NFSD: Fix zero-length NFSv3 WRITEs

The Linux NFS server currently responds to a zero-length NFSv3 WRITE
request with NFS3ERR_IO. It responds to a zero-length NFSv4 WRITE
with NFS4_OK and count of zero.

RFC 1813 says of the WRITE procedure's @count argument:

count
         The number of bytes of data to be written. If count is
         0, the WRITE will succeed and return a count of 0,
         barring errors due to permissions checking.

RFC 8881 has similar language for NFSv4, though NFSv4 removed the
explicit @count argument because that value is already contained in
the opaque payload array.

The synthetic client pynfs's WRT4 and WRT15 tests do emit zero-
length WRITEs to exercise this spec requirement. Commit fdec611
("nfsd4: zero-length WRITE should succeed") addressed the same
problem there with the same fix.

But interestingly the Linux NFS client does not appear to emit zero-
length WRITEs, instead squelching them. I'm not aware of a test that
can generate such WRITEs for NFSv3, so I wrote a naive C program to
generate a zero-length WRITE and test this fix.

Fixes: 8154ef2 ("NFSD: Clean up legacy NFS WRITE argument XDR decoders")
Reported-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: stable@vger.kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs3proc.c

@@ -202,15 +202,11 @@ nfsd3_proc_write(struct svc_rqst *rqstp)
  fh_copy(&resp->fh, &argp->fh);
  resp->committed = argp->stable;
  nvecs = svc_fill_write_vector(rqstp, &argp->payload);
- if (!nvecs) {
- resp->status = nfserr_io;
- goto out;
- }
+
  resp->status = nfsd_write(rqstp, &resp->fh, argp->offset,
  rqstp->rq_vec, nvecs, &cnt,
  resp->committed, resp->verf);
  resp->count = cnt;
-out:
  return rpc_success;
 }
 

fs/nfsd/nfsproc.c

@@ -235,10 +235,6 @@ nfsd_proc_write(struct svc_rqst *rqstp)
  argp->len, argp->offset);
 
  nvecs = svc_fill_write_vector(rqstp, &argp->payload);
- if (!nvecs) {
- resp->status = nfserr_io;
- goto out;
- }
 
  resp->status = nfsd_write(rqstp, fh_copy(&resp->fh, &argp->fh),
  argp->offset, rqstp->rq_vec, nvecs,
@@ -247,7 +243,6 @@ nfsd_proc_write(struct svc_rqst *rqstp)
  resp->status = fh_getattr(&resp->fh, &resp->stat);
  else if (resp->status == nfserr_jukebox)
  return rpc_drop_reply;
-out:
  return rpc_success;
 }