On Tue, Dec 29, 2020 at 04:25:47AM -0800, L.A. Walsh wrote:

xfs_io checks for CAP_SYS_ADMIN in order to open a
file_by_inode -- however, if the file one is opening
is owned by the user performing the call, the call should
not fail.

No. xfs_open_by_handle() requires root permissions because it
bypasses lots of security checks, such as parent directory
permissions, ACLs and security labels.

e.g. backups under a root-only directory heirarchy should not be
accessible to users because users are not allowed to traverse into
those root:root 0700 backup directories because permissions on the 
directory inodes do not allow non-root users to enter them.

Hence ...

(i.e. it opens the user's own file).

... the user doesn't actually own that file, even though it has
their own UID in it...

It gets rid of some unnecessary error messages if you
run xfs_restore to restore one of your own files.

That's not really a user case xfs_restore is intended to support.
It's an admin tool to be run by admins, not end users....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com