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, 2 insertions, 0 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5a958b17f9f..5157bd9eee3 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1819,6 +1819,8 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
 
 #define JOBCTL_PENDING_MASK	JOBCTL_STOP_PENDING
 
+extern bool task_set_jobctl_pending(struct task_struct *task,
+				    unsigned int mask);
 extern void task_clear_jobctl_pending(struct task_struct *task,
 				      unsigned int mask);