Re: [PATCH] overlayfs: ignore empty NFSv4 ACLs in ext4 upperdir

J. Bruce Fields
2016-12-05T16:26:00Z

On Mon, Dec 05, 2016 at 04:36:03PM +0100, Miklos Szeredi wrote:
> On Mon, Dec 5, 2016 at 4:19 PM, J. Bruce Fields <bfields@fieldses.org> wrote:
> >> Can NFS people comment on this? Where does the nfs4_acl come from?
> >
> > This is the interface the NFS client provides for applications to modify
> > NFSv4 ACLs on servers that support them.
>
> Fine, but why are we seeing this xattr on exports where no xattrs are
> set on the exported fs?

I don't know. I took another look at the original patch and don't see
any details on the server setup: which server is it (knfsd, ganesha,
netapp, ...)? How is it configured?

> >> What can overlayfs do if it's a non-empty ACL?
> >
> > As little as possible. You can't copy it up, can you? So any attempt
> > to support it is going to be incomplete.
>
> Right.
>
> >
> >> Does knfsd translate posix ACL into NFS acl? If so, we can translate
> >> back. Should we do a generic POSIX<->NFS acl translator?
> >
> > knsd does translate between POSIX and NFSv4 ACLs. It's a complicated
>
> This does explain the nfs4_acl xattr on the client. Question: if it's
> empty, why have it at all?

I'm honestly not sure what's going on there. I'd be curious to see a
network trace if possible.

> > algorithm, and lossy (in the NFSv4->POSIX direction). The client
> > developers have been understandably reluctant to have anything to do
> > with it.
> >
> > So, I think listxattr should omit system.nfs4_acl, and attempts to
> > set/get the attribute should error out. The same should apply to any
> > "system." attribute not supported by both filesystems, I think?
>
> Basically that's what happens now. The problem is that nfsv4 mounts
> seem always have these xattrs, even when the exported fs doesn't have
> anything.

I said "both", that's a logical "and". Whether or not nfs claims
support would then be irrelevant in this case, since ext4 doesn't
support system.nfs4_acl.

> We could do the copy up even if the NFS4->POSIX translation was
> possible (which is the case with POSIX ACL translated by knfsd). We'd
> just get back the original ACL, so that's OK.

Note that knfsd is an exception, most NFSv4-acl-supporting servers
aren't translating from POSIX ACLs.

> NFS is only supported as lower (read-only) layer, so we don't care
> about changing the ACL on the server.

Out of curiosity, how do you check permissions after copy up?

The client doesn't do much permissions-checking normally, because it's
hard to get right--even in the absence of ACLs, it may not understand
the server's owners and groups completely.

I guess that's fine, you may be happy to let people write to the file
without permissions to the lower file, since the writes aren't going
there anyway.

So, I don't know what want here.

You're not going to want to use the ACL for actual permission-checking,
and you can't modify it, so it doesn't seem very useful.

--b.