Re: [PATCH 3/3] xfs: actually account for quota changes in xfs_swap_extents

[Christoph Hellwig <hch@infradead.org>]

On Wed, May 06, 2020 at 09:34:24AM -0700, Darrick J. Wong wrote:
> On Wed, May 06, 2020 at 07:57:28AM -0700, Christoph Hellwig wrote:
> > On Mon, May 04, 2020 at 06:10:29PM -0700, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@oracle.com>
> > > 
> > > Currently, xfs_swap_extents neither checks for sufficient quota
> > > reservation nor does it actually update quota counts when swapping the
> > > extent forks.  While the primary known user of extent swapping (xfs_fsr)
> > > is careful to ensure that the user/group/project ids of both files
> > > match, this is not required by the kernel.  Consequently, unprivileged
> > > userspace can cause the quota counts to be incorrect.
> > 
> > Wouldn't be the right fix to enforce an id match?  I think that is a
> > very sensible limitation.
> 
> One could do that, but at a cost of breaking any userspace program that
> was using XFS_IOC_SWAPEXT and was not aware that the ids had to match
> (possibly due to the lack of documentation...)

I don't really expect that to be the case.  I'd throw in the check
and a printk_once warning, and I bet a beer at the next conference
(if there ever is one :)) that no one will trigger it.