Re: [PATCH] xfs: don't expose misaligned extszinherit hints to userspace

On Fri, Jul 09, 2021 at 07:25:09AM +0100, Christoph Hellwig wrote:

On Thu, Jul 08, 2021 at 09:12:09PM -0700, Darrick J. Wong wrote:

quoted

From: Darrick J. Wong <djwong@kernel.org>

Commit 603f000b15f2 changed xfs_ioctl_setattr_check_extsize to reject an
attempt to set an EXTSZINHERIT extent size hint on a directory with
RTINHERIT set if the hint isn't a multiple of the realtime extent size.
However, I have recently discovered that it is possible to change the
realtime extent size when adding a rt device to a filesystem, which
means that the existence of directories with misaligned inherited hints
is not an accident.

As a result, it's possible that someone could have set a valid hint and
added an rt volume with a different rt extent size, which invalidates
the ondisk hints.  After such a sequence, FSGETXATTR will report a
misaligned hint, which FSSETXATTR will trip over, causing confusion if
the user was doing the usual GET/SET sequence to change some other
attribute.  Change xfs_fill_fsxattr to omit the hint if it isn't aligned
properly.

Looks sensible, but maybe we need a pr_info_ratelimited to inform
the user of this case?

Why?  Now that I've established that the system administrator is and
always has been allowed to invalidate the extent size hints when
realtime volumes are in play, I don't think we need to spam the kernel
log about the admin's strange choices.

The only reason I put that xfs_info_once thing in 603f00 is because I
mistakenly thought that the only way we could end up with a fs like that
was due to gaps in user input sanitization, i.e. fs isn't supposed to be
weird, but it is anyway.

--D