[PATCH v8 3/8] libfs: Introduce case-insensitive string comparison helper

[Gabriel Krisman Bertazi <krisman@collabora.com>]

generic_ci_match can be used by case-insensitive filesystems to compare
strings under lookup with dirents in a case-insensitive way.  This
function is currently reimplemented by each filesystem supporting
casefolding, so this reduces code duplication in filesystem-specific
code.

Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>

---
Changes since v5:
  (eric)
  - Rename variable err -> res
  - Retype de_name_len from size_t to u32
  - Bring WARN_ON_ONCE(!fscrypt_has_encryption_key) from f2fs implementation
---
 fs/libfs.c         | 68 ++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h |  4 +++
 2 files changed, 72 insertions(+)

diff --git a/fs/libfs.c b/fs/libfs.c
index 974125270a42..f1d88cd903f8 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1465,6 +1465,74 @@ static const struct dentry_operations generic_ci_dentry_ops = {
 	.d_hash = generic_ci_d_hash,
 	.d_compare = generic_ci_d_compare,
 };
+
+/**
+ * generic_ci_match() - Match a name (case-insensitively) with a dirent.
+ * @parent: Inode of the parent of the dirent under comparison
+ * @name: name under lookup.
+ * @folded_name: Optional pre-folded name under lookup
+ * @de_name: Dirent name.
+ * @de_name_len: dirent name length.
+ *
+ *
+ * Test whether a case-insensitive directory entry matches the filename
+ * being searched.  If @folded_name is provided, it is used instead of
+ * recalculating the casefold of @name.
+ *
+ * Return: > 0 if the directory entry matches, 0 if it doesn't match, or
+ * < 0 on error.
+ */
+int generic_ci_match(const struct inode *parent,
+		     const struct qstr *name,
+		     const struct qstr *folded_name,
+		     const u8 *de_name, u32 de_name_len)
+{
+	const struct super_block *sb = parent->i_sb;
+	const struct unicode_map *um = sb->s_encoding;
+	struct fscrypt_str decrypted_name = FSTR_INIT(NULL, de_name_len);
+	struct qstr dirent = QSTR_INIT(de_name, de_name_len);
+	int res, match = false;
+
+	if (IS_ENCRYPTED(parent)) {
+		const struct fscrypt_str encrypted_name =
+			FSTR_INIT((u8 *) de_name, de_name_len);
+
+		if (WARN_ON_ONCE(!fscrypt_has_encryption_key(parent)))
+			return -EINVAL;
+
+		decrypted_name.name = kmalloc(de_name_len, GFP_KERNEL);
+		if (!decrypted_name.name)
+			return -ENOMEM;
+		res = fscrypt_fname_disk_to_usr(parent, 0, 0, &encrypted_name,
+						&decrypted_name);
+		if (res < 0)
+			goto out;
+		dirent.name = decrypted_name.name;
+		dirent.len = decrypted_name.len;
+	}
+
+	if (folded_name->name)
+		res = utf8_strncasecmp_folded(um, folded_name, &dirent);
+	else
+		res = utf8_strncasecmp(um, name, &dirent);
+
+	if (!res)
+		match = true;
+	else if (res < 0 && !sb_has_strict_encoding(sb)) {
+		/*
+		 * In non-strict mode, fallback to a byte comparison if
+		 * the names have invalid characters.
+		 */
+		res = 0;
+		match = ((name->len == dirent.len) &&
+			 !memcmp(name->name, dirent.name, dirent.len));
+	}
+
+out:
+	kfree(decrypted_name.name);
+	return (res >= 0) ? match : res;
+}
+EXPORT_SYMBOL(generic_ci_match);
 #endif
 
 #ifdef CONFIG_FS_ENCRYPTION
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e2d892b201b0..4c2b781f667a 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3359,6 +3359,10 @@ extern int generic_file_fsync(struct file *, loff_t, loff_t, int);
 extern int generic_check_addressable(unsigned, u64);
 
 extern void generic_set_encrypted_ci_d_ops(struct dentry *dentry);
+extern int generic_ci_match(const struct inode *parent,
+			    const struct qstr *name,
+			    const struct qstr *folded_name,
+			    const u8 *de_name, u32 de_name_len);
 
 #ifdef CONFIG_MIGRATION
 extern int buffer_migrate_page(struct address_space *,
-- 
2.36.1