linux-ext4 - Re: [PATCH 2/6] fs: FS_IOC

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Date: Tue, 6 Feb 2024 04:00:13 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Amir Goldstein <amir73il@...il.com>
Cc: Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-btrfs@...r.kernel.org, linux-xfs@...r.kernel.org, 
	linux-ext4@...r.kernel.org, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Dave Chinner <dchinner@...hat.com>, "Darrick J. Wong" <djwong@...nel.org>, 
	Theodore Ts'o <tytso@....edu>, linux-fsdevel@...r.kernel.or, Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [PATCH 2/6] fs: FS_IOC_GETUUID

On Tue, Feb 06, 2024 at 10:24:45AM +0200, Amir Goldstein wrote:
> On Tue, Feb 6, 2024 at 12:49 AM Kent Overstreet
> <kent.overstreet@...ux.dev> wrote:
> >
> > On Tue, Feb 06, 2024 at 09:17:58AM +1100, Dave Chinner wrote:
> > > On Mon, Feb 05, 2024 at 03:05:13PM -0500, Kent Overstreet wrote:
> > > > Add a new generic ioctls for querying the filesystem UUID.
> > > >
> > > > These are lifted versions of the ext4 ioctls, with one change: we're not
> > > > using a flexible array member, because UUIDs will never be more than 16
> > > > bytes.
> > > >
> > > > This patch adds a generic implementation of FS_IOC_GETFSUUID, which
> > > > reads from super_block->s_uuid; FS_IOC_SETFSUUID is left for individual
> > > > filesystems to implement.
> > > >
> 
> It's fine to have a generic implementation, but the filesystem should
> have the option to opt-in for a specific implementation.
> 
> There are several examples, even with xfs and btrfs where ->s_uuid
> does not contain the filesystem's UUID or there is more than one
> uuid and ->s_uuid is not the correct one to expose to the user.

Yeah, some of you were smoking some good stuff from the stories I've
been hearing...

> A model like ioctl_[gs]etflags() looks much more appropriate
> and could be useful for network filesystems/FUSE as well.

A filesystem needs to store two UUIDs (that identify the filesystem as a
whole).

 - Your internal UUID, which can never change because it's referenced in
   various other on disk data structures
 - Your external UUID, which identifies the filesystem to the outside
   world. Users want to be able to change this - which is why it has to
   be distinct from the internal UUID.

The internal UUID must never be exposed to the outside world, and that
includes the VFS; storing your private UUID in sb->s_uuid is wrong -
separation of concerns.

yes, I am aware of fscrypt, and yes, someone's going to have to fix
that.

This interface is only for the external/public UUID.