Re: FS_IOC_GETFSLABEL and FS_IOC_SETFSLABEL

[Darrick J. Wong]

On Wed, Jan 01, 2020 at 07:39:20PM +0100, Pali Rohár wrote:
> 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.

"Null terminated sequence of bytes*" is more or less what xfsprogs do,
and it looks like btrfs does that as well.

(* with the idiotic exception that if the label is exactly 256 bytes long
then the array is not required to have a null terminator, because btrfs
encoded that quirk of their ondisk format into the API. <grumble>)

So for VFAT, I think you can use the same code that does the name
encoding transformations for iocharset= to handle labels, right?

> > 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=).

I'm confused by this statement... but I think we're saying the same
thing?

--D

> 
> -- 
> Pali Rohár
> pali.rohar@xxxxxxxxx