signal: Handle ignored signals in do_sigaction(action != SIG_IGN)

When a real handler (including SIG_DFL) is installed for a signal, which
had previously SIG_IGN set, then the list of ignored posix timers has to be
checked for timers which are affected by this change.

Add a list walk function which checks for the matching signal number and if
found requeues the timers signal, so the timer is rearmed on signal
delivery.

Rearming the timer right away is not possible because that requires to drop
sighand lock.

No functional change as the counter part which queues the timers on the
ignored list is still missing.

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/20241105064214.054091076@linutronix.de

Author: KAGA-KOKO

kernel/signal.c

@@ -2002,7 +2002,54 @@ int posixtimer_send_sigqueue(struct k_itimer *tmr)
 	unlock_task_sighand(t, &flags);
 	return ret;
 }
-#endif /* CONFIG_POSIX_TIMERS */
+
+static void posixtimer_sig_unignore(struct task_struct *tsk, int sig)
+{
+	struct hlist_head *head = &tsk->signal->ignored_posix_timers;
+	struct hlist_node *tmp;
+	struct k_itimer *tmr;
+
+	if (likely(hlist_empty(head)))
+		return;
+
+	/*
+	 * Rearming a timer with sighand lock held is not possible due to
+	 * lock ordering vs. tmr::it_lock. Just stick the sigqueue back and
+	 * let the signal delivery path deal with it whether it needs to be
+	 * rearmed or not. This cannot be decided here w/o dropping sighand
+	 * lock and creating a loop retry horror show.
+	 */
+	hlist_for_each_entry_safe(tmr, tmp , head, ignored_list) {
+		struct task_struct *target;
+
+		/*
+		 * tmr::sigq.info.si_signo is immutable, so accessing it
+		 * without holding tmr::it_lock is safe.
+		 */
+		if (tmr->sigq.info.si_signo != sig)
+			continue;
+
+		hlist_del_init(&tmr->ignored_list);
+
+		/* This should never happen and leaks a reference count */
+		if (WARN_ON_ONCE(!list_empty(&tmr->sigq.list)))
+			continue;
+
+		/*
+		 * Get the target for the signal. If target is a thread and
+		 * has exited by now, drop the reference count.
+		 */
+		guard(rcu)();
+		target = posixtimer_get_target(tmr);
+		if (target)
+			posixtimer_queue_sigqueue(&tmr->sigq, target, tmr->it_pid_type);
+		else
+			posixtimer_putref(tmr);
+	}
+}
+#else /* CONFIG_POSIX_TIMERS */
+static inline void posixtimer_sig_unignore(struct task_struct *tsk, int sig) { }
+#endif /* !CONFIG_POSIX_TIMERS */
 
 void do_notify_pidfd(struct task_struct *task)
 {
@@ -4180,6 +4227,8 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
 	sigaction_compat_abi(act, oact);
 
 	if (act) {
+		bool was_ignored = k->sa.sa_handler == SIG_IGN;
+
 		sigdelsetmask(&act->sa.sa_mask,
 			      sigmask(SIGKILL) | sigmask(SIGSTOP));
 		*k = *act;
@@ -4200,6 +4249,8 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
 			flush_sigqueue_mask(p, &mask, &p->signal->shared_pending);
 			for_each_thread(p, t)
 				flush_sigqueue_mask(p, &mask, &t->pending);
+		} else if (was_ignored) {
+			posixtimer_sig_unignore(p, sig);
 		}
 	}