Re: F_RDLCK lock to FreeBSD NFS server fails to R/O target file [PATCH]

John wrote:
> After pondering the best way to allow the VOP_ACCESS() call to
> only query for the permissions really needed, I've come up with
> a patch that minimally adds one parameter to the nlm_get_vfs_state()
> function call with the lock type from the original argp.
> 
> http://people.freebsd.org/~jwd/nlm_prot_impl.c.accmode.patch
> 
> I'd appreciate a review and seeing what might be required to commit
> this prior to 9 release.
> 
> Thanks,
> John
> 
> ----- John's Original Message -----
> > Hi Fellow NFS'ers,
> >
> >    I believe I have found the problem we've been having with read
> >    locks
> > while attaching to a FreeBSD NFS server.
> >
> >    In sys/nlm/nlm_prot_impl.c, function nlm_get_vfs_state(), there
> >    is a call
> > to VOP_ACCESS() as follows:
> >
> >         /*
> >          * Check cred.
> >          */
> >         NLM_DEBUG(3, "nlm_get_vfs_state(): Calling
> >         VOP_ACCESS(VWRITE) with cred->cr_uid=%d\n",cred->cr_uid);
> >         error = VOP_ACCESS(vs->vs_vp, VWRITE, cred, curthread);
> >         if (error) {
> >                 NLM_DEBUG(3, "nlm_get_vfs_state(): caller_name = %s
> >                 VOP_ACCESS() returns %d\n",
> >                 host->nh_caller_name, error);
> >                 goto out;
> >         }
> >
> >   The file being accessed is read only to the user, and open()ed
> >   with
> > O_RDONLY. The lock being requested is for a read.
> >
> > fd = open(filename, O_RDONLY, 0);
> > ...
> >
> > lblk.l_type = F_RDLCK;
> > lblk.l_start = 0;
> > lblk.l_whence= SEEK_SET;
> > lblk.l_len = 0;
> > lblk.l_pid = 0;
> > rc = fcntl(fd, F_SETLK, &lblk);
> >
> >    Running the above from a remote system, the lock call fails with
> > errno set to ENOLCK. Given cred->cr_uid comes in as 227 which is
> > my uid on the remote system. Since the file is R/O to me, and the
> > VOP_ACCESS() is asking for VWRITE, it fails with errno 13, EACCES,
> > Permission denied.
> >
> >    The above operations work correctly to some of our other
> > favorite big-name nfs vendors :-)
> >
> >    Opinions on the "correct" way to fix this?
> >
> > 1. Since we're only asking for a read lock, why do we need to ask
> >    for VWRITE? I may not understand an underlying requirement for
> >    the VWRITE so please feel free to educate me if needed.
> >
> >    Something like: request == F_RDLCK ? VREAD : VWRITE
> >    (need to figure out where to get the request from in this
> >    context).
> >
> > 2. Attempt VWRITE, fallback to VREAD... seems off to me though.
> >
I think I prefer this approach because it will never fail unless it
would fail without the patch. For the case where the client uid has
write but not read permission and attempts an F_RDLCK, I believe your
patch would fail the request whereas it will succeed without your patch.
(Although I'll admit I didn't actually test this.;-)

Doing VOP_ACCESS(..VWRITE..) first and only doing a VOP_ACCESS(..VREAD..)
if the VOP_ACCESS(..VWRITE..) fails will preserve current behaviour fot
the non-failing case, but allow an F_RDLCK for uids with read-only access.

I've attached a patch that does this variant. John, could you test this
patch?

And does anyone have an opinion w.r.t. which variant of the patch is
more appropriate?

Thanks in advance for your help, rick

> > 3. Other?
> >
> >    I appreciate any thoughts on this.
> >
> > Thanks,
> > John
> >
> >    While they might not follow style(9) completely, I've uploaded
> > my patch to nlm_prot.impl.c with the NLM_DEBUG() calls i've added.
> > I'd appreciate it if someone would consider committing them so
> > who ever debugs this file next will have them available.
> >
> > http://people.freebsd.org/~jwd/nlm_prot_impl.c.patch
> >
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