The EVFILT_USER implementation does not quite align with FreeBSD.
One difference is that FreeBSD updates the `data' and `udata' fields
when a user event is triggered. FreeBSD also preserves `fflags' when
the event is "untriggered". The diff below fixes these.

In addition, use kn_sfflags and kn_sdata instead of kn_fflags and
kn_data to make the implementation a bit more similar to FreeBSD.

OK?

Index: sys/kern/kern_event.c
===================================================================
RCS file: src/sys/kern/kern_event.c,v
retrieving revision 1.204
diff -u -p -r1.204 kern_event.c
--- sys/kern/kern_event.c	15 May 2025 19:56:08 -0000	1.204
+++ sys/kern/kern_event.c	18 May 2025 17:17:04 -0000
@@ -809,8 +809,6 @@ filt_userattach(struct knote *kn)
 	}
 
 	kn->kn_ptr.p_useract = ((kn->kn_sfflags & NOTE_TRIGGER) != 0);
-	kn->kn_fflags = kn->kn_sfflags & NOTE_FFLAGSMASK;
-	kn->kn_data = kn->kn_sdata;
 
 	return (0);
 }
@@ -837,29 +835,25 @@ filt_usermodify(struct kevent *kev, stru
 	case NOTE_FFNOP:
 		break;
 	case NOTE_FFAND:
-		kn->kn_fflags &= fflags;
+		kn->kn_sfflags &= fflags;
 		break;
 	case NOTE_FFOR:
-		kn->kn_fflags |= fflags;
+		kn->kn_sfflags |= fflags;
 		break;
 	case NOTE_FFCOPY:
-		kn->kn_fflags = fflags;
+		kn->kn_sfflags = fflags;
 		break;
 	default:
 		/* ignored, should not happen */
 		break;
 	}
 
-	if (kev->flags & EV_ADD) {
-		kn->kn_data = kev->data;
-		kn->kn_udata = kev->udata;
-	}
+	kn->kn_sdata = kev->data;
+	kn->kn_udata = kev->udata;
 
 	/* Allow clearing of an activated event. */
-	if (kev->flags & EV_CLEAR) {
+	if (kev->flags & EV_CLEAR)
 		kn->kn_ptr.p_useract = 0;
-		kn->kn_data = 0;
-	}
 
 	return (kn->kn_ptr.p_useract);
 }
@@ -872,11 +866,10 @@ filt_userprocess(struct knote *kn, struc
 	active = kn->kn_ptr.p_useract;
 	if (active && kev != NULL) {
 		*kev = kn->kn_kevent;
-		if (kn->kn_flags & EV_CLEAR) {
+		kev->fflags = kn->kn_sfflags & NOTE_FFLAGSMASK;
+		kev->data = kn->kn_sdata;
+		if (kn->kn_flags & EV_CLEAR)
 			kn->kn_ptr.p_useract = 0;
-			kn->kn_fflags = 0;
-			kn->kn_data = 0;
-		}
 	}
 
 	return (active);
Index: regress/sys/kern/kqueue/kqueue-user.c
===================================================================
RCS file: src/regress/sys/kern/kqueue/kqueue-user.c,v
retrieving revision 1.1
diff -u -p -r1.1 kqueue-user.c
--- regress/sys/kern/kqueue/kqueue-user.c	10 May 2025 09:44:39 -0000	1.1
+++ regress/sys/kern/kqueue/kqueue-user.c	18 May 2025 17:17:04 -0000
@@ -44,10 +44,7 @@ do_user(void)
 	n = kevent(kq, NULL, 0, kev, 2, &ts);
 	ASSX(n == 0);
 
-	/*
-	 * Activate the event.
-	 * Fields `data' and `udata' do not get updated without EV_ADD.
-	 */
+	/* Activate the event. This updates `data' and `udata'. */
 	EV_SET(&kev[0], 1, EVFILT_USER, 0, NOTE_TRIGGER | NOTE_FFNOP,
 	    123, &dummy);
 	n = kevent(kq, kev, 1, NULL, 0, NULL);
@@ -58,20 +55,6 @@ do_user(void)
 	ASSX(n == 1);
 	ASSX(kev[0].ident == 1);
 	ASSX(kev[0].fflags == NOTE_FFLAGSMASK);
-	ASSX(kev[0].data == 0);
-	ASSX(kev[0].udata == NULL);
-
-	/* Activate the event. Update `data' and `udata'. */
-	EV_SET(&kev[0], 1, EVFILT_USER, EV_ADD, NOTE_TRIGGER | NOTE_FFNOP,
-	    123, &dummy);
-	n = kevent(kq, kev, 1, NULL, 0, NULL);
-	ASSX(n == 0);
-
-	/* Check active events. */
-	n = kevent(kq, NULL, 0, kev, 2, &ts);
-	ASSX(n == 1);
-	ASSX(kev[0].ident == 1);
-	ASSX(kev[0].fflags == NOTE_FFLAGSMASK);
 	ASSX(kev[0].data == 123);
 	ASSX(kev[0].udata == &dummy);
 
@@ -122,8 +105,8 @@ do_user(void)
 	ASSX(n == 1);
 	ASSX(kev[0].ident == 2);
 	ASSX(kev[0].fflags == 0x11);
-	ASSX(kev[0].data == 42);
-	ASSX(kev[0].udata == &dummy);
+	ASSX(kev[0].data == 24);
+	ASSX(kev[0].udata == &dummy2);
 
 	n = kevent(kq, NULL, 0, kev, 2, &ts);
 	ASSX(n == 0);
@@ -132,9 +115,9 @@ do_user(void)
 	n = kevent(kq, kev, 1, kev, 2, &ts);
 	ASSX(n == 1);
 	ASSX(kev[0].ident == 2);
-	ASSX(kev[0].fflags == 0);
-	ASSX(kev[0].data == 0);
-	ASSX(kev[0].udata == &dummy);
+	ASSX(kev[0].fflags == 0x11);
+	ASSX(kev[0].data == 9);
+	ASSX(kev[0].udata == &dummy2);
 
 	EV_SET(&kev[0], 2, EVFILT_USER, EV_DELETE, 0, 0, NULL);
 	n = kevent(kq, kev, 1, kev, 2, &ts);
@@ -154,7 +137,7 @@ do_user(void)
 	ASSX(kev[0].ident == 1);
 	ASSX(kev[0].fflags == 0x0faaf0);
 	ASSX(kev[0].data == 0);
-	ASSX(kev[0].udata == &dummy);
+	ASSX(kev[0].udata == NULL);
 
 	/* Test event limit. */
 	for (i = 0;; i++) {