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

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Date:   Tue, 07 May 2019 10:24:58 +1000
From:   NeilBrown <neilb@...e.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>,
        Patrick Plagwitz <Patrick_Plagwitz@....de>,
        "linux-unionfs\@vger.kernel.org" <linux-unionfs@...r.kernel.org>,
        Linux NFS list <linux-nfs@...r.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [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@...redi.hu> wrote:
>> >> > On Tue, Dec 6, 2016 at 12:24 AM, Andreas Grünbacher
>> >> > <andreas.gruenbacher@...il.com> wrote:
>> >> >> 2016-12-06 0:19 GMT+01:00 Andreas Grünbacher <andreas.gruenbacher@...il.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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