job control: introduce task_set_jobctl_pending()

task->jobctl currently hosts JOBCTL_STOP_PENDING and will host TRAP
pending bits too.  Setting pending conditions on a dying task may make
the task unkillable.  Currently, each setting site is responsible for
checking for the condition but with to-be-added job control traps this
becomes too fragile.

This patch adds task_set_jobctl_pending() which should be used when
setting task->jobctl bits to schedule a stop or trap.  The function
performs the followings to ease setting pending bits.

* Sanity checks.

* If fatal signal is pending or PF_EXITING is set, no bit is set.

* STOP_SIGMASK is automatically cleared if new value is being set.

do_signal_stop() and ptrace_attach() are updated to use
task_set_jobctl_pending() instead of setting STOP_PENDING explicitly.
The surrounding structures around setting are changed to fit
task_set_jobctl_pending() better but there should be no userland
visible behavior difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index eb191116edf7..0c37d999c8b8 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -256,10 +256,10 @@ static int ptrace_attach(struct task_struct *task)
          * The following task_is_stopped() test is safe as both transitions
          * in and out of STOPPED are protected by siglock.
          */
-        if (task_is_stopped(task)) {
-                task->jobctl |= JOBCTL_STOP_PENDING | JOBCTL_TRAPPING;
+        if (task_is_stopped(task) &&
+            task_set_jobctl_pending(task,
+                                    JOBCTL_STOP_PENDING | JOBCTL_TRAPPING))
                 signal_wake_up(task, 1);
-        }
 
         spin_unlock(&task->sighand->siglock);