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>

3 files changed, 45 insertions, 9 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5a958b17f9fe..5157bd9eee37 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);
 
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);
 
diff --git a/kernel/signal.c b/kernel/signal.c
index 637a171b65b6..9ab91c516c3f 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -224,6 +224,39 @@ static inline void print_dropped_signal(int sig)
 }
 
 /**
+ * task_set_jobctl_pending - set jobctl pending bits
+ * @task: target task
+ * @mask: pending bits to set
+ *
+ * Clear @mask from @task->jobctl.  @mask must be subset of
+ * %JOBCTL_PENDING_MASK | %JOBCTL_STOP_CONSUME | %JOBCTL_STOP_SIGMASK |
+ * %JOBCTL_TRAPPING.  If stop signo is being set, the existing signo is
+ * cleared.  If @task is already being killed or exiting, this function
+ * becomes noop.
+ *
+ * CONTEXT:
+ * Must be called with @task->sighand->siglock held.
+ *
+ * RETURNS:
+ * %true if @mask is set, %false if made noop because @task was dying.
+ */
+bool task_set_jobctl_pending(struct task_struct *task, unsigned int mask)
+{
+        BUG_ON(mask & ~(JOBCTL_PENDING_MASK | JOBCTL_STOP_CONSUME |
+                        JOBCTL_STOP_SIGMASK | JOBCTL_TRAPPING));
+        BUG_ON((mask & JOBCTL_TRAPPING) && !(mask & JOBCTL_PENDING_MASK));
+
+        if (unlikely(fatal_signal_pending(task) || (task->flags & PF_EXITING)))
+                return false;
+
+        if (mask & JOBCTL_STOP_SIGMASK)
+                task->jobctl &= ~JOBCTL_STOP_SIGMASK;
+
+        task->jobctl |= mask;
+        return true;
+}
+
+/**
  * task_clear_jobctl_trapping - clear jobctl trapping bit
  * @task: target task
  *
@@ -1902,19 +1935,20 @@ static int do_signal_stop(int signr)
                 else
                         WARN_ON_ONCE(!task_ptrace(current));
 
-                current->jobctl &= ~JOBCTL_STOP_SIGMASK;
-                current->jobctl |= signr | gstop;
-                sig->group_stop_count = 1;
+                sig->group_stop_count = 0;
+
+                if (task_set_jobctl_pending(current, signr | gstop))
+                        sig->group_stop_count++;
+
                 for (t = next_thread(current); t != current;
                      t = next_thread(t)) {
-                        t->jobctl &= ~JOBCTL_STOP_SIGMASK;
                         /*
                          * Setting state to TASK_STOPPED for a group
                          * stop is always done with the siglock held,
                          * so this check has no races.
                          */
-                        if (!(t->flags & PF_EXITING) && !task_is_stopped(t)) {
-                                t->jobctl |= signr | gstop;
+                        if (!task_is_stopped(t) &&
+                            task_set_jobctl_pending(t, signr | gstop)) {
                                 sig->group_stop_count++;
                                 signal_wake_up(t, 0);
                         }