linux-kernel - Re: [PATCH 03/10] ptrace: implement PTRACE

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Message-ID: <20110526091401.GD9715@htj.dyndns.org>
Date:	Thu, 26 May 2011 11:14:01 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	jan.kratochvil@...hat.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	indan@....nu, bdonlan@...il.com
Subject: Re: [PATCH 03/10] ptrace: implement PTRACE_SEIZE

Hello,

On Wed, May 25, 2011 at 08:29:19PM +0200, Oleg Nesterov wrote:
> > > And. Currently there is no way to detach a zombie leader. Perhaps we
> > > should change do_wait(), but it is not clear what should we do if the
> > > tracer is the real parent (we already discussed this a bit).
> >
> > Hmmm... maybe just allow detaching zombie leader?
> 
> Yes, I think we should do this.
> 
> If we change PTRACE_DETACH (or add the new request) to allow this, then
> I think it it should detach any zombie, leader or not.

I think we can just make PTRACE_DETACH to succeed for zombies.  No
reason to add a new request for this.

> Or we can change do_wait() to detach a zombie leader. In this case it
> is not clear what should we do if the debugger is the real parent.
> Perhaps do_wait() should do the same: detach a leader (but not reap).
> When the last thread does, the real parent will be notified again.
> IOW, wait(tgid) can succeed twice.

Just letting PTRACE_DETACH work for zombies sounds much simpler to me.

> > As it's guaranteed to be not running, we don't have problem with
> > ptrace_disable.
> 
> Agreed. In fact it can be running, but it can't return to the user-space,
> and I think this is enough.
> 
> ptrace_detach()->ptrace_disable() can race with SIGKILL anyway, this means
> it should safe to call it if the tracee is exiting/exited.

Yeap, unless userland gets to run again, I don't think there's any
problem.

Thanks.

-- 
tejun
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