Hello! While I was reading a new patch series for exfat filesystem driver I saw how is proposed implementation for converting exfat's UTF-16LE filenames to userspace and so I decided to investigate what filesystems which are already part of Linux kernel are doing. I looked at filesystems supported by Linux kernel which do not store filenames as sequence of octets, but rather expect that on-disk format of filenames is according to some encoding. Below is list of these filesystems with its native encoding: befs UTF-8 cifs UTF-16LE msdos unspecified OEM codepage vfat unspecified OEM codepage or UTF-16LE hfs octets hfsplus UTF-16BE-NFD-Apple isofs octets or UTF-16BE jfs UTF-16LE ntfs UTF-16LE udf Latin1 or UTF-16BE Filesystems msdos, vfat, hfs and isofs are bogus as their filesystem structure does not say in which encoding is filename stored. For vfat and isofs there is information if it is UTF-16LE or some unspecified encoding. User who access such filesystem must know under which encoding he stored data on it. For this purpose there is for vfat and hfs mount option codepage=<codepage>. All other filesystems stores in their structures encoding of filenames. Either implicitly (hfsplus is always in UTF-16BE with modified Apple's NFD normalization) or explicitly (in UDF is byte which says if filename is in Latin1 or in UTF-16BE). As passing UTF-16(LE|BE) buffers is not possible via null term strings for any VFS sycall, Linux kernel translates these Unicode filenames to some charset. It is done by various mount options. I looked which mount options are understood by our Linux filesystems implementations. In all next paragraphs by filesystem I would mean Linux driver implementation (and not structure on disk), so do not be confused. Below is table: befs iocharset=<charset> cifs iocharset=<charset> msdos (unsupported) vfat utf8=0|no|false|1|yes|true OR utf8 OR iocharset=<charset> hfs iocharset=<charset> hfsplus nls=<charset> isofs iocharset=<charset> OR utf8 jfs iocharset=<charset> ntfs nls=<charset> OR iochrset=<charset> OR utf8 udf utf8 OR iocharset=<charset> Filesystem msdos does not support specifying OEM codepage encoding. It passthrough 8bit buffer to userspace and expects that userspace understand correct OEM codepage. There is no support for reencoding it to UTF-8 (or any other charset). Same applies for isofs when Joliet structure is not stored on filesystem. Filesystem vfat has the most obscure way how to specifying charset. Details are in mount(8) manual page. What is important: option iocharset=utf8 is buggy and may break filesystem consistency (it allows to create two directory entries which would differ only in case sensitivity which is not allowed by FAT specification). Due to this problem there is a fix, mount option utf8=1 (or utf8=yes or utf8=true or just utf8) which do what you have would expect from iocharset=utf8 if it was not buggy. Filesystem ntfs has option iocharset=<charset> which is just alias for nls=<charset> and says that iocharset= is deprecated. Same applies for option utf8 which is just alias for nls=utf8. Filesystems isofs and udf have two ways how to specify UTF-8 encoding. First way is via utf8 mount option and second one via iocharset=utf8 option. Looks like that difference is only one, iocharset=utf8 supports only Uncicode code points up to the U+FFFF (limited to 3 byte long UTF-8 sequences, like utf8/utf8mb3 encoding in MySQL/MariaDB) and utf8 option supports also code points above U+FFFF, so full Unicode and not just limited subset. Filesystem cifs in UTF-8 mode (via iocharset=utf8) always supports code points above U+FFFF. But remaining filesystems befs, hfs, hfsplus, jfs and ntfs seems to supports only Unicode code points up to the U+FFFF. So effectively they do not support UTF-16, but effectively just UCS-2. This limitation comes from Kernel NLS table framework/API which is limited to 16bit integers and therefore maximal Unicode code point is U+FFFF. Filesystems cifs, isofs, udf and vfat has own special code to work with surrogate pairs and do not use limited NLS table functions. There are also functions utf8s_to_utf16s() and utf16s_to_utf8s() for this purpose. And here I see these improvements for all above filesystems: 1) Unify mount options for specifying charset. Currently all filesystems except msdos and hfsplus have mount option iocharset=<charset>. hfsplus has nls=<charset> and msdos does not implement re-encoding support. Plus vfat, udf and isofs have broken iocharset=utf8 option (but working utf8 option) And ntfs has deprecated iocharset=<charset> option. I would suggest following changes for unification: * Add a new alias iocharset= for hfsplus which would do same as nls= * Make iocharset=utf8 option for vfat, udf and isofs to do same as utf8 * Un-deprecate iocharset=<charset> option for ntfs This would cause that all filesystems would have iocharset=<charset> option which would work for any charset, including iocharset=utf8. And it would fix also broken iocharset=utf8 for vfat, udf and isofs. 2) Add support for Unicode code points above U+FFFF for filesystems befs, hfs, hfsplus, jfs and ntfs, so iocharset=utf8 option would work also with filenames in userspace which would be 4 bytes long UTF-8. 3) Add support for iocharset= and codepage= options for msdos filesystem. It shares lot of pars of code with vfat driver. What do you think about these improvements? First improvement should be relatively simple and if we agree that this unification of mount option iocharset= make sense, I could do it. -- Pali Rohár pali.rohar@xxxxxxxxx
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