agree with Andy, SS_FORCE looks simpler and better to me.
>
> But perhaps you missed the most important thing, that
> it is not possible to change the altstack in sighandler - you'll
> get EPERM, even with my patch.

See above.

> But with SS_FORCE this is
> exactly not the case.

Yes, and SS_FORCE means "I know what I do", looks very simple.

> Also it would be interesting to know what do you think about
> just removing the EPERM check instead of this all.

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.

Oleg.