linux-kernel - Re: [PATCHSET] ptrace,signal: Improve ptrace and job control interaction

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Date:	Fri, 25 Mar 2011 15:26:30 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	jan.kratochvil@...hat.com, vda.linux@...glemail.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, indan@....nu, roland@...k.frob.com
Subject: Re: [PATCHSET] ptrace,signal: Improve ptrace and job control
 interaction

Hello, Oleg.

On Wed, Mar 23, 2011 at 07:38:37PM +0100, Oleg Nesterov wrote:
> But of course we need more changes. In particular, there is still the
> small problem with the CLD_CONTINUED notification.
> 
> __ptrace_unlink() does signal_wake_up() if it adds SIGNAL_STOP_STOPPED.
> This is correct, but it should also add TIF_SIGPENDING if
> (signal->flags & SIGNAL_CLD_MASK) != 0.
> 
> Otherwise, if the stopped tracee was PTRACE_CONT'ed and then SIGCONT
> ends the group-stop, the real_parent won't be notified after detach.

Heh, that's an interesting one.  I don't think it has much to do with
__ptrace_unlink() tho.  Isn't the proper solution using something akin
to signal_wake_up() in SIGCONT generation path in prepare_signal()?

Explicit wake_up_state() without kick_process() is okay there because
if the code assumes that the tasks are guaranteed to pass through
signal delivery path whenever event worthy of notification happens
(either SIGNAL_STOP_STOPPED or group_stop_count is set).  PTRACE_CONT
breaks that as the tracee could be running in userland and thus the
solution is to add kick_process() as in signal_wake_up().

Am I making any sense?

> Unfortunately, this means that recalc_sigpending_tsk() has to check
> SIGNAL_CLD_MASK as well. Do you see another solution?

Hmmm... I think the above subtle breakage exists for !ptrace case too.
Please consider the following scenario.

* SIGSTOP is sent to a task and group stop is initiated.

* Before the task participates in group stop, SIGCONT is sent.

* Before CLD_STOPPED notification for the incomplete-stop/cont
  sequence can be made, recalc_sigpending() happens.

* CLD_STOPPED notification is pending but TIF_SIGPENDING isn't set and
  the task isn't in signal delivery path and can continue execution.

It's a pretty convoluted extremely unlikely corner case tho.  Anyways,
adding SIGNAL_CLD_MASK test to recalc_sigpending() should solve it.

> There is another case. SIGCONT can hit the stopped-but-running-task,
> but I don't think we should try to set TIF_SIGPENDING in this case,
> you are going to add the trap later.

Hmmm... As I wrote above, I think we should do it regardless of the
new trap.

Thanks.

-- 
tejun
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