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    SubjectRe: [PATCH] overlayfs: ignore empty NFSv4 ACLs in ext4 upperdir
    On Fri, May 03 2019, J. Bruce Fields wrote:

    > On Thu, May 02, 2019 at 12:02:33PM +1000, NeilBrown wrote:
    >> On Tue, Dec 06 2016, J. Bruce Fields wrote:
    >>
    >> > On Tue, Dec 06, 2016 at 02:18:31PM +0100, Andreas Gruenbacher wrote:
    >> >> On Tue, Dec 6, 2016 at 11:08 AM, Miklos Szeredi <miklos@szeredi.hu> wrote:
    >> >> > On Tue, Dec 6, 2016 at 12:24 AM, Andreas Grünbacher
    >> >> > <andreas.gruenbacher@gmail.com> wrote:
    >> >> >> 2016-12-06 0:19 GMT+01:00 Andreas Grünbacher <andreas.gruenbacher@gmail.com>:
    >> >> >
    >> >> >>> It's not hard to come up with a heuristic that determines if a
    >> >> >>> system.nfs4_acl value is equivalent to a file mode, and to ignore the
    >> >> >>> attribute in that case. (The file mode is transmitted in its own
    >> >> >>> attribute already, so actually converting .) That way, overlayfs could
    >> >> >>> still fail copying up files that have an actual ACL. It's still an
    >> >> >>> ugly hack ...
    >> >> >>
    >> >> >> Actually, that kind of heuristic would make sense in the NFS client
    >> >> >> which could then hide the "system.nfs4_acl" attribute.
    >> >> >
    >> >> > Even simpler would be if knfsd didn't send the attribute if not
    >> >> > necessary. Looks like there's code actively creating the nfs4_acl on
    >> >> > the wire even if the filesystem had none:
    >> >> >
    >> >> > pacl = get_acl(inode, ACL_TYPE_ACCESS);
    >> >> > if (!pacl)
    >> >> > pacl = posix_acl_from_mode(inode->i_mode, GFP_KERNEL);
    >> >> >
    >> >> > What's the point?
    >> >>
    >> >> That's how the protocol is specified.
    >> >
    >> > Yep, even if we could make that change to nfsd it wouldn't help the
    >> > client with the large number of other servers that are out there
    >> > (including older knfsd's).
    >> >
    >> > --b.
    >> >
    >> >> (I'm not saying that that's very helpful.)
    >> >>
    >> >> Andreas
    >>
    >> Hi everyone.....
    >> I have a customer facing this problem, and so stumbled onto the email
    >> thread.
    >> Unfortunately it didn't resolve anything. Maybe I can help kick things
    >> along???
    >>
    >> The core problem here is that NFSv4 and ext4 use different and largely
    >> incompatible ACL implementations. There is no way to accurately
    >> translate from one to the other in general (common specific examples
    >> can be converted).
    >>
    >> This means that either:
    >> 1/ overlayfs cannot use ext4 for upper and NFS for lower (or vice
    >> versa) or
    >> 2/ overlayfs need to accept that sometimes it cannot copy ACLs, and
    >> that is OK.
    >>
    >> Silently not copying the ACLs is probably not a good idea as it might
    >> result in inappropriate permissions being given away. So if the
    >> sysadmin wants this (and some clearly do), they need a way to
    >> explicitly say "I accept the risk".
    >
    > So, I feel like silently copying ACLs up *also* carries a risk, if that
    > means switching from server-enforcement to client-enforcement of those
    > permissions.

    Interesting perspective .... though doesn't NFSv4 explicitly allow
    client-side ACL enforcement in the case of delegations?
    Not sure how relevant that is....

    It seems to me we have two options:
    1/ declare the NFSv4 doesn't work as a lower layer for overlayfs and
    recommend people use NFSv3, or
    2/ Modify overlayfs to work with NFSv4 by ignoring nfsv4 ACLs either
    2a/ always - and ignore all other acls and probably all system. xattrs,
    or
    2b/ based on a mount option that might be
    2bi/ general "noacl" or might be
    2bii/ explicit "noxattr=system.nfs4acl"

    I think that continuing to discuss the miniature of the options isn't
    going to help. No solution is perfect - we just need to clearly
    document the implications of whatever we come up with.

    I lean towards 2a, but I be happy with with any '2' and '1' won't kill
    me.

    Do we have a vote? Or does someone make an executive decision??

    NeilBrown
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