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

Andrey Ryabinin
2015-11-09T15:12:32Z

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