f2fs: avoid garbage lenghs in dentries

This patch fixes to eliminate garbage name lengths in dentries in order
to provide correct answers of readdir.

For example, if a valid dentry consists of:
 bitmap : 1   1 1 1
 len    : 32  0 x 0,

readdir can start with second bit_pos having len = 0.
Or, it can start with third bit_pos having garbage.

In both of cases, we should avoid to try filling dentries.
So, this patch not only removes any garbage length, but also avoid entering
zero length case in readdir.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/dir.c

@@ -511,8 +511,12 @@ void f2fs_update_dentry(nid_t ino, umode_t mode, struct f2fs_dentry_ptr *d,
 	memcpy(d->filename[bit_pos], name->name, name->len);
 	de->ino = cpu_to_le32(ino);
 	set_de_type(de, mode);
-	for (i = 0; i < slots; i++)
+	for (i = 0; i < slots; i++) {
 		test_and_set_bit_le(bit_pos + i, (void *)d->bitmap);
+		/* avoid wrong garbage data for readdir */
+		if (i)
+			(de + i)->name_len = 0;
+	}
 }
 
 /*
@@ -792,6 +796,12 @@ bool f2fs_fill_dentries(struct dir_context *ctx, struct f2fs_dentry_ptr *d,
 			break;
 
 		de = &d->dentry[bit_pos];
+		if (de->name_len == 0) {
+			bit_pos++;
+			ctx->pos = start_pos + bit_pos;
+			continue;
+		}
+
 		if (de->file_type < F2FS_FT_MAX)
 			d_type = f2fs_filetype_table[de->file_type];
 		else