Re: [PATCH 03/10] ptrace: implement PTRACE_SEIZE

[Oleg Nesterov <oleg@redhat.com>]

On 05/24, Tejun Heo wrote:
>
> Hello, Oleg.
>
> On Mon, May 23, 2011 at 02:43:14PM +0200, Oleg Nesterov wrote:
> > On 05/18, Tejun Heo wrote:
> > > I've been thinking about Jan's suggestion to make ATTACH and DETACH
> > > not require tracee to trap. We already have this for DETACH for cases
> > > where the tracer is killed
> >
> > Yes, I still think that the new DETACH_XXX request which doesn't need
> > the stopped tracee makes sense. Yes, we have PTRACE_INTERRUPT. But please
> > recall the previous discussion, it is possible that the tracee can't
> > react to PTRACE_INTERRUPT and trap because it waits for other threads
> > we are tracing.
>
> Yeah, untrapped DETACH sounds nice but as you've already acknowledged
> in another reply, we have those nasty disable traps.

Yes. Which I never thought about, I alway assume PTRACE_DETACH_ASYNC
is trivial, but it is not. Lets forget it for now.

> > 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.

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.

> 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.

Oleg.