Re: FS_IOC_GETFSLABEL and FS_IOC_SETFSLABEL
From: Pali RohÃr
Date: Thu Jan 02 2020 - 17:08:07 EST
On Thursday 02 January 2020 13:57:54 Darrick J. Wong wrote:
> On Wed, Jan 01, 2020 at 07:39:20PM +0100, Pali RohÃr 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?
Yes I can! But I need to process also codepage= transformation (details
in email <20191228200523.eaxpwxkpswzuihow@pali>). And I already have
this implementation in progress.
> > > 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
Theodore suggested to use UTF-8 encoding for FS_IOC_GETFSLABEL. And I
suggested to use iocharset= encoding for FS_IOC_GETFSLABEL. You said to
use for VFAT "same code that does the name encoding", so if I'm
understanding correctly, yes it is the same thing (as VFAT use
iocharset= and codepage= mount options for name encoding). Right?
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