Messages in this thread | | | From | Andrey Ryabinin <> | Subject | ptrace() hangs on attempt to seize/attach stopped & frozen task | Date | Mon, 9 Nov 2015 18:12:50 +0300 |
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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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