diff --git a/include/linux/rfkill.h b/include/linux/rfkill.h
index 2ce29831feb61d9c0d7aefa5f0f3446f17519220..f3d5812693d6bd04aca163e75df61118e891dfcd 100644
--- a/include/linux/rfkill.h
+++ b/include/linux/rfkill.h
@@ -82,6 +82,20 @@ struct rfkill_event {
 	__u8  soft, hard;
 } __packed;
 
+/*
+ * We are planning to be backward and forward compatible with changes
+ * to the event struct, by adding new, optional, members at the end.
+ * When reading an event (whether the kernel from userspace or vice
+ * versa) we need to accept anything that's at least as large as the
+ * version 1 event size, but might be able to accept other sizes in
+ * the future.
+ *
+ * One exception is the kernel -- we already have two event sizes in
+ * that we've made the 'hard' member optional since our only option
+ * is to ignore it anyway.
+ */
+#define RFKILL_EVENT_SIZE_V1	8
+
 /* ioctl for turning off rfkill-input (if present) */
 #define RFKILL_IOC_MAGIC	'R'
 #define RFKILL_IOC_NOINPUT	1
diff --git a/net/rfkill/core.c b/net/rfkill/core.c
index 79693fe2001e50df79d259a41dec1f6aa28b9370..47497c97c8d98b6fbed4ac0aae45f13ec10049d8 100644
--- a/net/rfkill/core.c
+++ b/net/rfkill/core.c
@@ -1076,10 +1076,16 @@ static ssize_t rfkill_fop_write(struct file *file, const char __user *buf,
 	struct rfkill_event ev;
 
 	/* we don't need the 'hard' variable but accept it */
-	if (count < sizeof(ev) - 1)
+	if (count < RFKILL_EVENT_SIZE_V1 - 1)
 		return -EINVAL;
 
-	if (copy_from_user(&ev, buf, sizeof(ev) - 1))
+	/*
+	 * Copy as much data as we can accept into our 'ev' buffer,
+	 * but tell userspace how much we've copied so it can determine
+	 * our API version even in a write() call, if it cares.
+	 */
+	count = min(count, sizeof(ev));
+	if (copy_from_user(&ev, buf, count))
 		return -EFAULT;
 
 	if (ev.op != RFKILL_OP_CHANGE && ev.op != RFKILL_OP_CHANGE_ALL)