ptrace: make wait_on_bit(JOBCTL_TRAPPING_BIT) in ptrace_attach() killable

ptrace_attach() can hang waiting for STOPPED -> TRACED transition if the
tracee gets frozen in between, change wait_on_bit() to use TASK_KILLABLE.

This doesn't really solve the problem(s) and we probably need to fix the
freezer.  In particular, note that this means that pm freezer will fail if
it races attach-to-stopped-task.

And otoh perhaps we can just remove JOBCTL_TRAPPING_BIT altogether, it is
not clear if we really need to hide this transition from debugger, WNOHANG
after PTRACE_ATTACH can fail anyway if it races with SIGCONT.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Roland McGrath <roland@hack.frob.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Pedro Alves <palves@redhat.com>
Cc: Jan Kratochvil <jan.kratochvil@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/ptrace.c

@@ -364,8 +364,14 @@ unlock_creds:
 	mutex_unlock(&task->signal->cred_guard_mutex);
 out:
 	if (!retval) {
-		wait_on_bit(&task->jobctl, JOBCTL_TRAPPING_BIT,
-			    TASK_UNINTERRUPTIBLE);
+		/*
+		 * We do not bother to change retval or clear JOBCTL_TRAPPING
+		 * if wait_on_bit() was interrupted by SIGKILL. The tracer will
+		 * not return to user-mode, it will exit and clear this bit in
+		 * __ptrace_unlink() if it wasn't already cleared by the tracee;
+		 * and until then nobody can ptrace this task.
+		 */
+		wait_on_bit(&task->jobctl, JOBCTL_TRAPPING_BIT, TASK_KILLABLE);
 		proc_ptrace_connector(task, PTRACE_ATTACH);
 	}