[PATCH 1/2] ptrace: make wait_on_bit(JOBCTL_TRAPPING_BIT) in ptrace_attach() killable

[Oleg Nesterov <oleg@redhat.com>]

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.

Reported-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/ptrace.c |   10 ++++++++--
 1 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index c8e0e05..80b3604 100644
--- a/kernel/ptrace.c
+++ b/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);
 	}
 
-- 
1.5.5.1