Index: sys/fs/tmpfs/tmpfs.h
===================================================================
--- sys/fs/tmpfs/tmpfs.h
+++ sys/fs/tmpfs/tmpfs.h
@@ -37,6 +37,7 @@
 #ifndef _FS_TMPFS_TMPFS_H_
 #define _FS_TMPFS_TMPFS_H_
 
+#include <sys/cdefs.h>
 #include <sys/queue.h>
 #include <sys/tree.h>
 
@@ -393,12 +394,12 @@
  * This structure maps a file identifier to a tmpfs node.  Used by the
  * NFS code.
  */
-struct tmpfs_fid {
-	uint16_t		tf_len;
-	uint16_t		tf_pad;
-	ino_t			tf_id;
-	unsigned long		tf_gen;
+struct tmpfs_fid_data {
+	ino_t			tfd_id;
+	unsigned long		tfd_gen;
 };
+_Static_assert(sizeof(struct tmpfs_fid_data) <= MAXFIDSZ,
+    "(struct tmpfs_fid_data) is larger than (struct fid).fid_data");
 
 struct tmpfs_dir_cursor {
 	struct tmpfs_dirent	*tdc_current;
Index: sys/fs/tmpfs/tmpfs_vfsops.c
===================================================================
--- sys/fs/tmpfs/tmpfs_vfsops.c
+++ sys/fs/tmpfs/tmpfs_vfsops.c
@@ -566,24 +566,29 @@
 tmpfs_fhtovp(struct mount *mp, struct fid *fhp, int flags,
     struct vnode **vpp)
 {
-	struct tmpfs_fid *tfhp;
+	struct tmpfs_fid_data tfd;
 	struct tmpfs_mount *tmp;
 	struct tmpfs_node *node;
 	int error;
 
+	if (fhp->fid_len != sizeof(struct tmpfs_fid_data))
+		return (EINVAL);
+
+	/*
+	 * Copy from fid_data onto the stack to avoid unaligned pointer use.
+	 * See the comment in sys/mount.h on struct fid for details.
+	 */
+	memcpy(&tfd, fhp->fid_data, fhp->fid_len);
+
 	tmp = VFS_TO_TMPFS(mp);
 
-	tfhp = (struct tmpfs_fid *)fhp;
-	if (tfhp->tf_len != sizeof(struct tmpfs_fid))
+	if (tfd.tfd_id >= tmp->tm_nodes_max)
 		return (EINVAL);
 
-	if (tfhp->tf_id >= tmp->tm_nodes_max)
-		return (EINVAL);
-
 	TMPFS_LOCK(tmp);
 	LIST_FOREACH(node, &tmp->tm_nodes_used, tn_entries) {
-		if (node->tn_id == tfhp->tf_id &&
-		    node->tn_gen == tfhp->tf_gen) {
+		if (node->tn_id == tfd.tfd_id &&
+		    node->tn_gen == tfd.tfd_gen) {
 			tmpfs_ref_node(node);
 			break;
 		}
Index: sys/fs/tmpfs/tmpfs_vnops.c
===================================================================
--- sys/fs/tmpfs/tmpfs_vnops.c
+++ sys/fs/tmpfs/tmpfs_vnops.c
@@ -1435,16 +1435,28 @@
 
 static int
 tmpfs_vptofh(struct vop_vptofh_args *ap)
+/*
+vop_vptofh {
+	IN struct vnode *a_vp;
+	IN struct fid *a_fhp;
+};
+*/
 {
-	struct tmpfs_fid *tfhp;
+	struct tmpfs_fid_data tfd;
 	struct tmpfs_node *node;
+	struct fid *fhp;
 
-	tfhp = (struct tmpfs_fid *)ap->a_fhp;
 	node = VP_TO_TMPFS_NODE(ap->a_vp);
+	fhp = ap->a_fhp;
+	fhp->fid_len = sizeof(tfd);
 
-	tfhp->tf_len = sizeof(struct tmpfs_fid);
-	tfhp->tf_id = node->tn_id;
-	tfhp->tf_gen = node->tn_gen;
+	/*
+	 * Copy into fid_data from the stack to avoid unaligned pointer use.
+	 * See the comment in sys/mount.h on struct fid for details.
+	 */
+	tfd.tfd_id = node->tn_id;
+	tfd.tfd_gen = node->tn_gen;
+	memcpy(fhp->fid_data, &tfd, fhp->fid_len);
 
 	return (0);
 }
Index: sys/sys/mount.h
===================================================================
--- sys/sys/mount.h
+++ sys/sys/mount.h
@@ -57,6 +57,13 @@
 /*
  * File identifier.
  * These are unique per filesystem on a single machine.
+ *
+ * Note that fid_data is at a 32-bit offset, so care must be taken to avoid
+ * unaligned access to fields within an embedded struct, which is undefined
+ * behavior.  For example, if fid_data is used for a struct with 64-bit
+ * fields, it is undefined behavior to access the fields through a pointer
+ * to fid_data on 64-bit platforms.  Use memcpy to load/store the struct
+ * instead.
  */
 #define	MAXFIDSZ	16