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Re: [PATCH][usercr]: Ghost tasks must be detached [message #41606 is a reply to message #41591]

Thu, 10 February 2011 02:44

Sukadev Bhattiprolu is currently offline

Sukadev Bhattiprolu
Messages: 413
Registered: August 2006

Senior Member

Oren Laadan [orenl@cs.columbia.edu] wrote:
|
|
| > But if that is true, I need to investigate why Louis Rilling was getting
| > the crash in Jun 2010 - which he tried to fix here:
| >
| > http://lkml.org/lkml/2010/6/16/295
|
| I see. So basically there is a kerenl bug that can be potentially
| exposed by the c/r code. Therefore, we need to fix the kernel bug...
| (and until such a fix makes it to mainline, we'll add it as part of
| the linux-cr patchset).

Yes, but there is more than one problem (at least in our C/R kernel).

There is the bug that Louis Rilling reported and Eric has a fix for.
Even if we apply Eric's fix to the C/R kernel, we still will have
another problem if do_ghost_task() sets ->exit_signal to -1.

Consider this in 2.6.37:

Container-init: Ghost child of container-init

do_ghost_task()
zap_pid_ns..()
Send SIGKILL

do_wait()
- adds self to ->wait_chldexit queue
- goes through do_wait_thread() - finds that
it has at least one child (on tsk->children),
but it has not yet exited
- so waits for the child to exit
wakes up for SIGKILL
->exit_signal = -1
do_exit()

Note that exit_notify() does not notify parent when the ghost process
exits, because ->exit_signal is -1.

So you may ask how did the container-init have a ghost child. That was
due to a bug in usercr :-).

But my point is such a userspace bug can leave the above container init
unkillable.

Note that this does not happen with normal threads which set ->exit_signal
to -1 . That is because of the following two pieces of code in copy_process():

/* ok, now we should be set up.. */
p->exit_signal = (clone_flags & CLONE_THREAD) ? -1 : (clone_flags & CSIGNAL);

and

/* CLONE_PARENT re-uses the old parent */
if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
p->real_parent = current->real_parent;
p->parent_exec_id = current->parent_exec_id;

With this our container-init above will not have any children to wait
for in do_wait_thread().

Sukadev
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[Message index]

		Re: [PATCH][usercr]: Ghost tasks must be detached By: Oren Laadan on Sat, 05 February 2011 18:55
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Sukadev Bhattiprolu on Sat, 05 February 2011 21:40
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Oren Laadan on Sat, 05 February 2011 22:02
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Oren Laadan on Sat, 05 February 2011 22:33
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Sukadev Bhattiprolu on Wed, 09 February 2011 02:09
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Oren Laadan on Wed, 09 February 2011 03:35
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Sukadev Bhattiprolu on Thu, 10 February 2011 02:44
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Oren Laadan on Thu, 10 February 2011 03:53
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Sukadev Bhattiprolu on Thu, 10 February 2011 06:17
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Oren Laadan on Thu, 10 February 2011 14:56
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Sukadev Bhattiprolu on Thu, 10 February 2011 17:42
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Sukadev Bhattiprolu on Wed, 16 February 2011 20:10
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Louis Rilling on Thu, 17 February 2011 15:21
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Sukadev Bhattiprolu on Mon, 21 February 2011 20:40
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Louis Rilling on Tue, 22 February 2011 10:28
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Louis Rilling on Wed, 09 February 2011 12:01
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Oren Laadan on Wed, 09 February 2011 12:18
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Louis Rilling on Wed, 09 February 2011 12:35
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Louis Rilling on Wed, 09 February 2011 12:37
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Sukadev Bhattiprolu on Wed, 09 February 2011 19:02
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Louis Rilling on Thu, 10 February 2011 10:23
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Sukadev Bhattiprolu on Thu, 10 February 2011 17:54
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Louis Rilling on Thu, 10 February 2011 18:04
		Re: [PATCH][usercr]: Ghost tasks must be detached By: Sukadev Bhattiprolu on Thu, 10 February 2011 22:31

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