linux-kernel - ptrace() hangs on attempt to seize/attach stopped & frozen task

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Date:	Mon, 9 Nov 2015 18:12:50 +0300
From:	Andrey Ryabinin <aryabinin@...tuozzo.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Roland McGrath <roland@...k.frob.com>, Tejun Heo <tj@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: ptrace() hangs on attempt to seize/attach stopped & frozen task

Hi,

So, the ptrace() hangs if we try to attach to stopped task from freezing cgroup.
It seems this was introduced by 5d8f72b55c2756("freezer: change ptrace_stop/do_signal_stop to use freezable_schedule()").
See below for the exact scenario and small script to reproduce this.


Tracee:                                                                 Tracer:
static bool do_signal_stop(int signr)
	__set_current_state(TASK_STOPPED);
	freezable_schedule();
		freezer_do_not_count();
		schedule(); /* waiting for wake up */

									ptrace_attach()
										if (task_is_stopped(task) &&
										    task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING))
											signal_wake_up_state(task, __TASK_STOPPED);

		/* woken up by ptrace_attach() */
		freezer_count();
			__refrigerator()
										/* And here we will hang, because tracee is now frozen in __refrigerator() */
										wait_on_bit(&task->jobctl, JOBCTL_TRAPPING_BIT,
												TASK_UNINTERRUPTIBLE);



Reproduecer:
-----------
#!/bin/bash                                                                                                                                                                                                          

sleep 100 &
pid=$!
kill -SIGSTOP $pid
mkdir -p /sys/fs/cgroup/freezer/test
echo $pid > /sys/fs/cgroup/freezer/test/tasks
echo FROZEN > /sys/fs/cgroup/freezer/test/freezer.state

echo '#include <stdlib.h>                                                                                                                                                                                            
#include <sys/ptrace.h>                                                                                                                                                                                              
                                                                                                                                                                                                                     
int main(int argc, char  **argv)                                                                                                                                                                                     
{                                                                                                                                                                                                                    
  int pid = atoi(argv[1]);                                                                                                                                                                                           
  return ptrace(PTRACE_SEIZE, pid, NULL, 0);                                                                                                                                                                         
}                                                                                                                                                                                                                    
' | gcc -x c -
./a.out $pid


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