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

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Date:	Mon, 1 Feb 2016 22:01:49 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Oleg Nesterov <oleg@...hat.com>
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

01.02.2016 21:52, Oleg Nesterov пишет:
> Stas, I probably missed something, but I don't understand your concerns,
>
> On 02/01, Stas Sergeev 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.
> Nothing? restore_sigaltstack() should work as expected?
That's likely the reason for EPERM: restore_sigaltstack()
does the job, so manual modifications are disallowed.
Allowing them will bring in the surprises where the changes
done by the user are ignored.

>>> 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();
> Yes, or
>
> 	sigaltstack({ DISABLE | FORCE}, &old_ss);
> 	swapcontext();
> 	sigaltstack(&old_ss, NULL);
> 	rt_sigreturn();
>
> and if you are going to return from sighandler you do not even need the 2nd
> sigaltstack(), you can rely on sigreturn.
Yes, that's what I do in my app already.
But its only there when SA_SIGINFO is used.

>> What's at the end? Do we want a surprise for the user
>> that he's new_sas got ignored?
> Can't understand.... do you mean "set up new_sas" will be ignored because
> rt_sigreturn() does restore_sigaltstack() ? I see no problem here...
Allowing the modifications that were previously EPERMed
but will now be silently ignored, may be seen as a problem.
But if it isn't - fine, lets code that.