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

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Date:	Mon, 1 Feb 2016 21:41:14 +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 22:29, Oleg Nesterov пишет:
> >>>
> >>>	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 ?
> AFAIK without SA_SIGINFO you get sigreturn instead of
> rt_sigreturn, which doesn't seem to do restore_altstack().
> Or am I wrong?
>
> Hmm:
>
>         /* Set up the stack frame */
>         if (is_ia32_frame()) {
>                 if (ksig->ka.sa.sa_flags & SA_SIGINFO)
>                         return ia32_setup_rt_frame(usig, ksig, cset, regs);
>                 else
>                         return ia32_setup_frame(usig, ksig, cset, regs);

Ah, ia32... So this is even more confusing.

> >>>>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.
> Before we return, the signals are usually blocked.
> So whatever is after return is most important.

Yes, but I still can't understand your "silently ignored". At least how does
this differ from the case when a non-SA_ONSTACK signal handler does
sigaltstack() and then rt_sigreturn() restores the old stack.

Oleg.