linux-kernel - Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE

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Date:	Sun, 20 Feb 2011 18:48:27 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Jan Kratochvil <jan.kratochvil@...hat.com>,
	torvalds@...ux-foundation.org, Tejun Heo <tj@...nel.org>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after
	PTRACE_ATTACH

On 02/20, Denys Vlasenko wrote:
>
> On Sunday 20 February 2011 10:40, Jan Kratochvil wrote:
> > Sure by default GDB does not do anything special, it will respawn (using
> > PTRACE_CONT(SIGSTOP)) any SIGSTOP it sees due to the default setting of:
> > 	(gdb) handle SIGSTOP
> > 	Signal        Stop  Print Pass to program Description
> > 	SIGSTOP       Yes Yes Yes   Stopped (signal)
> >
> > Therefore there happens the double SIGSTOP reporting as discussed before:
> > 	(gdb) run
> > 	Starting program: /bin/sleep 1h
> > 	# external kill -STOP <inferior pid>
> > 	Program received signal SIGSTOP, Stopped (signal).
> > 	# State:	t (tracing stop)
> > 	(gdb) continue
> > 	Continuing.
> > 	Program received signal SIGSTOP, Stopped (signal).
> > 	# State:	t (tracing stop)
> > 	(gdb) continue
> > 	Continuing.
> > 	# State:	S (sleeping)
> >
> > Your proposal is I expect:
> > 	(gdb) run
> > 	Starting program: /bin/sleep 1h
> > 	# external kill -STOP <inferior pid>
> > 	Program received signal SIGSTOP, Stopped (signal).
> > 	# State:	t (tracing stop)
> > 	(gdb) continue
> > 	Continuing.
> > 	# State:	T (stopped)
>
> Not exactly. Even after we fix kernel so that it properly preserves
> group-stop across ptrace-stops, gdb will still see TWO
> waitpid:SIGSTOP events, not one.

Yes, I didn't notice the second report doesn't show SIGSTOP twice.
The only important change is

	(gdb) continue
	Continuing.
-	# State:        S (sleeping)
+	# State:        T (stopped)


> I think you can use similar trick in gdb, so that second message says
> "Program stopped due to signal SIGSTOP, Stopped (signal)",
> not "Program received signal SIGSTOP, Stopped (signal)".

Agreed.

Oleg.

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