diff --git a/sys/kern/sys_process.c b/sys/kern/sys_process.c
--- a/sys/kern/sys_process.c
+++ b/sys/kern/sys_process.c
@@ -190,8 +190,21 @@
 	if (regset == NULL)
 		return (EINVAL);
 
+	if (regset->get == NULL)
+		return (EINVAL);
+
+	size = regset->size;
+	/*
+	 * The regset is dynamically sized, e.g. the size could change
+	 * depending on the hardware, or may have a per-thread size.
+	 */
+	if (size == 0) {
+		if (!regset->get(regset, td, NULL, &size))
+			return (EINVAL);
+	}
+
 	if (iov->iov_base == NULL) {
-		iov->iov_len = regset->size;
+		iov->iov_len = size;
 		if (iov->iov_len == 0)
 			return (EINVAL);
 
@@ -199,14 +212,10 @@
 	}
 
 	/* The length is wrong, return an error */
-	if (iov->iov_len != regset->size)
-		return (EINVAL);
-
-	if (regset->get == NULL)
+	if (iov->iov_len != size)
 		return (EINVAL);
 
 	error = 0;
-	size = regset->size;
 	p = td->td_proc;
 
 	/* Drop the proc lock while allocating the temp buffer */
@@ -218,7 +227,7 @@
 	if (!regset->get(regset, td, buf, &size)) {
 		error = EINVAL;
 	} else {
-		KASSERT(size == regset->size,
+		KASSERT(size == regset->size || regset->size == 0,
 		    ("%s: Getter function changed the size", __func__));
 
 		iov->iov_len = size;
@@ -245,14 +254,23 @@
 	if (regset == NULL)
 		return (EINVAL);
 
+	size = regset->size;
+	/*
+	 * The regset is dynamically sized, e.g. the size could change
+	 * depending on the hardware, or may have a per-thread size.
+	 */
+	if (size == 0) {
+		if (!regset->get(regset, td, NULL, &size))
+			return (EINVAL);
+	}
+
 	/* The length is wrong, return an error */
-	if (iov->iov_len != regset->size)
+	if (iov->iov_len != size)
 		return (EINVAL);
 
 	if (regset->set == NULL)
 		return (EINVAL);
 
-	size = regset->size;
 	p = td->td_proc;
 
 	/* Drop the proc lock while allocating the temp buffer */