linux-kernel - [patch v6.1 17/20] signal: Queue ignored posixtimers on ignore list

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Message-ID: <8734k9qrcr.ffs@tglx>
Date: Sat, 02 Nov 2024 22:05:08 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Anna-Maria Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker
 <frederic@...nel.org>, John Stultz <jstultz@...gle.com>, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Stephen Boyd
 <sboyd@...nel.org>, Eric Biederman <ebiederm@...ssion.com>, Oleg Nesterov
 <oleg@...hat.com>
Subject: [patch v6.1 17/20] signal: Queue ignored posixtimers on ignore list

Queue posixtimers which have their signal ignored on the ignored list:

   1) When the timer fires and the signal has SIG_IGN set

   2) When SIG_IGN is installed via sigaction() and a timer signal
      is already queued

This completes the SIG_IGN handling and such timers are not longer self
rearmed which avoids pointless wakeups.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
V6.1: Handle oneshot timer expiry or transitioning from periodic to
      oneshot after a rearming correctly. - Frederic
---
 kernel/signal.c |   70 ++++++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 65 insertions(+), 5 deletions(-)
---

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -731,6 +731,16 @@ void signal_wake_up_state(struct task_st
 		kick_process(t);
 }
 
+static inline void posixtimer_sig_ignore(struct task_struct *tsk, struct sigqueue *q);
+
+static void sigqueue_free_ignored(struct task_struct *tsk, struct sigqueue *q)
+{
+	if (likely(!(q->flags & SIGQUEUE_PREALLOC) || q->info.si_code != SI_TIMER))
+		__sigqueue_free(q);
+	else
+		posixtimer_sig_ignore(tsk, q);
+}
+
 /* Remove signals in mask from the pending set and queue. */
 static void flush_sigqueue_mask(struct task_struct *p, sigset_t *mask, struct sigpending *s)
 {
@@ -747,7 +757,7 @@ static void flush_sigqueue_mask(struct t
 	list_for_each_entry_safe(q, n, &s->list, list) {
 		if (sigismember(mask, q->info.si_signo)) {
 			list_del_init(&q->list);
-			__sigqueue_free(q);
+			sigqueue_free_ignored(p, q);
 		}
 	}
 }
@@ -1964,7 +1974,7 @@ int posixtimer_send_sigqueue(struct k_it
 	int sig = q->info.si_signo;
 	struct task_struct *t;
 	unsigned long flags;
-	int ret, result;
+	int result;
 
 	guard(rcu)();
 
@@ -1981,13 +1991,48 @@ int posixtimer_send_sigqueue(struct k_it
 	 */
 	tmr->it_sigqueue_seq = tmr->it_signal_seq;
 
-	ret = 1; /* the signal is ignored */
 	if (!prepare_signal(sig, t, false)) {
 		result = TRACE_SIGNAL_IGNORED;
+
+		/* Paranoia check. Try to survive. */
+		if (WARN_ON_ONCE(!list_empty(&q->list)))
+			goto out;
+
+		/* Periodic timers with SIG_IGN are queued on the ignored list */
+		if (tmr->it_status == POSIX_TIMER_REQUEUE_PENDING) {
+			/*
+			 * Already queued means the timer was rearmed after
+			 * the previous expiry got it on the ignore list.
+			 * Nothing to do for that case.
+			 */
+			if (hlist_unhashed(&tmr->ignored_list)) {
+				/*
+				 * Take a signal reference and queue it on
+				 * the ignored list.
+				 */
+				posixtimer_sigqueue_getref(q);
+				posixtimer_sig_ignore(t, tmr);
+			}
+		} else if (!hlist_unhashed(&tmr->ignored_list)) {
+			/*
+			 * Covers the case where a timer was periodic and
+			 * then signal was ignored. Then it was rearmed as
+			 * oneshot timer. The previous signal is invalid
+			 * now, and the oneshot signal has to be dropped.
+			 * Remove it from the ignored list and drop the
+			 * reference count as the signal is not longer
+			 * queued.
+			 */
+			hlist_del_init(&tmr->ignored_list);
+			posixtimer_putref(tmr);
+		}
 		goto out;
 	}
 
-	ret = 0;
+	/* This should never happen and leaks a reference count */
+	if (WARN_ON_ONCE(!hlist_unhashed(&tmr->ignored_list)))
+		hlist_del_init(&tmr->ignored_list);
+
 	if (unlikely(!list_empty(&q->list))) {
 		/* This holds a reference count already */
 		result = TRACE_SIGNAL_ALREADY_PENDING;
@@ -2000,7 +2045,21 @@ int posixtimer_send_sigqueue(struct k_it
 out:
 	trace_signal_generate(sig, &q->info, t, tmr->it_pid_type != PIDTYPE_PID, result);
 	unlock_task_sighand(t, &flags);
-	return ret;
+	return 0;
+}
+
+static inline void posixtimer_sig_ignore(struct task_struct *tsk, struct sigqueue *q)
+{
+	struct k_itimer *tmr = container_of(q, struct k_itimer, sigq);
+
+	/*
+	 * Only enqueue periodic timer signals to the ignored list. For
+	 * oneshot timers, drop the reference count.
+	 */
+	if (tmr->it_status == POSIX_TIMER_REQUEUE_PENDING)
+		hlist_add_head(&tmr->ignored_list, &tsk->signal->ignored_posix_timers);
+	else
+		posixtimer_putref(tmr);
 }
 
 static void posixtimer_sig_unignore(struct task_struct *tsk, int sig)
@@ -2048,6 +2107,7 @@ static void posixtimer_sig_unignore(stru
 	}
 }
 #else /* CONFIG_POSIX_TIMERS */
+static inline void posixtimer_sig_ignore(struct task_struct *tsk, struct sigqueue *q) { }
 static inline void posixtimer_sig_unignore(struct task_struct *tsk, int sig) { }
 #endif /* !CONFIG_POSIX_TIMERS */