linux-kernel - Re: Q: SEGSEGV && uc_mcontext->ip (Was: Signal delivery order)

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Message-ID: <20090317104330.GA13912@redhat.com>
Date:	Tue, 17 Mar 2009 11:43:31 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Gábor Melis <mega@...es.hu>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Friesen <cfriesen@...tel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: SEGSEGV && uc_mcontext->ip (Was: Signal delivery order)

On 03/17, Gábor Melis wrote:
>
> On Martes 17 Marzo 2009, Oleg Nesterov wrote:
> > On 03/17, Gábor Melis wrote:
> > > On Martes 17 Marzo 2009, Oleg Nesterov wrote:
> > > > But this doesn't look very nice. So, perhaps we can do another
> > > > change?
> > > >
> > > > 	--- arch/x86/mm/fault.c
> > > > 	+++ arch/x86/mm/fault.c
> > > > 	@@ -177,6 +177,13 @@ static void force_sig_info_fault(int si_
> > > > 	 {
> > > > 		siginfo_t info;
> > > >
> > > > 	+	current->saved_sigmask = current->blocked;
> > > > 	+	spin_lock_irq(&current->sighand->siglock);
> > > > 	+	siginitsetinv(&current->blocked, sigmask(si_signo) |
> > > > 	+			sigmask(SIGKILL) | sigmask(SIGSTOP));
> > > > 	+	spin_unlock_irq(&current->sighand->siglock);
> > > > 	+	set_restore_sigmask();
> > > > 	+
> > > > 		info.si_signo = si_signo;
> > > > 		info.si_errno = 0;
> > > > 		info.si_code = si_code;
> > > >
> > > > But this is a user-visible change, all signals will be blocked
> > > > until sigsegv_handler() returns. But with this change
> > > > sigsegv_handler() always has the "correct" rt_sigframe.
> > >
> > > As an application developer what I'd like to have is this:
> > > synchronously generated signals are delivered before asynchronously
> > > generated ones. That is, if a number of signals are generated but
> > > not yet delivered then the synchronously generated ones are
> > > delivered first. I guess, in the kernel this would mean that the
> > > private/non-private distinction is not enough.
> >
> > With the change like above, no other signal (except SIGKILL) can be
> > delivered until the signal handler returns.
>
> Surely, you don't mean the above literally:

I literally meant the above ;)

> it would violate the
> standard to prevent all other signals from being delivered until the
> sigsegv handler returns.

Yes. That is why I didn't send the patch but asked the question.

But, just in case, this will not happen if the signal was sent from
user-space via kill/tkill.

> While we are at it, an interesting case is when a synchronously
> generated signal and an asynchronously generated signal - that is also
> of the type that can be synchronously generated - are to be delivered.
> Say we have a fault and a sigsegv generated but some misguided soul
> pthread_kill()s with sigtrap. In this case the sigsegv shall be
> delivered first, and the async sigtrap later.

In this case we can do nothing. The second signal will be lost.
But this is not the problem. If sigsegv_handler() wants to play
with context, it should check SI_FROMKERNEL() first. If we lose
the SIGSEGV from the fault, it will be re-generated.

Oleg.

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