[nfsv4] Re: Comments for draft-rmacklem-nfsv4-posix-acls-02

[Pali Rohár <[e-mail]>]

Hello Rick, I'm sending comments for the second part of your email.

On Tuesday 06 August 2024 15:20:50 Rick Macklem wrote:
> On Tue, Aug 6, 2024 at 10:05 AM Pali Rohár <[e-mail]>
> wrote:
> > > A server that is configured for a acl_trueform_scope other that
> > > ACL_SCOPE_FILE_OBJECT MUST not support the posix_default_acl and
> > > posix_access_acl unless the true form for the file system is
> > > ACL_MODEL_POSIX_DRAFT.
> >
> > This is a restriction which basically means that no existing NFS4
> > server with native NFS4 ACL support can support "posix_access_acl".
> > In multiprotocol environment where is NFSv4.0 and NFSv4.1, the NFS4
> > server would have to provide server scope native NFS4 model (to support
> > NFSv4.1 as as your document is only for NFSv4.2) which implies that
> > server must not support POSIX ACL. And same applies in the multiprotocol
> > environment with NFS4 and SMB.
> >
> Not exactly. posix_access_acl refers to the proposed optional
> attribute for NFSv4.2 (same goes for acl_trueform_scope).
> What the server does w.r.t. "true form" is up to the server.
> Same goes for the scope of it.
> (For example, Linux server always use POSIX draft ACLs.)
> 
> The difference is that NFSv4.0 and 4.1 clients cannot know
> what the "true form" is and can only see NFSv4 ACLs (possibly
> mapped to/from POSIX draft ACLs).
> 
> What the above restriction is meant to do is allow a NFSv4.2
> client to know that the "true form" is ACL_MODEL_POSIX_DRAFT
> when the scope is ACL_SCOPE_FILE_SYSTEM or ACL_SCOPE_SERVER
> by seeing that posix_access_acl and posix_default_acl are
> supported_attrs for the file system.
> --> Note that acl_trueform_scope is "per server", so a client
>     only needs to GETATTR this attribute once for the server.
>     Then, if that is ACL_SCOPE_FILE_SYSTEM or ACL_SCOPE_SERVER,
>     it can tell that the "true form" is ACL_MODEL_POSIX_DRAFT
>     as soon as it sees posix_access_acl and posix_default_acl
>     in supported_attrs.
>     --> It does not have to bother to check acl_trueform,
>         which would often result in an extra RPC.

Ok, I will try to describe this issue in different way on example.

NFS4 client can decide that will not implement support for ACL and DACL
attributes because, NFS4 client is POSIX only and mapping from NFS4 to
POSIX cause issues for that client.

NFS4 client will implement this extension, so it would understand only
POSIX_ACCESS_ACL and POSIX_DEFAULT_ACL attributes.

There are existing NFS4 server which implements multiprotocol
environment. Such server would always use NFS4 true form ACLs and
nothing different (for example because POSIX ACL cannot store SMB ACLs).
So such server would return for acl_trueform_scope ACL_SCOPE_SERVER (or
ACL_SCOPE_FILE_SYSTEM in case server supports more filesystems and want
such feature).

Now according to your description, such server "MUST not support the
posix_default_acl and posix_access_acl" (because its true form is NFS4).

So it means for NFS4 clients which would implement only POSIX_ACCESS_ACL
and POSIX_DEFAULT_ACL attributes, those NFS4 servers are not able to
serve any ACL.

And if customers of such NFS4 server start asking for "ACL compatibility
with that NFS4 client" then such NFS4 server would ignore the part "MUST
not support the posix_default_acl and posix_access_acl" and will start
providing these attributes via some translation or mapping (it would be
even worse mess).

This is why I think it is too restrictive. It is disallowing NFS4
servers to provide compatibility for POSIX NFS4 clients.

> 
> > I think that this is too strict and what would happen is either that
> > this extension would not be implemented OR section option (which I think
> > everybody would do) is ignoring this restriction.
> >
> As above, this does not define what the server does for NFSv4.0
> and NFSv4.1, it just simplifies what a NFSv4.2 client needs to
> check, if it knows about and the server supports the extension
> proposed by this draft.
> 
> >
> > I guess that for compatibility with FreeBSD and Linux, servers would
> > implement it even on NFS4 server scope model.
> >
> Not sure what you mean by this.

I explained above the whole example. I did not mentioned in it explicit
client and servers as it nos tight to FreeBSD or Linux. Hopefully it
would be clear now what I mean.

> Linux always uses a "true form"
> of POSIX ACL and FreeBSD supports one of POSIX ACL or NFSv4 ACL
> on a per-file system basic (at least for now).
> - That is not changed by this extension. All this extension does
>   is allow NFSv4.2 clients to use the server more effectively
>   by choosing to get/set the "true form" and avoiding mapping,
>   assuming the client and server both implement the extension.
> Other file systems (IBM's GPFS seems to be the extant example
> I am aware of) also supports a "true form" on a per-file basis.
> (Again this is a server file system configuration choice and
> is not affected by whether or not this extension is supported
> by a NFSv4.2 server.)
> 
> 
> >
> > And I have there two additional points without quoting your document.
> >
> > 1)
> >
> > Your document does not describe what existing RFC 8881 acl and dacl
> > attributes should return when user sets new "posix_default_acl" or
> > "posix_access_acl" attribute. Also what should happen when another
> > client tries to change acl or dacl attribute when POSIX ACL is already
> > active.
> >
> The intent (if that is not clear, the words need to be improved)
> is that the server file system shall have one "true form" for each
> file object. If the attribute(s) for the other "true form" is used
> on the object (which assumes the server has chosen to put attributes for
> both in supported_attrs for the file syetem, the GETATTR/READIR/SETATTR
> will use mapping to translate it to/from the "true form".

Ok, I think this is important to know and was not clear for me.
So it means that if server's true form is POSIX and NFS4 client call
GETATTR(DACL) command then server is required to map its POSIX ACL into
NFS4 ACL. I think that this should be explicitly mentioned to prevent
any confusion.

> The intent is to discourage use of mapping. As such, the client using
> these extensions can avoid that by determining what the "true form" is
> for the object and using the appropriate attributes.

Now I understand. So NFS4 client without this extension would use just
ACL and DACL attributes. And NFS4 client with this extension can use
POSIX_ACCESS_ACL if wants and can benefit from it.

> My thinking is that servers like FreeBSD (where the "true form" is per-file
> system and mapping is not supported) would:
> (A) - Support posix_access_acl and posix_default_acl, but not acl, for file
>   systems that use POSIX ACLs as their "true form".

Why not ACL/DACL attribute at least via mapping? This will degrade NFS4
clients which use NFS4 true form ACL or similar (e.g. Windows or SMB).

RFC 8881 suggest implementations to implement ACL and DACL attributes
for access control. So servers which will not implement ACL and DACL
attribute would for access control would not be compatible with RFC 8881
clients which expects access control.

> (B) - Support acl, but not posix_access_acl and posix_default_acl, for file
>   systems that ise NFSv4 ACLs as their "true form".

That makes sense. Export POSIX_ACCESS_ACL/POSIX_DEFAULT_ACL only in case
POSIX is server's true form.

> For Linux servers, they might choose (A) above, or they might choose
> to support all of acl, posix_access_acl and posix_default_acl, where acl
> is supported via the mapping algorithms they already use.
> 
> It is the case of a per-file scope where things get difficult:
> The server will probably want to support all of acl, posix_access_acl
> and posix_default_acl, but since the "true form" is defined a one
> model for each file, then how should the attribute(s) for the other
> model be handled?
> - I would like to avoid mapping, so I think being able to return
>   "no ACL" would be preferred.

I think that this is against RFC 8881 where it is expected that server
construct ACL attribute which will describe the current access
permissions.

Returning "no ACL" for NFS4 clients which do not understand this
extension would be very confusing. User will ask for ACL, user will see
that it is empty and would think that nobody has access to that file.
And then would be surprised that somebody can access that file just
because some optional extension attribute returns some more information
about access, which the client does not implement. This can lead also to
security or audit issues.

> - I had thought a zero length ACL would work for this, but that no
>   longer appears to be the case.

Yes, it would not work.

> Maybe, defining an ACL with a single ACE that says "not valid" would
> work?

I think not. ACL and DACL attribute should always say current access
permissions for the object. And if server is using some other (e.g.
POSIX or any other strange scheme) then server should provide the best
effort translation for such clients.

> This could be the reply for GETATTR/READDIR and could be used to
> "erase" the ACL for SETATTR.
> --> I think this is what the next draft will propose.
> 
> 
> > If interop between mode, dacl and posix_default_acl, posix_access_acl
> > attributes is not explicitly defined in specification then every server
> > implementation will invent its own rules and it would just lead to the
> > mess in multiprotocol environment where clients would not be sure what
> > can happen. Like it is already with "mode" attribute in NFS v4.1 (see 2).
> >
> For posix_access_acl and posix_default_acl the intent is that the
> POSIX draft specification defines the interaction between mode and
> the POSIX ACL.

Ok. But the problem is that POSIX mode != NFS4 mode. Below in 2)
I described one difference. And because POSIX ACL does not define
interaction between NFS4 mode and POSIX ACL then I think it is required
to define it in extension specification to make it clear.

> For NFSv4 ACLs, you are correct. It is a mess.
> And I wish David N. the best of luck w.r.t. cleaning it up.

Interesting, is David working on cleaning it up? If you are interesting
I can provide feedback what needs to be fixed (something I have already
written in my first email to the list).

> (OpenZFS already has a knob that specifies 4 different ways
> of doing this. Another knob setting may be needed.)
> 
> Thanks for the comments, rick
> 
> >
> > 2)
> >
> > Then there is a problem with "group" bits of "mode" attribute.
> > In RFC 8881 section 6.2.4 is written:
> >
> > "Bits MODE4_RGRP, MODE4_WGRP, and MODE4_XGRP apply to principals
> > identified in the owner_group attribute but who are not identified in
> > the owner attribute."
> >
> > And in section 6.3.2.1 is written:
> >
> > "Some server implementations also add bits permitted to named users and
> > groups to the group bits (MODE4_RGRP, MODE4_WGRP, and MODE4_XGRP).
> > Implementations are discouraged from doing this."
> > "The same user confusion seen when fetching the mode also results if
> > setting the mode does not effectively control permissions for the owner,
> > group, and other users; this motivates some of the requirements that
> > follow."
> >
> > POSIX ACL with at least one named user or named group requires that
> > mode group bits refers to POSIX ACL mask, and not to the POSIX ACL
> > group_obj (which represents owner_group).
> >
> > Those RFC 8881 parts of NFS mode attribute are basically not compatible
> > with POSIX ACL. Your extension does not mention this problem and does
> > not handle it.
> >
> > Existing NFS4 servers which follows the RFC 8881 meaning of mode group
> > bits, would not be able to implement this your extension for POSIX ACL,
> > because existing servers cannot change behavior of mode attribute due
> > backward compatibility. I think that this is a big problem.
> > Servers in multiprotocol environment mostly follows the RFC 8881 advice
> > to never return something like "POSIX mask" in group bits.
> >
> > How to solve this problem. One option could be including all mode bits
> > into your new "posix_access_acl" attribute and let NFS4 clients to use
> > only "posix_access_acl" on chown operation, and never use existing
> > "mode" attribute at all (if the extension is supported by server).
> >
> > Another option could be to introduce a new "posix_mode" attribute which
> > would be strictly POSIX and old "mode" attribute can stay as before for
> > backward compatibility. Again clients would not use "mode" attribute
> > anymore.
> >
> >
> > Regards,
> > Pali
> >