Re: Better interop for NFS/SMB file share mode/reservation

Trond Myklebust
2019-04-29T00:57:53Z

On Sun, 2019-04-28 at 18:33 -0400, Amir Goldstein wrote:
> On Sun, Apr 28, 2019 at 6:08 PM Trond Myklebust <
> trondmy@hammerspace.com> wrote:
> > On Sun, 2019-04-28 at 18:00 -0400, Amir Goldstein wrote:
> > > On Sun, Apr 28, 2019 at 11:06 AM Trond Myklebust
> > > <trondmy@hammerspace.com> wrote:
> > > > On Sun, 2019-04-28 at 09:45 -0400, Amir Goldstein wrote:
> > > > > On Sun, Apr 28, 2019 at 8:09 AM Jeff Layton <
> > > > > jlayton@kernel.org>
> > > > > wrote:
> > > > > > On Sat, 2019-04-27 at 16:16 -0400, Amir Goldstein wrote:
> > > > > > > [adding back samba/nfs and fsdevel]
> > > > > > >
> > > > > >
> > > > > > cc'ing Pavel too -- he did a bunch of work in this area a
> > > > > > few
> > > > > > years
> > > > > > ago.
> > > > > >
> > > > > > > On Fri, Apr 26, 2019 at 6:22 PM Jeff Layton <
> > > > > > > jlayton@kernel.org>
> > > > > > > wrote:
> > > > > > > > On Fri, 2019-04-26 at 10:50 -0400, J. Bruce Fields
> > > > > > > > wrote:
> > > > > > > > > On Fri, Apr 26, 2019 at 04:11:00PM +0200, Amir
> > > > > > > > > Goldstein
> > > > > > > > > wrote:
> > > > > > > > > > On Fri, Apr 26, 2019, 4:00 PM J. Bruce Fields <
> > > > > > > > > > bfields@fieldses.org> wrote:
> > > > > > > > > >
> > > > > > > > That said, we could also look at a vfs-level mount
> > > > > > > > option
> > > > > > > > that
> > > > > > > > would
> > > > > > > > make the kernel enforce these for any opener. That
> > > > > > > > could
> > > > > > > > also
> > > > > > > > be useful,
> > > > > > > > and shouldn't be too hard to implement. Maybe even make
> > > > > > > > it
> > > > > > > > a
> > > > > > > > vfsmount-
> > > > > > > > level option (like -o ro is).
> > > > > > > >
> > > > > > >
> > > > > > > Yeh, I am humbly going to leave this struggle to someone
> > > > > > > else.
> > > > > > > Not important enough IMO and completely independent
> > > > > > > effort to
> > > > > > > the
> > > > > > > advisory atomic open&lock API.
> > > > > >
> > > > > > Having the kernel allow setting deny modes on any open call
> > > > > > is
> > > > > > a
> > > > > > non-
> > > > > > starter, for the reasons Bruce outlined earlier. This
> > > > > > _must_ be
> > > > > > restricted in some fashion or we'll be opening up a
> > > > > > ginormous
> > > > > > DoS
> > > > > > mechanism.
> > > > > >
> > > > > > My proposal was to make this only be enforced by
> > > > > > applications
> > > > > > that
> > > > > > explicitly opt-in by setting O_SH*/O_EX* flags. It wouldn't
> > > > > > be
> > > > > > too
> > > > > > difficult to also allow them to be enforced on a per-fs
> > > > > > basis
> > > > > > via
> > > > > > mount
> > > > > > option or something. Maybe we could expand the meaning of
> > > > > > '-o
> > > > > > mand'
> > > > > > ?
> > > > > >
> > > > > > How would you propose that we restrict this?
> > > > > >
> > > > >
> > > > > Our communication channel is broken.
> > > > > I did not intend to propose any implicit locking.
> > > > > If samba and nfsd can opt-in with O_SHARE flags, I do not
> > > > > understand why a mount option is helpful for the cause of
> > > > > samba/nfsd interop.
> > > > >
> > > > > If someone else is interested in samba/local interop than
> > > > > yes, a mount option like suggested by Pavel could be a good
> > > > > option,
> > > > > but it is an orthogonal effort IMO.
> > > >
> > > > If an NFS client 'opts in' to set share deny, then that still
> > > > makes
> > > > it
> > > > a non-optional lock for the other NFS clients, because all
> > > > ordinary
> > > > open() calls will be gated by the server whether or not their
> > > > application specifies the O_SHARE flag. There is no flag in the
> > > > NFS
> > > > protocol that could tell the server to ignore deny modes.
> > > >
> > > > IOW: it would suffice for 1 client to use O_SHARE|O_DENY* to
> > > > opt
> > > > all
> > > > the other clients in.
> > > >
> > >
> > > Sorry for being thick, I don't understand if we are in agreement
> > > or
> > > not.
> > >
> > > My understanding is that the network file server implementations
> > > (i.e. samba, knfds, Ganesha) will always use share/deny modes.
> > > So for example nfs v3 opens will always use O_DENY_NONE
> > > in order to have correct interop with samba and nfs v4.
> > >
> > > If I am misunderstanding something, please enlighten me.
> > > If there is a reason why mount option is needed for the sole
> > > purpose
> > > of interop between network filesystem servers, please enlighten
> > > me.
> > >
> > >
> >
> > Same difference. As long as nfsd and/or Ganesha are translating
> > OPEN4_SHARE_ACCESS_READ and OPEN4_SHARE_ACCESS_WRITE into share
> > access
> > locks, then those will conflict with any deny locks set by whatever
> > application that uses them.
> >
> > IOW: any open(O_RDONLY) and open(O_RDWR) will conflict with an
> > O_DENY_READ that is set on the server, and any open(O_WRONLY) and
> > open(O_RDWR) will conflict with an O_DENY_WRITE that is set on the
> > server. There is no opt-out for NFS clients on this issue, because
> > stateful NFSv4 opens MUST set one or more of
> > OPEN4_SHARE_ACCESS_READ
> > and OPEN4_SHARE_ACCESS_WRITE.
> >
>
> Urgh! I *think* I understand the confusion.
>
> I believe Jeff was talking about implementing a mount option
> similar to -o mand for local fs on the server.
> With that mount option, *any* open() by any app of file from
> that mount will use O_DENY_NONE to interop correctly with
> network servers that explicitly opt-in for interop on share modes.
> I agree its a nice feature that is easy to implement - not important
> for first version IMO.
>
> I *think* you are talking on nfs client mount option for
> opt-in/out of share modes? there was no such intention.
>

No. I'm saying that whether you intended to or not, you _are_
implementing a mandatory lock over NFS. No talk about O_SHARE flags and
it being an opt-in process for local applications changes the fact that
non-local applications (i.e. the ones that count ☺) are being subjected
to a mandatory lock with all the potential for denial of service that
implies.
So we need a mechanism beyond O_SHARE in order to ensure this system
cannot be used on sensitive files that need to be accessible to all. It
could be an export option, or a mount option, or it could be a more
specific mechanism (e.g. the setgid with no execute mode bit as using
in POSIX mandatory locks).

--
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com