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

From: NeilBrown
Date: Wed May 01 2019 - 22:03:03 EST

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@xxxxxxxxxx> wrote:
>> > On Tue, Dec 6, 2016 at 12:24 AM, Andreas GrÃnbacher
>> > <andreas.gruenbacher@xxxxxxxxx> wrote:
>> >> 2016-12-06 0:19 GMT+01:00 Andreas GrÃnbacher <andreas.gruenbacher@xxxxxxxxx>:
>> >
>> >>> 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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