diff --git a/sys/security/mac/mac_syscalls.h b/sys/security/mac/mac_syscalls.h
--- a/sys/security/mac/mac_syscalls.h
+++ b/sys/security/mac/mac_syscalls.h
@@ -23,6 +23,16 @@
 	    char **const u_string);
 void	free_copied_label(const struct mac *const mac);
 
+#ifdef COMPAT_FREEBSD32
+struct freebsd32_mac {
+	uint32_t	m_buflen;	/* size_t */
+	uint32_t	m_string;	/* char * */
+};
+
+int	mac_label_copyin32(const struct freebsd32_mac *const u_mac,
+	    struct mac *const mac, char **const u_string);
+#endif /* COMPAT_FREEBSD32 */
+
 int	mac_set_proc_prepare(struct thread *const td,
 	    const struct mac *const mac, void **const mac_set_proc_data);
 int	mac_set_proc_core(struct thread *const td, struct ucred *const newcred,
diff --git a/sys/security/mac/mac_syscalls.c b/sys/security/mac/mac_syscalls.c
--- a/sys/security/mac/mac_syscalls.c
+++ b/sys/security/mac/mac_syscalls.c
@@ -46,6 +46,7 @@
 #include "opt_mac.h"
 
 #include <sys/param.h>
+#include <sys/abi_compat.h>
 #include <sys/capsicum.h>
 #include <sys/fcntl.h>
 #include <sys/kernel.h>
@@ -86,16 +87,27 @@
  * after use by calling free_copied_label() (which see).  On success, 'u_string'
  * if not NULL is filled with the userspace address for 'u_mac->m_string'.
  */
-int
-mac_label_copyin(const struct mac *const u_mac, struct mac *const mac,
-    char **const u_string)
+static int
+mac_label_copyin_int(const void *const u_mac, struct mac *const mac,
+    char **const u_string, bool is_32bit)
 {
 	char *buffer;
 	int error;
 
-	error = copyin(u_mac, mac, sizeof(*mac));
-	if (error != 0)
-		return (error);
+	if (is_32bit) {
+		struct freebsd32_mac mac32;
+
+		error = copyin(u_mac, &mac32, sizeof(mac32));
+		if (error != 0)
+			return (error);
+
+		CP(mac32, *mac, m_buflen);
+		PTRIN_CP(mac32, *mac, m_string);
+	} else {
+		error = copyin(u_mac, mac, sizeof(*mac));
+		if (error != 0)
+			return (error);
+	}
 
 	error = mac_check_structmac_consistent(mac);
 	if (error != 0)
@@ -116,12 +128,26 @@
 	return (0);
 }
 
+int
+mac_label_copyin(const struct mac *const u_mac, struct mac *const mac,
+    char **const u_string)
+{
+	return (mac_label_copyin_int(u_mac, mac, u_string, false));
+}
+
 void
 free_copied_label(const struct mac *const mac)
 {
 	free(mac->m_string, M_MACTEMP);
 }
 
+int
+mac_label_copyin32(const struct freebsd32_mac *const u_mac,
+    struct mac *const mac, char **const u_string)
+{
+	return (mac_label_copyin_int(u_mac, mac, u_string, true));
+}
+
 int
 sys___mac_get_pid(struct thread *td, struct __mac_get_pid_args *uap)
 {