diff --git a/sys/fs/cd9660/cd9660_vnops.c b/sys/fs/cd9660/cd9660_vnops.c
--- a/sys/fs/cd9660/cd9660_vnops.c
+++ b/sys/fs/cd9660/cd9660_vnops.c
@@ -758,6 +758,9 @@
 	/* NOTREACHED */
 }
 
+_Static_assert(sizeof(struct ifid) <= sizeof(struct fid),
+    "struct ifid must be no larger than struct fid");
+
 /*
  * Vnode pointer to File handle
  */
diff --git a/sys/fs/cd9660/iso.h b/sys/fs/cd9660/iso.h
--- a/sys/fs/cd9660/iso.h
+++ b/sys/fs/cd9660/iso.h
@@ -267,7 +267,7 @@
 	u_short		ifid_pad;
 	cd_ino_t	ifid_ino;
 	long		ifid_start;
-};
+} __packed;
 
 #define VFSTOISOFS(mp)	((struct iso_mnt *)((mp)->mnt_data))