On Wednesday 01 January 2020 13:10:54 Theodore Y. Ts'o wrote: > On Tue, Dec 31, 2019 at 04:54:18PM -0600, Eric Sandeen wrote: > > > Because I was not able to find any documentation for it, what is format > > > of passed buffer... null-term string? fixed-length? and in which > > > encoding? utf-8? latin1? utf-16? or filesystem dependent? > > > > It simply copies the bits from the memory location you pass in, it knows > > nothing of encodings. > > > > For the most part it's up to the filesystem's own utilities to do any > > interpretation of the resulting bits on disk, null-terminating maximal-length > > label strings, etc. > > I'm not sure this is going to be the best API design choice. The > blkid library interprets the on disk format for each file syustem > knowing what is the "native" format for that particular file system. > This is mainly an issue only for the non-Linux file systems; for the > Linux file system, the party line has historically been that we don't > get involved with character encoding, but in practice, what that has > evolved into is that userspace has standardized on UTF-8, and that's > what we pass into the kernel from userspace by convention. > > But the problem is that if the goal is to make FS_IOC_GETFSLABEL and > FS_IOC_SETFSLABEL work without the calling program knowing what file > system type a particular pathname happens to be, then it would be > easist for the userspace program if it can expect that it can always > pass in a null-terminated UTF-8 string, and get back a null-terminated > UTF-8. I bet that in practice, that is what most userspace programs > are going to be do anyway, since it works that way for all other file > system syscalls. > > So for a file system which is a non-Linux-native file system, if it > happens to store the its label using utf-16, or some other > Windows-system-silliness, it would work a lot better if it assumed > that it was passed in utf-8, and stored in the the Windows file system > using whatever crazy encoding Windows wants to use. Otherwise, why > bother uplifting the ioctl to one which is file system independent, if > the paramters are defined to be file system *dependent*? Exactly. In another email I wrote that for those non-Linux-native filesystem could be used encoding specified in iocharset= mount parameter. I think it is better as usage of one fixing encoding (e.g. UTF-8) if other filesystem strings are propagated to userspace in other encoding (as specified by iocharset=). -- Pali Rohár pali.rohar@xxxxxxxxx
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