[PATCH RFC 02/12] xprtrdma: Replace use of xdr_stream_pos in rpcrdma_marshal_req

[Chuck Lever <chuck.lever@oracle.com>]

This is a latent bug. xdr_stream_pos works by subtracting
xdr_stream::nwords from xdr_buf::len. But xdr_stream::nwords is not
initialized by xdr_init_encode().

It works today only because all fields in rpcrdma_req::rl_stream
are initialized to zero by rpcrdma_req_create, making the
subtraction in xdr_stream_pos always a no-op.

I found this issue via code inspection. It was introduced by commit
39f4cd9e9982 ("xprtrdma: Harden chunk list encoding against send
buffer overflow"), but the code has changed enough since then that
this fix can't be automatically applied to stable.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/sunrpc/xprtrdma/rpc_rdma.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c b/net/sunrpc/xprtrdma/rpc_rdma.c
index 85115a2..608b9ac 100644
--- a/net/sunrpc/xprtrdma/rpc_rdma.c
+++ b/net/sunrpc/xprtrdma/rpc_rdma.c
@@ -867,9 +867,9 @@ static bool rpcrdma_prepare_msg_sges(struct rpcrdma_xprt *r_xprt,
 	if (ret)
 		goto out_err;
 
-	trace_xprtrdma_marshal(rqst, xdr_stream_pos(xdr), rtype, wtype);
+	trace_xprtrdma_marshal(rqst, req->rl_hdrbuf.len, rtype, wtype);
 
-	ret = rpcrdma_prepare_send_sges(r_xprt, req, xdr_stream_pos(xdr),
+	ret = rpcrdma_prepare_send_sges(r_xprt, req, req->rl_hdrbuf.len,
 					&rqst->rq_snd_buf, rtype);
 	if (ret)
 		goto out_err;