FUSE: regression when clearing setuid bits on chown

[Jeff Layton <jlayton@redhat.com>]

Hi Miklos,

I think we've found a "regression" that has crept in due to this patch:

commit a09f99eddef44035ec764075a37bace8181bec38
Author: Miklos Szeredi <mszeredi@redhat.com>
Date:   Sat Oct 1 07:32:32 2016 +0200

    fuse: fix killing s[ug]id in setattr
    
Basically, the pjdfstests set the ownership of a file to 06555, and then
chowns it (as root) to a new uid/gid. Prior to the patch above, fuse
would send down a setattr with both the uid/gid change and a new mode.
Now, it just sends down the uid/gid change.

Technically this is NOTABUG, since POSIX doesn't _require_ that we clear
these bits for a privileged process, but Linux (wisely) has done that
and I think we don't want to change that behavior here.

So, the issue I think is the use of should_remove_suid, which will
always return 0 when the process has CAP_FSETID. That's appropriate (I
think) for write/truncate, but not chown, where we want to ignore that.

Thoughts on the right fix here? A simple fix would be to add an
"override" bool to should_remove_suid, but maybe there's some more
elegant way to do it?

-- 
Jeff Layton <jlayton@redhat.com>