posix-timers: Handle ignored list on delete and exit

To handle posix timer signals on sigaction(SIG_IGN) properly, the timers
will be queued on a separate ignored list.

Add the necessary cleanup code for timer_delete() and exit_itimers().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/20241105064213.987530588@linutronix.de

Author: KAGA-KOKO

include/linux/posix-timers.h

@@ -152,7 +152,8 @@ static inline void posix_cputimers_init_work(void) { }
 
 /**
  * struct k_itimer - POSIX.1b interval timer structure.
- * @list:		List head for binding the timer to signals->posix_timers
+ * @list:		List node for binding the timer to tsk::signal::posix_timers
+ * @ignored_list:	List node for tracking ignored timers in tsk::signal::ignored_posix_timers
  * @t_hash:		Entry in the posix timer hash table
  * @it_lock:		Lock protecting the timer
  * @kclock:		Pointer to the k_clock struct handling this timer
@@ -176,6 +177,7 @@ static inline void posix_cputimers_init_work(void) { }
  */
 struct k_itimer {
 	struct hlist_node	list;
+	struct hlist_node	ignored_list;
 	struct hlist_node	t_hash;
 	spinlock_t		it_lock;
 	const struct k_clock	*kclock;

kernel/time/posix-timers.c

@@ -1027,6 +1027,18 @@ int common_timer_del(struct k_itimer *timer)
 	return 0;
 }
 
+/*
+ * If the deleted timer is on the ignored list, remove it and
+ * drop the associated reference.
+ */
+static inline void posix_timer_cleanup_ignored(struct k_itimer *tmr)
+{
+	if (!hlist_unhashed(&tmr->ignored_list)) {
+		hlist_del_init(&tmr->ignored_list);
+		posixtimer_putref(tmr);
+	}
+}
+
 static inline int timer_delete_hook(struct k_itimer *timer)
 {
 	const struct k_clock *kc = timer->kclock;
@@ -1059,6 +1071,7 @@ SYSCALL_DEFINE1(timer_delete, timer_t, timer_id)
 
 	spin_lock(&current->sighand->siglock);
 	hlist_del(&timer->list);
+	posix_timer_cleanup_ignored(timer);
 	spin_unlock(&current->sighand->siglock);
 	/*
 	 * A concurrent lookup could check timer::it_signal lockless. It
@@ -1110,6 +1123,8 @@ static void itimer_delete(struct k_itimer *timer)
 	}
 	hlist_del(&timer->list);
 
+	posix_timer_cleanup_ignored(timer);
+
 	/*
 	 * Setting timer::it_signal to NULL is technically not required
 	 * here as nothing can access the timer anymore legitimately via
@@ -1142,6 +1157,19 @@ void exit_itimers(struct task_struct *tsk)
 	/* The timers are not longer accessible via tsk::signal */
 	while (!hlist_empty(&timers))
 		itimer_delete(hlist_entry(timers.first, struct k_itimer, list));
+
+	/*
+	 * There should be no timers on the ignored list. itimer_delete() has
+	 * mopped them up.
+	 */
+	if (!WARN_ON_ONCE(!hlist_empty(&tsk->signal->ignored_posix_timers)))
+		return;
+
+	hlist_move_list(&tsk->signal->ignored_posix_timers, &timers);
+	while (!hlist_empty(&timers)) {
+		posix_timer_cleanup_ignored(hlist_entry(timers.first, struct k_itimer,
+							ignored_list));
+	}
 }
 
 SYSCALL_DEFINE2(clock_settime, const clockid_t, which_clock,