hrtimers: Make callback function pointer private

Make the struct hrtimer::function field private, to prevent users from
changing this field in an unsafe way. hrtimer_update_function() should be
used if the callback function needs to be changed.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/all/7d0e6e0c5c59a64a9bea940051aac05d750bc0c2.1738746927.git.namcao@linutronix.de

Author: covanam

include/linux/hrtimer_types.h

@@ -39,7 +39,7 @@ enum hrtimer_restart {
 struct hrtimer {
 	struct timerqueue_node		node;
 	ktime_t				_softexpires;
-	enum hrtimer_restart		(*function)(struct hrtimer *);
+	enum hrtimer_restart		(*__private function)(struct hrtimer *);
 	struct hrtimer_clock_base	*base;
 	u8				state;
 	u8				is_rel;

include/trace/events/timer.h

@@ -235,7 +235,7 @@ TRACE_EVENT(hrtimer_start,
 
 	TP_fast_assign(
 		__entry->hrtimer	= hrtimer;
-		__entry->function	= hrtimer->function;
+		__entry->function	= ACCESS_PRIVATE(hrtimer, function);
 		__entry->expires	= hrtimer_get_expires(hrtimer);
 		__entry->softexpires	= hrtimer_get_softexpires(hrtimer);
 		__entry->mode		= mode;
@@ -271,7 +271,7 @@ TRACE_EVENT(hrtimer_expire_entry,
 	TP_fast_assign(
 		__entry->hrtimer	= hrtimer;
 		__entry->now		= *now;
-		__entry->function	= hrtimer->function;
+		__entry->function	= ACCESS_PRIVATE(hrtimer, function);
 	),
 
 	TP_printk("hrtimer=%p function=%ps now=%llu",

kernel/time/hrtimer.c

@@ -1316,7 +1316,7 @@ void hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim,
 	struct hrtimer_clock_base *base;
 	unsigned long flags;
 
-	if (WARN_ON_ONCE(!timer->function))
+	if (WARN_ON_ONCE(!ACCESS_PRIVATE(timer, function)))
 		return;
 	/*
 	 * Check whether the HRTIMER_MODE_SOFT bit and hrtimer.is_soft
@@ -1629,9 +1629,9 @@ static void __hrtimer_setup(struct hrtimer *timer,
 	timerqueue_init(&timer->node);
 
 	if (WARN_ON_ONCE(!function))
-		timer->function = hrtimer_dummy_timeout;
+		ACCESS_PRIVATE(timer, function) = hrtimer_dummy_timeout;
 	else
-		timer->function = function;
+		ACCESS_PRIVATE(timer, function) = function;
 }
 
 /**
@@ -1743,7 +1743,7 @@ static void __run_hrtimer(struct hrtimer_cpu_base *cpu_base,
 	raw_write_seqcount_barrier(&base->seq);
 
 	__remove_hrtimer(timer, base, HRTIMER_STATE_INACTIVE, 0);
-	fn = timer->function;
+	fn = ACCESS_PRIVATE(timer, function);
 
 	/*
 	 * Clear the 'is relative' flag for the TIME_LOW_RES case. If the

kernel/time/timer_list.c

@@ -46,7 +46,7 @@ static void
 print_timer(struct seq_file *m, struct hrtimer *taddr, struct hrtimer *timer,
 	    int idx, u64 now)
 {
-	SEQ_printf(m, " #%d: <%p>, %ps", idx, taddr, timer->function);
+	SEQ_printf(m, " #%d: <%p>, %ps", idx, taddr, ACCESS_PRIVATE(timer, function));
 	SEQ_printf(m, ", S:%02x", timer->state);
 	SEQ_printf(m, "\n");
 	SEQ_printf(m, " # expires at %Lu-%Lu nsecs [in %Ld to %Ld nsecs]\n",