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diff --git a/sys/dev/null/null.c b/sys/dev/null/null.c
--- a/sys/dev/null/null.c
+++ b/sys/dev/null/null.c
@@ -4,6 +4,7 @@
* Copyright (c) 2000 Mark R. V. Murray & Jeroen C. van Gelderen
* Copyright (c) 2001-2004 Mark R. V. Murray
* Copyright (c) 2014 Eitan Adler
+ * Copyright (c) 2025 Pietro Cerutti
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
@@ -39,6 +40,7 @@
#include <sys/disk.h>
#include <sys/bus.h>
#include <sys/filio.h>
+#include <sys/event.h>
#include <machine/bus.h>
#include <machine/vmparam.h>
@@ -53,12 +55,28 @@
static d_ioctl_t null_ioctl;
static d_ioctl_t zero_ioctl;
static d_read_t zero_read;
+static d_kqfilter_t full_kqfilter;
+static d_kqfilter_t null_kqfilter;
+static d_kqfilter_t zero_kqfilter;
+static int immediate_event(struct knote *kn, long hint);
+static int never_event(struct knote *kn, long hint);
+
+static const struct filterops immediate_filterops = {
+ .f_isfd = 1,
+ .f_event = immediate_event
+};
+
+static const struct filterops never_filterops = {
+ .f_isfd = 1,
+ .f_event = never_event
+};
static struct cdevsw full_cdevsw = {
.d_version = D_VERSION,
.d_read = zero_read,
.d_write = full_write,
.d_ioctl = zero_ioctl,
+ .d_kqfilter = full_kqfilter,
.d_name = "full",
};
@@ -67,6 +85,7 @@
.d_read = (d_read_t *)nullop,
.d_write = null_write,
.d_ioctl = null_ioctl,
+ .d_kqfilter = null_kqfilter,
.d_name = "null",
};
@@ -75,6 +94,7 @@
.d_read = zero_read,
.d_write = null_write,
.d_ioctl = zero_ioctl,
+ .d_kqfilter = zero_kqfilter,
.d_name = "zero",
.d_flags = D_MMAP_ANON,
};
@@ -197,5 +217,65 @@
return (0);
}
+static int
+immediate_event(struct knote *kn, long hint)
+{
+
+ return 1;
+}
+
+static int
+never_event(struct knote *kn, long hint)
+{
+
+ return 0;
+}
+
+static int
+full_kqfilter(struct cdev *dev, struct knote *kn)
+{
+
+ switch (kn->kn_filter) {
+ case EVFILT_READ:
+ kn->kn_fop = &immediate_filterops;
+ return (0);
+ case EVFILT_WRITE:
+ kn->kn_fop = &never_filterops;
+ return (0);
+ default:
+ return(EOPNOTSUPP);
+ }
+}
+
+static int
+null_kqfilter(struct cdev *dev, struct knote *kn)
+{
+
+ switch (kn->kn_filter) {
+ case EVFILT_READ:
+ case EVFILT_WRITE:
+ kn->kn_fop = &immediate_filterops;
+ return (0);
+ default:
+ return(EOPNOTSUPP);
+ }
+}
+
+static int
+zero_kqfilter(struct cdev *dev, struct knote *kn)
+{
+
+ switch (kn->kn_filter) {
+ case EVFILT_READ:
+ kn->kn_fop = &never_filterops;
+ return (0);
+ case EVFILT_WRITE:
+ kn->kn_fop = &immediate_filterops;
+ return (0);
+ default:
+ return(EOPNOTSUPP);
+ }
+}
+
DEV_MODULE(null, null_modevent, NULL);
MODULE_VERSION(null, 1);

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