unionfs, tmpfs, and xattrs

[Bill Nottingham]

Stephen Smalley (sds at tycho.nsa.gov) said: 
> > Can you, in the kernel, easily check to see if xattrs are supported for a
> > filesystem? No.
> 
> SELinux xattrs are _always_ supported for every filesystem by
> definition, because their values are actually provided by the SELinux
> module.  All data must be labeled.

Then the filesystem should have a getxattr() method... that's all
that I'm saying here. Having filesystems that return xattrs, but,
claim they don't in their own methods, is somewhat disingenious.

> > > > I could theoretically patch unionfs to call the vfs method, but... ew.
> > 
> > listxattr isn't exported as a vfs method, and even just using the vfs_get/setxattr
> > methods doesn't appear to work correctly.
> 
> Not sure what issue you are encountering with using vfs_getxattr; nfsd
> uses it.

Locks. Could be some other stuck locks, will investigate some more.

> For listxattr, introducing a vfs_listxattr should be
> straightforward and reasonable if there is a user for it; I think the
> absence is just due to a lack of a user.

If we're going to have the filesystem's own getxattr() methods not
actually tell whether the FS returns an xattr, I think wrapping all
the calls is needed...

Bill