diff --git a/fs/afs/dir.c b/fs/afs/dir.c
index 9068d5578a26..7bd659ad959e 100644
--- a/fs/afs/dir.c
+++ b/fs/afs/dir.c
@@ -350,7 +350,7 @@ static int afs_dir_iterate_block(struct afs_vnode *dvnode,
 				 unsigned blkoff)
 {
 	union afs_xdr_dirent *dire;
-	unsigned offset, next, curr;
+	unsigned offset, next, curr, nr_slots;
 	size_t nlen;
 	int tmp;
 
@@ -363,13 +363,12 @@ static int afs_dir_iterate_block(struct afs_vnode *dvnode,
 	     offset < AFS_DIR_SLOTS_PER_BLOCK;
 	     offset = next
 	     ) {
-		next = offset + 1;
-
 		/* skip entries marked unused in the bitmap */
 		if (!(block->hdr.bitmap[offset / 8] &
 		      (1 << (offset % 8)))) {
 			_debug("ENT[%zu.%u]: unused",
 			       blkoff / sizeof(union afs_xdr_dir_block), offset);
+			next = offset + 1;
 			if (offset >= curr)
 				ctx->pos = blkoff +
 					next * sizeof(union afs_xdr_dirent);
@@ -381,35 +380,39 @@ static int afs_dir_iterate_block(struct afs_vnode *dvnode,
 		nlen = strnlen(dire->u.name,
 			       sizeof(*block) -
 			       offset * sizeof(union afs_xdr_dirent));
+		if (nlen > AFSNAMEMAX - 1) {
+			_debug("ENT[%zu]: name too long (len %u/%zu)",
+			       blkoff / sizeof(union afs_xdr_dir_block),
+			       offset, nlen);
+			return afs_bad(dvnode, afs_file_error_dir_name_too_long);
+		}
 
 		_debug("ENT[%zu.%u]: %s %zu \"%s\"",
 		       blkoff / sizeof(union afs_xdr_dir_block), offset,
 		       (offset < curr ? "skip" : "fill"),
 		       nlen, dire->u.name);
 
-		/* work out where the next possible entry is */
-		for (tmp = nlen; tmp > 15; tmp -= sizeof(union afs_xdr_dirent)) {
-			if (next >= AFS_DIR_SLOTS_PER_BLOCK) {
-				_debug("ENT[%zu.%u]:"
-				       " %u travelled beyond end dir block"
-				       " (len %u/%zu)",
+		nr_slots = afs_dir_calc_slots(nlen);
+		next = offset + nr_slots;
+		if (next > AFS_DIR_SLOTS_PER_BLOCK) {
+			_debug("ENT[%zu.%u]:"
+			       " %u extends beyond end dir block"
+			       " (len %zu)",
+			       blkoff / sizeof(union afs_xdr_dir_block),
+			       offset, next, nlen);
+			return afs_bad(dvnode, afs_file_error_dir_over_end);
+		}
+
+		/* Check that the name-extension dirents are all allocated */
+		for (tmp = 1; tmp < nr_slots; tmp++) {
+			unsigned int ix = offset + tmp;
+			if (!(block->hdr.bitmap[ix / 8] & (1 << (ix % 8)))) {
+				_debug("ENT[%zu.u]:"
+				       " %u unmarked extension (%u/%u)",
 				       blkoff / sizeof(union afs_xdr_dir_block),
-				       offset, next, tmp, nlen);
-				return afs_bad(dvnode, afs_file_error_dir_over_end);
-			}
-			if (!(block->hdr.bitmap[next / 8] &
-			      (1 << (next % 8)))) {
-				_debug("ENT[%zu.%u]:"
-				       " %u unmarked extension (len %u/%zu)",
-				       blkoff / sizeof(union afs_xdr_dir_block),
-				       offset, next, tmp, nlen);
+				       offset, tmp, nr_slots);
 				return afs_bad(dvnode, afs_file_error_dir_unmarked_ext);
 			}
-
-			_debug("ENT[%zu.%u]: ext %u/%zu",
-			       blkoff / sizeof(union afs_xdr_dir_block),
-			       next, tmp, nlen);
-			next++;
 		}
 
 		/* skip if starts before the current position */
diff --git a/fs/afs/dir_edit.c b/fs/afs/dir_edit.c
index 2ffe09abae7f..f4600c1353ad 100644
--- a/fs/afs/dir_edit.c
+++ b/fs/afs/dir_edit.c
@@ -215,8 +215,7 @@ void afs_edit_dir_add(struct afs_vnode *vnode,
 	}
 
 	/* Work out how many slots we're going to need. */
-	need_slots = round_up(12 + name->len + 1 + 4, AFS_DIR_DIRENT_SIZE);
-	need_slots /= AFS_DIR_DIRENT_SIZE;
+	need_slots = afs_dir_calc_slots(name->len);
 
 	meta_page = kmap(page0);
 	meta = &meta_page->blocks[0];
@@ -393,8 +392,7 @@ void afs_edit_dir_remove(struct afs_vnode *vnode,
 	}
 
 	/* Work out how many slots we're going to discard. */
-	need_slots = round_up(12 + name->len + 1 + 4, AFS_DIR_DIRENT_SIZE);
-	need_slots /= AFS_DIR_DIRENT_SIZE;
+	need_slots = afs_dir_calc_slots(name->len);
 
 	meta_page = kmap(page0);
 	meta = &meta_page->blocks[0];
diff --git a/fs/afs/xdr_fs.h b/fs/afs/xdr_fs.h
index c926430fd08a..8ca868164507 100644
--- a/fs/afs/xdr_fs.h
+++ b/fs/afs/xdr_fs.h
@@ -58,7 +58,8 @@ union afs_xdr_dirent {
 		/* When determining the number of dirent slots needed to
 		 * represent a directory entry, name should be assumed to be 16
 		 * bytes, due to a now-standardised (mis)calculation, but it is
-		 * in fact 20 bytes in size.
+		 * in fact 20 bytes in size.  afs_dir_calc_slots() should be
+		 * used for this.
 		 *
 		 * For names longer than (16 or) 20 bytes, extra slots should
 		 * be annexed to this one using the extended_name format.
@@ -101,4 +102,15 @@ struct afs_xdr_dir_page {
 	union afs_xdr_dir_block	blocks[AFS_DIR_BLOCKS_PER_PAGE];
 };
 
+/*
+ * Calculate the number of dirent slots required for any given name length.
+ * The calculation is made assuming the part of the name in the first slot is
+ * 16 bytes, rather than 20, but this miscalculation is now standardised.
+ */
+static inline unsigned int afs_dir_calc_slots(size_t name_len)
+{
+	name_len++; /* NUL-terminated */
+	return 1 + ((name_len + 15) / AFS_DIR_DIRENT_SIZE);
+}
+
 #endif /* XDR_FS_H */
diff --git a/include/trace/events/afs.h b/include/trace/events/afs.h
index 4eef374d4413..4a5cc8c64be3 100644
--- a/include/trace/events/afs.h
+++ b/include/trace/events/afs.h
@@ -231,6 +231,7 @@ enum afs_file_error {
 	afs_file_error_dir_bad_magic,
 	afs_file_error_dir_big,
 	afs_file_error_dir_missing_page,
+	afs_file_error_dir_name_too_long,
 	afs_file_error_dir_over_end,
 	afs_file_error_dir_small,
 	afs_file_error_dir_unmarked_ext,
@@ -488,6 +489,7 @@ enum afs_cb_break_reason {
 	EM(afs_file_error_dir_bad_magic,	"DIR_BAD_MAGIC")	\
 	EM(afs_file_error_dir_big,		"DIR_BIG")		\
 	EM(afs_file_error_dir_missing_page,	"DIR_MISSING_PAGE")	\
+	EM(afs_file_error_dir_name_too_long,	"DIR_NAME_TOO_LONG")	\
 	EM(afs_file_error_dir_over_end,		"DIR_ENT_OVER_END")	\
 	EM(afs_file_error_dir_small,		"DIR_SMALL")		\
 	EM(afs_file_error_dir_unmarked_ext,	"DIR_UNMARKED_EXT")	\