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

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Date:	Mon, 1 Feb 2016 20:29:36 +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 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.

Unlikely. Suppose you do sigalstack() and then a non SA_ONSTACK signal handler
runs and calls sigaltstack() again. This won't fail, but restore_sigaltstack()
will restore the old alt stack after return.

I too do not know why uc_stack exists, in fact I do not know about it until
today when I read your patch ;) But it is here, and I do not think SS_FORCE
can add more confusion than we already have.

> >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.

Hmm. how this connects to SA_SIGINFO ?

> >>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.

Still can't understand. The 2nd sigaltstack() is no longer EPERMed because
application used SS_FORCED before that and disabled altstack.

And it is not ignored, it actually changes alt stack. Until we return from
handler.

Oleg.