ext4: replace strcmp with direct comparison for '.' and '..'

In a discussion over a proposed patch, "ext4: replace strcpy() with
'.' assignment"[1], I had asserted that directory entries in ext4 were
not NUL terminated, and hence it was safe to replace strcpy() with a
direct assignment.  As it turns out, this was incorrect.  It's true
for all all directory entries *except* for '.' and '..' where the
kernel was using strcmp() and where e2fsck actually checks and offers
to fix things if '.'  and '..' are not NUL terminated.

[1] https://lore.kernel.org/r/202505191316.JJMnPobO-lkp@intel.com

We can't change this without breaking old kernel versions, but in the
spirit of "be liberal in what you receive", use direct comparison of
de->name_len and de->name[0,1] instead of strcmp().  This has the side
benefit of reducing the compiled text size by 96 bytes on x86_64.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Link: https://patch.msgid.link/20250712181249.434530-1-tytso@mit.edu
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: tytso

fs/ext4/namei.c

@@ -3082,7 +3082,8 @@ bool ext4_empty_dir(struct inode *inode)
 	de = (struct ext4_dir_entry_2 *) bh->b_data;
 	if (ext4_check_dir_entry(inode, NULL, de, bh, bh->b_data, bh->b_size,
 				 0) ||
-	    le32_to_cpu(de->inode) != inode->i_ino || strcmp(".", de->name)) {
+	    le32_to_cpu(de->inode) != inode->i_ino || de->name_len != 1 ||
+	    de->name[0] != '.') {
 		ext4_warning_inode(inode, "directory missing '.'");
 		brelse(bh);
 		return false;
@@ -3091,7 +3092,8 @@ bool ext4_empty_dir(struct inode *inode)
 	de = ext4_next_entry(de, sb->s_blocksize);
 	if (ext4_check_dir_entry(inode, NULL, de, bh, bh->b_data, bh->b_size,
 				 offset) ||
-	    le32_to_cpu(de->inode) == 0 || strcmp("..", de->name)) {
+	    le32_to_cpu(de->inode) == 0 || de->name_len != 2 ||
+	    de->name[0] != '.' || de->name[1] != '.') {
 		ext4_warning_inode(inode, "directory missing '..'");
 		brelse(bh);
 		return false;
@@ -3532,7 +3534,7 @@ static struct buffer_head *ext4_get_first_dir_block(handle_t *handle,
 		if (ext4_check_dir_entry(inode, NULL, de, bh, bh->b_data,
 					 bh->b_size, 0) ||
 		    le32_to_cpu(de->inode) != inode->i_ino ||
-		    strcmp(".", de->name)) {
+		    de->name_len != 1 || de->name[0] != '.') {
 			EXT4_ERROR_INODE(inode, "directory missing '.'");
 			brelse(bh);
 			*retval = -EFSCORRUPTED;
@@ -3543,7 +3545,8 @@ static struct buffer_head *ext4_get_first_dir_block(handle_t *handle,
 		de = ext4_next_entry(de, inode->i_sb->s_blocksize);
 		if (ext4_check_dir_entry(inode, NULL, de, bh, bh->b_data,
 					 bh->b_size, offset) ||
-		    le32_to_cpu(de->inode) == 0 || strcmp("..", de->name)) {
+		    le32_to_cpu(de->inode) == 0 || de->name_len != 2 ||
+		    de->name[0] != '.' || de->name[1] != '.') {
 			EXT4_ERROR_INODE(inode, "directory missing '..'");
 			brelse(bh);
 			*retval = -EFSCORRUPTED;