[PATCH v3 1/9] libfs: Create the helper function generic_ci_validate_strict_name()

["André Almeida" <andrealmeid@igalia.com>]

Create a helper function for filesystems do the checks required for
casefold directories and strict encoding.

Suggested-by: Gabriel Krisman Bertazi <gabriel@krisman.be>
Signed-off-by: André Almeida <andrealmeid@igalia.com>
---
Changes from v2:
- Moved function to libfs and adpated its name
- Wrapped at 72 chars column
- Decomposed the big if (...) to be more clear
---
 fs/libfs.c         | 38 ++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h |  1 +
 2 files changed, 39 insertions(+)

diff --git a/fs/libfs.c b/fs/libfs.c
index 8aa34870449f..99fb36b48708 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1928,6 +1928,44 @@ int generic_ci_match(const struct inode *parent,
 	return !res;
 }
 EXPORT_SYMBOL(generic_ci_match);
+
+/**
+ * generic_ci_validate_strict_name - Check if a given name is suitable
+ * for a directory
+ *
+ * This functions checks if the proposed filename is valid for the
+ * parent directory. That means that only valid UTF-8 filenames will be
+ * accepted for casefold directories from filesystems created with the
+ * strict encoding flag.  That also means that any name will be
+ * accepted for directories that doesn't have casefold enabled, or
+ * aren't being strict with the encoding.
+ *
+ * @dir: inode of the directory where the new file will be created
+ * @name: name of the new file
+ *
+ * Return:
+ * * True if the filename is suitable for this directory. It can be
+ * true if a given name is not suitable for a strict encoding
+ * directory, but the directory being used isn't strict
+ * * False if the filename isn't suitable for this directory. This only
+ * happens when a directory is casefolded and the filesystem is strict
+ * about its encoding.
+ */
+bool generic_ci_validate_strict_name(struct inode *dir, struct qstr *name)
+{
+	if (!IS_CASEFOLDED(dir) || !sb_has_strict_encoding(dir->i_sb))
+		return true;
+
+	/*
+	 * A casefold dir must have a encoding set, unless the filesystem
+	 * is corrupted
+	 */
+	if (WARN_ON_ONCE(!dir->i_sb->s_encoding))
+		return true;
+
+	return utf8_validate(dir->i_sb->s_encoding, name);
+}
+EXPORT_SYMBOL(generic_ci_validate_strict_name);
 #endif
 
 #ifdef CONFIG_FS_ENCRYPTION
diff --git a/include/linux/fs.h b/include/linux/fs.h
index fd34b5755c0b..937142950dfe 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3385,6 +3385,7 @@ 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);
+bool generic_ci_validate_strict_name(struct inode *dir, struct qstr *name);
 
 static inline bool sb_has_encoding(const struct super_block *sb)
 {
-- 
2.46.0