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

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Message-ID: <20090317092546.GA9356@redhat.com>
Date:	Tue, 17 Mar 2009 10:25:46 +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:
> >
> > 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.

Probably it is better to just change force_sig_info(), in this case
SIGFPE/etc will have the same behaviour.

> The only thing that
> worries me is this remark from Oleg
> (http://marc.info/?l=linux-kernel&m=123711058421913&w=2):
>
> "But please note that it is still possible to hit is_signal_blocked()
> even with test_with_kill(), but the probability is very low."

Sorry for confusion. Initially I misread test_with_kill() case, and then
forgot to remove this part. I think this is not possible.

Oleg.

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