linux-kernel - Re: FS_IOC_GETFSLABEL and FS_IOC

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Message-ID: <20200107231522.GC472641@magnolia>
Date:   Tue, 7 Jan 2020 15:15:22 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Pali Rohár <pali.rohar@...il.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        Eric Sandeen <sandeen@...hat.com>,
        Andreas Dilger <adilger@...ger.ca>,
        David Sterba <dsterba@...e.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: FS_IOC_GETFSLABEL and FS_IOC_SETFSLABEL

On Thu, Jan 02, 2020 at 11:08:00PM +0100, Pali Rohár wrote:
> On Thursday 02 January 2020 13:57:54 Darrick J. Wong wrote:
> > 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?
> 
> Yes I can! But I need to process also codepage= transformation (details
> in email <20191228200523.eaxpwxkpswzuihow@...i>). And I already have
> this implementation in progress.

<nod>

> > > > 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?
> 
> 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?

Right.

--D

> -- 
> Pali Rohár
> pali.rohar@...il.com