[PATCH] ext4: introduce linear search for dentries

Theodore Ts'o
2025-02-12T16:45:02Z

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 5c26d2f1d3f5 ("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 231825b2e1ff.

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: 5c26d2f1d3f5 ("unicode: Don't special case ignorable code points")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
fs/ext4/namei.c | 14 ++++++++++----
include/linux/fs.h | 6 +++++-
2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 536d56d15072..820e7ab7f3a3 100644
--- a/fs/ext4/namei.c
+++ b/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);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 2c3b2f8a621f..a10edf804140 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1258,11 +1258,15 @@ 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)

+#define sb_no_casefold_compat_fallback(sb) \
+ (sb->s_encoding_flags & SB_ENC_NO_COMPAT_FALLBACK_FL)
+
/*
* Umount options
*/
--
2.45.2