XFS reports lchmod failure, but changes file system contents

[Al Viro]

On Wed, Feb 12, 2020 at 08:15:08PM +0100, Florian Weimer wrote:

> | Further, I've found some inconsistent behavior with ext4: chmod on the
> | magic symlink fails with EOPNOTSUPP as in Florian's test, but fchmod
> | on the O_PATH fd succeeds and changes the symlink mode. This is with
> | 5.4. Cany anyone else confirm this? Is it a problem?
> 
> It looks broken to me because fchmod (as an inode-changing operation)
> is not supposed to work on O_PATH descriptors.

Why?  O_PATH does have an associated inode just fine; where does
that "not supposed to" come from?