xfs: loudly complain about defunct mount options

Apparently we can never deprecate mount options in this project, because
it will invariably turn out that some foolish userspace depends on some
behavior and break. From Oleksandr Natalenko:

In v6.18, the attr2 XFS mount option is removed. This may silently
break system boot if the attr2 option is still present in /etc/fstab
for rootfs.

Consider Arch Linux that is being set up from scratch with / being
formatted as XFS. The genfstab command that is used to generate
/etc/fstab produces something like this by default:

/dev/sda2 on / type xfs (rw,relatime,attr2,discard,inode64,logbufs=8,logbsize=32k,noquota)

Once the system is set up and rebooted, there's no deprecation warning
seen in the kernel log:

# cat /proc/cmdline
root=UUID=77b42de2-397e-47ee-a1ef-4dfd430e47e9 rootflags=discard rd.luks.options=discard quiet

# dmesg | grep -i xfs
[ 2.409818] SGI XFS with ACLs, security attributes, realtime, scrub, repair, quota, no debug enabled
[ 2.415341] XFS (sda2): Mounting V5 Filesystem 77b42de2-397e-47ee-a1ef-4dfd430e47e9
[ 2.442546] XFS (sda2): Ending clean mount

Although as per the deprecation intention, it should be there.

Vlastimil (in Cc) suggests this is because xfs_fs_warn_deprecated()
doesn't produce any warning by design if the XFS FS is set to be
rootfs and gets remounted read-write during boot. This imposes two
problems:

1) a user doesn't see the deprecation warning; and
2) with v6.18 kernel, the read-write remount fails because of unknown
attr2 option rendering system unusable:

systemd[1]: Switching root.
systemd-remount-fs[225]: /usr/bin/mount for / exited with exit status 32.

# mount -o rw /
mount: /: fsconfig() failed: xfs: Unknown parameter 'attr2'.

Thorsten (in Cc) suggested reporting this as a user-visible regression.

From my PoV, although the deprecation is in place for 5 years already,
it may not be visible enough as the warning is not emitted for rootfs.
Considering the amount of systems set up with XFS on /, this may
impose a mass problem for users.

Vlastimil suggested making attr2 option a complete noop instead of
removing it.

IOWs, the initrd mounts the root fs with (I assume) no mount options,
and mount -a remounts with whatever options are in fstab. However,
XFS doesn't complain about deprecated mount options during a remount, so
technically speaking we were not warning all users in all combinations
that they were heading for a cliff.

Gotcha!!

Now, how did 'attr2' get slurped up on so many systems? The old code
would put that in /proc/mounts if the filesystem happened to be in attr2
mode, even if user hadn't mounted with any such option. IOWs, this is
because someone thought it would be a good idea to advertise system
state via /proc/mounts.

The easy way to fix this is to reintroduce the four mount options but
map them to a no-op option that ignores them, and hope that nobody's
depending on attr2 to appear in /proc/mounts. (Hint: use the fsgeometry
ioctl). But we've learned our lesson, so complain as LOUDLY as possible
about the deprecation.

Lessons learned:

1. Don't expose system state via /proc/mounts; the only strings that
ought to be there are options *explicitly* provided by the user.
2. Never tidy, it's not worth the stress and irritation.

Reported-by: Vlastimil Babka <vbabka@suse.cz>
Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Cc: stable@vger.kernel.org # v6.18-rc1
Fixes: b9a176e54162f8 ("xfs: remove deprecated mount options")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>