ext4: introduce linear search for dentries

This patch addresses an issue where some files in case-insensitive
directories become inaccessible due to changes in how the kernel
function, utf8_casefold(), generates case-folded strings from the
commit 5c26d2f ("unicode: Don't special case ignorable code
points").

There are good reasons why this change should be made; it's actually
quite stupid that Unicode seems to think that the characters ❤ and ❤️
should be casefolded.  Unfortimately because of the backwards
compatibility issue, this commit was reverted in 231825b.

This problem is addressed by instituting a brute-force linear fallback
if a lookup fails on case-folded directory, which does result in a
performance hit when looking up files affected by the changing how
thekernel treats ignorable Uniode characters, or when attempting to
look up non-existent file names.  So this fallback can be disabled by
setting an encoding flag if in the future, the system administrator or
the manufacturer of a mobile handset or tablet can be sure that there
was no opportunity for a kernel to insert file names with incompatible
encodings.

Fixes: 5c26d2f ("unicode: Don't special case ignorable code points")
Signed-off-by: Theodore Ts'o <[email protected]>
Reviewed-by: Gabriel Krisman Bertazi <[email protected]>

Author: tytso

fs/ext4/namei.c

@@ -1462,7 +1462,8 @@ static bool ext4_match(struct inode *parent,
 		 * sure cf_name was properly initialized before
 		 * considering the calculated hash.
 		 */
-		if (IS_ENCRYPTED(parent) && fname->cf_name.name &&
+		if (sb_no_casefold_compat_fallback(parent->i_sb) &&
+		    IS_ENCRYPTED(parent) && fname->cf_name.name &&
 		    (fname->hinfo.hash != EXT4_DIRENT_HASH(de) ||
 		     fname->hinfo.minor_hash != EXT4_DIRENT_MINOR_HASH(de)))
 			return false;
@@ -1595,10 +1596,15 @@ static struct buffer_head *__ext4_find_entry(struct inode *dir,
 		 * return.  Otherwise, fall back to doing a search the
 		 * old fashioned way.
 		 */
-		if (!IS_ERR(ret) || PTR_ERR(ret) != ERR_BAD_DX_DIR)
+		if (IS_ERR(ret) && PTR_ERR(ret) == ERR_BAD_DX_DIR)
+			dxtrace(printk(KERN_DEBUG "ext4_find_entry: dx failed, "
+				       "falling back\n"));
+		else if (!sb_no_casefold_compat_fallback(dir->i_sb) &&
+			 *res_dir == NULL && IS_CASEFOLDED(dir))
+			dxtrace(printk(KERN_DEBUG "ext4_find_entry: casefold "
+				       "failed, falling back\n"));
+		else
 			goto cleanup_and_exit;
-		dxtrace(printk(KERN_DEBUG "ext4_find_entry: dx failed, "
-			       "falling back\n"));
 		ret = NULL;
 	}
 	nblocks = dir->i_size >> EXT4_BLOCK_SIZE_BITS(sb);

include/linux/fs.h

@@ -1258,11 +1258,19 @@ extern int send_sigurg(struct file *file);
 #define SB_NOUSER       BIT(31)
 
 /* These flags relate to encoding and casefolding */
-#define SB_ENC_STRICT_MODE_FL	(1 << 0)
+#define SB_ENC_STRICT_MODE_FL		(1 << 0)
+#define SB_ENC_NO_COMPAT_FALLBACK_FL	(1 << 1)
 
 #define sb_has_strict_encoding(sb) \
 	(sb->s_encoding_flags & SB_ENC_STRICT_MODE_FL)
 
+#if IS_ENABLED(CONFIG_UNICODE)
+#define sb_no_casefold_compat_fallback(sb) \
+	(sb->s_encoding_flags & SB_ENC_NO_COMPAT_FALLBACK_FL)
+#else
+#define sb_no_casefold_compat_fallback(sb) (1)
+#endif
+
 /*
  *	Umount options
  */