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

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Message-ID: <20110220212053.GA23412@host1.dyn.jankratochvil.net>
Date:	Sun, 20 Feb 2011 22:20:53 +0100
From:	Jan Kratochvil <jan.kratochvil@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Tejun Heo <tj@...nel.org>, Roland McGrath <roland@...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 Sun, 20 Feb 2011 21:38:19 +0100, Oleg Nesterov wrote:
> On 02/20, Jan Kratochvil wrote:
> > If I do (on kernel-debug-2.6.35.11-83.fc14.x86_64)
> > 	ptrace (PTRACE_ATTACH);
> > 	sleep (1);
> > 	ptrace (PTRACE_DETACH, 0);
> >
> > even without the wait() it really has no effect.
> 
> Well. what does this "has no effect" mean? ;) I am totally confused.
> We were talking about the case when the tracee was stopped before
> attach, right?

No, the case it is not `(T) stopped'.  I was surprised by this ptrace behavior
but it is offtopic and not useful so let's drop it.

> So. So far I assume you are not against this change ;)

No, although you should provide the patch in advance, it would be nice to also
post it first to <gdb@...rceware.org> for comments.

Now if new GDB should allow inferior functions calls on previously
`(T) stopped' process doing PTRACE_CONT(SIGCONT) for executing the call should
be harmless but how to make it `(T) stopped' afterwards?  PTRACE_CONT(SIGSTOP)
right after the inferior call will make the old kernels run the inferior - we
do not want that.  GDB can only wait till the end of debugging session and do
PTRACE_DETACH(SIGSTOP).  But we are back at the point if GDB crashes in
between the inferior will accidentally resume.

(This is the ``(T) stopped' preservation after _exit()' thread along claimed
to be unrelated.)

Thanks,
Jan
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