XFS reports lchmod failure, but changes file system contents

[Florian Weimer]

* 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?

It fails on most file systems right now.  I thought that was expected.
Other system calls (fsetxattr IIRC) do not work on O_PATH descriptors,
either.  I assumed that an O_PATH descriptor was not intending to
confer that capability.  Even openat fails.

Although fchmod does succeed on read-only descriptors, which is a bit
strange.