NFSD: Extract the svcxdr_init_encode() helper

NFSD initializes an encode xdr_stream only after the RPC layer has
already inserted the RPC Reply header. Thus it behaves differently
than xdr_init_encode does, which assumes the passed-in xdr_buf is
entirely devoid of content.

nfs4proc.c has this server-side stream initialization helper, but
it is visible only to the NFSv4 code. Move this helper to a place
that can be accessed by NFSv2 and NFSv3 server XDR functions.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4proc.c

@@ -2262,25 +2262,6 @@ static bool need_wrongsec_check(struct svc_rqst *rqstp)
 	return !(nextd->op_flags & OP_HANDLES_WRONGSEC);
 }
 
-static void svcxdr_init_encode(struct svc_rqst *rqstp,
-			       struct nfsd4_compoundres *resp)
-{
-	struct xdr_stream *xdr = &resp->xdr;
-	struct xdr_buf *buf = &rqstp->rq_res;
-	struct kvec *head = buf->head;
-
-	xdr->buf = buf;
-	xdr->iov = head;
-	xdr->p   = head->iov_base + head->iov_len;
-	xdr->end = head->iov_base + PAGE_SIZE - rqstp->rq_auth_slack;
-	/* Tail and page_len should be zero at this point: */
-	buf->len = buf->head[0].iov_len;
-	xdr_reset_scratch_buffer(xdr);
-	xdr->page_ptr = buf->pages - 1;
-	buf->buflen = PAGE_SIZE * (1 + rqstp->rq_page_end - buf->pages)
-		- rqstp->rq_auth_slack;
-}
-
 #ifdef CONFIG_NFSD_V4_2_INTER_SSC
 static void
 check_if_stalefh_allowed(struct nfsd4_compoundargs *args)
@@ -2335,10 +2316,14 @@ nfsd4_proc_compound(struct svc_rqst *rqstp)
 	struct nfsd_net *nn = net_generic(SVC_NET(rqstp), nfsd_net_id);
 	__be32		status;
 
-	svcxdr_init_encode(rqstp, resp);
-	resp->tagp = resp->xdr.p;
+	resp->xdr = &rqstp->rq_res_stream;
+
+	/* reserve space for: NFS status code */
+	xdr_reserve_space(resp->xdr, XDR_UNIT);
+
+	resp->tagp = resp->xdr->p;
 	/* reserve space for: taglen, tag, and opcnt */
-	xdr_reserve_space(&resp->xdr, 8 + args->taglen);
+	xdr_reserve_space(resp->xdr, XDR_UNIT * 2 + args->taglen);
 	resp->taglen = args->taglen;
 	resp->tag = args->tag;
 	resp->rqstp = rqstp;
@@ -2444,7 +2429,7 @@ nfsd4_proc_compound(struct svc_rqst *rqstp)
 encode_op:
 		if (op->status == nfserr_replay_me) {
 			op->replay = &cstate->replay_owner->so_replay;
-			nfsd4_encode_replay(&resp->xdr, op);
+			nfsd4_encode_replay(resp->xdr, op);
 			status = op->status = op->replay->rp_status;
 		} else {
 			nfsd4_encode_operation(resp, op);

fs/nfsd/nfs4state.c

@@ -2903,7 +2903,7 @@ gen_callback(struct nfs4_client *clp, struct nfsd4_setclientid *se, struct svc_r
 static void
 nfsd4_store_cache_entry(struct nfsd4_compoundres *resp)
 {
-	struct xdr_buf *buf = resp->xdr.buf;
+	struct xdr_buf *buf = resp->xdr->buf;
 	struct nfsd4_slot *slot = resp->cstate.slot;
 	unsigned int base;
 
@@ -2973,7 +2973,7 @@ nfsd4_replay_cache_entry(struct nfsd4_compoundres *resp,
 			 struct nfsd4_sequence *seq)
 {
 	struct nfsd4_slot *slot = resp->cstate.slot;
-	struct xdr_stream *xdr = &resp->xdr;
+	struct xdr_stream *xdr = resp->xdr;
 	__be32 *p;
 	__be32 status;
 
@@ -3708,7 +3708,7 @@ nfsd4_sequence(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 {
 	struct nfsd4_sequence *seq = &u->sequence;
 	struct nfsd4_compoundres *resp = rqstp->rq_resp;
-	struct xdr_stream *xdr = &resp->xdr;
+	struct xdr_stream *xdr = resp->xdr;
 	struct nfsd4_session *session;
 	struct nfs4_client *clp;
 	struct nfsd4_slot *slot;

fs/nfsd/nfssvc.c

@@ -997,7 +997,7 @@ int nfsd_dispatch(struct svc_rqst *rqstp, __be32 *statp)
 	 * NFSv4 does some encoding while processing
 	 */
 	p = resv->iov_base + resv->iov_len;
-	resv->iov_len += sizeof(__be32);
+	svcxdr_init_encode(rqstp);
 
 	*statp = proc->pc_func(rqstp);
 	if (*statp == rpc_drop_reply || test_bit(RQ_DROPME, &rqstp->rq_flags))
@@ -1052,7 +1052,7 @@ int nfssvc_decode_voidarg(struct svc_rqst *rqstp, __be32 *p)
  */
 int nfssvc_encode_voidres(struct svc_rqst *rqstp, __be32 *p)
 {
-        return xdr_ressize_check(rqstp, p);
+	return 1;
 }
 
 int nfsd_pool_stats_open(struct inode *inode, struct file *file)

fs/nfsd/xdr4.h

@@ -698,7 +698,7 @@ struct nfsd4_compoundargs {
 
 struct nfsd4_compoundres {
 	/* scratch variables for XDR encode */
-	struct xdr_stream		xdr;
+	struct xdr_stream		*xdr;
 	struct svc_rqst *		rqstp;
 
 	u32				taglen;

include/linux/sunrpc/svc.h

@@ -248,6 +248,7 @@ struct svc_rqst {
 	size_t			rq_xprt_hlen;	/* xprt header len */
 	struct xdr_buf		rq_arg;
 	struct xdr_stream	rq_arg_stream;
+	struct xdr_stream	rq_res_stream;
 	struct page		*rq_scratch_page;
 	struct xdr_buf		rq_res;
 	struct page		*rq_pages[RPCSVC_MAXPAGES + 1];
@@ -574,4 +575,28 @@ static inline void svcxdr_init_decode(struct svc_rqst *rqstp)
 	xdr_set_scratch_page(xdr, rqstp->rq_scratch_page);
 }
 
+/**
+ * svcxdr_init_encode - Prepare an xdr_stream for svc Reply encoding
+ * @rqstp: controlling server RPC transaction context
+ *
+ */
+static inline void svcxdr_init_encode(struct svc_rqst *rqstp)
+{
+	struct xdr_stream *xdr = &rqstp->rq_res_stream;
+	struct xdr_buf *buf = &rqstp->rq_res;
+	struct kvec *resv = buf->head;
+
+	xdr_reset_scratch_buffer(xdr);
+
+	xdr->buf = buf;
+	xdr->iov = resv;
+	xdr->p   = resv->iov_base + resv->iov_len;
+	xdr->end = resv->iov_base + PAGE_SIZE - rqstp->rq_auth_slack;
+	buf->len = resv->iov_len;
+	xdr->page_ptr = buf->pages - 1;
+	buf->buflen = PAGE_SIZE * (1 + rqstp->rq_page_end - buf->pages);
+	buf->buflen -= rqstp->rq_auth_slack;
+	xdr->rqst = NULL;
+}
+
 #endif /* SUNRPC_SVC_H */