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

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Message-ID: <20110214174503.GB15847@redhat.com>
Date:	Mon, 14 Feb 2011 18:45:03 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Roland McGrath <roland@...hat.com>, jan.kratochvil@...hat.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after
	PTRACE_ATTACH

On 02/14, Tejun Heo wrote:
>
> On Mon, Feb 14, 2011 at 06:20:52PM +0100, Denys Vlasenko wrote:
> > >> 23:02:15.622112 nanosleep({30, 0}, NULL) = ? ERESTART_RESTARTBLOCK (To be restarted)
> > >> 23:02:23.781165 --- SIGSTOP (Stopped (signal)) @ 0 (0) ---
> > >> 23:02:23.781251 --- SIGSTOP (Stopped (signal)) @ 0 (0) ---
> > >>     (I forgot again why we see it twice. Another quirk I guess...)
> > >> 23:02:23.781310 restart_syscall(<... resuming interrupted call ...>) = 0
> > >> 23:02:45.622433 close(1)                = 0
> > >> 23:02:45.622743 close(2)                = 0
> > >> 23:02:45.622885 exit_group(0)           = ?
> ...
> > > This can be fixed by updating strace, right?  strace can look at the
> > > wait(2) exit code and if the tracee stopped for group stop, wait for
> > > the tracee to be continued instead of issuing PTRACE_SYSCALL.
> >
> > But tracee didn't stop _yet_. Signal is not delivered _yet_, debugger
> > can decide at this point whether to deliver it:
> > ptrace(PTRACE_SYSCALL, $PID, 0x1, SIGSTOP)
> > or ignore:
> > ptrace(PTRACE_SYSCALL, $PID, 0x1, 0)
> >
> > strace has to deliver SIGSTOP if it wants to make program run exactly
> > as it would run without strace. So it tries to do so.
> > Currently, ptrace machinery doesn't react as strace, its user, expects it to.
>
> Okay, maybe I'm missing something but so once SIGSTOP is determined to
> be delivered, then the tracee enters group stop and that's the second
> SIGSTOP notification you get.

Yes, this is correct.

But my head spins ;) I have already lost the picture.

> At that point, strace should wait for
> the tracee to be continued by SIGCONT.  That should work, right?

Again, given that strace is the real parent, in this particular
case I think strace can work as you suggest.

Oleg.

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