posix-timers: Move sequence logic into struct k_itimer

The posix timer signal handling uses siginfo::si_sys_private for handling
the sequence counter check. That indirection is not longer required and the
sequence count value at signal queueing time can be stored in struct
k_itimer itself.

This removes the requirement of treating siginfo::si_sys_private special as
it's now always zero as the kernel does not touch it anymore.

Suggested-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://lore.kernel.org/all/20241105064213.852619866@linutronix.de

Author: KAGA-KOKO

include/linux/posix-timers.h

@@ -162,6 +162,7 @@ static inline void posix_cputimers_init_work(void) { }
  * @it_overrun:		The overrun counter for pending signals
  * @it_overrun_last:	The overrun at the time of the last delivered signal
  * @it_signal_seq:	Sequence count to control signal delivery
+ * @it_sigqueue_seq:	The sequence count at the point where the signal was queued
  * @it_sigev_notify:	The notify word of sigevent struct for signal delivery
  * @it_interval:	The interval for periodic timers
  * @it_signal:		Pointer to the creators signal struct
@@ -184,6 +185,7 @@ struct k_itimer {
 	s64			it_overrun;
 	s64			it_overrun_last;
 	unsigned int		it_signal_seq;
+	unsigned int		it_sigqueue_seq;
 	int			it_sigev_notify;
 	enum pid_type		it_pid_type;
 	ktime_t			it_interval;

include/uapi/asm-generic/siginfo.h

@@ -46,7 +46,7 @@ union __sifields {
 		__kernel_timer_t _tid;	/* timer id */
 		int _overrun;		/* overrun count */
 		sigval_t _sigval;	/* same as below */
-		int _sys_private;       /* not to be passed to user */
+		int _sys_private;       /* Not used by the kernel. Historic leftover. Always 0. */
 	} _timer;
 
 	/* POSIX.1b signals */

kernel/signal.c

@@ -1976,12 +1976,10 @@ int posixtimer_send_sigqueue(struct k_itimer *tmr)
 		return -1;
 
 	/*
-	 * Update @q::info::si_sys_private for posix timer signals with
-	 * sighand locked to prevent a race against dequeue_signal() which
-	 * decides based on si_sys_private whether to invoke
-	 * posixtimer_rearm() or not.
+	 * Update @tmr::sigqueue_seq for posix timer signals with sighand
+	 * locked to prevent a race against dequeue_signal().
 	 */
-	q->info.si_sys_private = tmr->it_signal_seq;
+	tmr->it_sigqueue_seq = tmr->it_signal_seq;
 
 	ret = 1; /* the signal is ignored */
 	if (!prepare_signal(sig, t, false)) {

kernel/time/posix-timers.c

@@ -259,7 +259,7 @@ static bool __posixtimer_deliver_signal(struct kernel_siginfo *info, struct k_it
 	 * since the signal was queued. In either case, don't rearm and
 	 * drop the signal.
 	 */
-	if (timr->it_signal_seq != info->si_sys_private || WARN_ON_ONCE(!timr->it_signal))
+	if (timr->it_signal_seq != timr->it_sigqueue_seq || WARN_ON_ONCE(!timr->it_signal))
 		return false;
 
 	if (!timr->it_interval || WARN_ON_ONCE(timr->it_status != POSIX_TIMER_REQUEUE_PENDING))
@@ -297,9 +297,6 @@ bool posixtimer_deliver_signal(struct kernel_siginfo *info, struct sigqueue *tim
 	posixtimer_putref(timr);
 
 	spin_lock(&current->sighand->siglock);
-
-	/* Don't expose the si_sys_private value to userspace */
-	info->si_sys_private = 0;
 	return ret;
 }