Re: [PATCH v1 3/3] xfs: Add a testcase to check remount with noattr2 on a v5 xfs

Dave Chinner
2025-02-12T21:47:08Z

On Wed, Feb 12, 2025 at 12:39:58PM +0000, Nirjhar Roy (IBM) wrote:
> This testcase reproduces the following bug:
> Bug:
> mount -o remount,noattr2 <device> <mount_point> succeeds
> unexpectedly on a v5 xfs when CONFIG_XFS_SUPPORT_V4 is set.

AFAICT, this is expected behaviour. Remount intentionally ignores
options that cannot be changed.

> Ideally the above mount command should always fail with a v5 xfs
> filesystem irrespective of whether CONFIG_XFS_SUPPORT_V4 is set
> or not.

No, we cannot fail remount when invalid options are passed to the
kernel by the mount command for historical reasons. i.e. the mount
command has historically passed invalid options to the kernel on
remount, but expects the kernel to apply just the new options that
they understand and ignore the rest without error.

i.e. to keep compatibility with older userspace, we cannot fail a
remount because userspace passed an option the kernel does not
understand or cannot change.

Hence, in this case, XFS emits a deprecation warning for the noattr2
mount option on remount (because it is understood), then ignores
because it it isn't a valid option that remount can change.

-Dave.
--
Dave Chinner
david@fromorbit.com