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Date:   Thu, 02 May 2019 12:02:33 +1000
From:   NeilBrown <>
To:     "J. Bruce Fields" <>,
        Andreas Gruenbacher <>
Cc:     Miklos Szeredi <>,
        Andreas Grünbacher 
        Patrick Plagwitz <>,
        "linux-unionfs\" <>,
        Linux NFS list <>,
        Linux FS-devel Mailing List <>,
        Linux Kernel Mailing List <>
Subject: Re: [PATCH] overlayfs: ignore empty NFSv4 ACLs in ext4 upperdir

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 <> wrote:
>> > On Tue, Dec 6, 2016 at 12:24 AM, Andreas Grünbacher
>> > <> wrote:
>> >> 2016-12-06 0:19 GMT+01:00 Andreas Grünbacher <>:
>> >
>> >>> 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
 Unfortunately it didn't resolve anything.  Maybe I can help kick things

 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".  If only standard Unix permissions
 are used, there is no risk, so this seems reasonable.

 So I would like to propose a new option for overlayfs
    nocopyupacl:   when overlayfs is copying a file (or directory etc)
        from the lower filesystem to the upper filesystem, it does not
        copy extended attributes with the "system." prefix.  These are
        used for storing ACL information and this is sometimes not
        compatible between different filesystem types (e.g. ext4 and
        NFSv4).  Standard Unix ownership permission flags (rwx) *are*
        copied so this option does not risk giving away inappropriate
        permissions unless the lowerfs uses unusual ACLs.

 Miklos: would you find that acceptable?



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