SUNRPC: Fix encoding of accepted but unsuccessful RPC replies

Jiri Slaby says:
> I bisected to this ... as it breaks nfs3-only servers in 6.3.
> I.e. /etc/nfs.conf containing:
> [nfsd]
> vers4=no
>
> The client sees:
>  mount("10.0.2.15:/tmp", "/mnt", "nfs", 0, "vers=4.2,addr=10.0.2.15,clientad"...) = -1 EIO (Input/output error)
>  write(2, "mount.nfs: mount system call fai"..., 45
>  mount.nfs: mount system call failed for /mnt
>
> And the kernel says:
>  nfs4_discover_server_trunking unhandled error -5. Exiting with error EIO
Reported-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://bugzilla.suse.com/show_bug.cgi?id=1210995
Fixes: 4bcf0343 ("SUNRPC: Set rq_accept_statp inside ->accept methods")
Tested-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svc.c

@@ -1416,7 +1416,7 @@ svc_process_common(struct svc_rqst *rqstp)
 	/* Only RPCv2 supported */
 	xdr_stream_encode_u32(xdr, RPC_VERSION);
 	xdr_stream_encode_u32(xdr, RPC_VERSION);
-	goto sendit;
+	return 1;	/* don't wrap */
 
 err_bad_auth:
 	dprintk("svc: authentication failed (%d)\n",
@@ -1432,7 +1432,7 @@ svc_process_common(struct svc_rqst *rqstp)
 err_bad_prog:
 	dprintk("svc: unknown program %d\n", rqstp->rq_prog);
 	serv->sv_stats->rpcbadfmt++;
-	xdr_stream_encode_u32(xdr, RPC_PROG_UNAVAIL);
+	*rqstp->rq_accept_statp = rpc_prog_unavail;
 	goto sendit;
 
 err_bad_vers:
@@ -1440,7 +1440,12 @@ svc_process_common(struct svc_rqst *rqstp)
 		       rqstp->rq_vers, rqstp->rq_prog, progp->pg_name);
 
 	serv->sv_stats->rpcbadfmt++;
-	xdr_stream_encode_u32(xdr, RPC_PROG_MISMATCH);
+	*rqstp->rq_accept_statp = rpc_prog_mismatch;
+
+	/*
+	 * svc_authenticate() has already added the verifier and
+	 * advanced the stream just past rq_accept_statp.
+	 */
 	xdr_stream_encode_u32(xdr, process.mismatch.lovers);
 	xdr_stream_encode_u32(xdr, process.mismatch.hivers);
 	goto sendit;
@@ -1449,19 +1454,19 @@ svc_process_common(struct svc_rqst *rqstp)
 	svc_printk(rqstp, "unknown procedure (%d)\n", rqstp->rq_proc);
 
 	serv->sv_stats->rpcbadfmt++;
-	xdr_stream_encode_u32(xdr, RPC_PROC_UNAVAIL);
+	*rqstp->rq_accept_statp = rpc_proc_unavail;
 	goto sendit;
 
 err_garbage_args:
 	svc_printk(rqstp, "failed to decode RPC header\n");
 
 	serv->sv_stats->rpcbadfmt++;
-	xdr_stream_encode_u32(xdr, RPC_GARBAGE_ARGS);
+	*rqstp->rq_accept_statp = rpc_garbage_args;
 	goto sendit;
 
 err_system_err:
 	serv->sv_stats->rpcbadfmt++;
-	xdr_stream_encode_u32(xdr, RPC_SYSTEM_ERR);
+	*rqstp->rq_accept_statp = rpc_system_err;
 	goto sendit;
 }