nfsd: return RESOURCE not GARBAGE_ARGS on too many ops

A client that sends more than a hundred ops in a single compound
currently gets an rpc-level GARBAGE_ARGS error.

It would be more helpful to return NFS4ERR_RESOURCE, since that gives
the client a better idea how to recover (for example by splitting up the
compound into smaller compounds).

This is all a bit academic since we've never actually seen a reason for
clients to send such long compounds, but we may as well fix it.

While we're there, just use NFSD4_MAX_OPS_PER_COMPOUND == 16, the
constant we already use in the 4.1 case, instead of hard-coding 100.
Chances anyone actually uses even 16 ops per compound are small enough
that I think there's a neglible risk or any regression.

This fixes pynfs test COMP6.

Reported-by: "Lu, Xinyu" <luxy.fnst@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4proc.c

@@ -1703,6 +1703,9 @@ nfsd4_proc_compound(struct svc_rqst *rqstp)
 	status = nfserr_minor_vers_mismatch;
 	if (nfsd_minorversion(args->minorversion, NFSD_TEST) <= 0)
 		goto out;
+	status = nfserr_resource;
+	if (args->opcnt > NFSD_MAX_OPS_PER_COMPOUND)
+		goto out;
 
 	status = nfs41_check_op_ordering(args);
 	if (status) {

fs/nfsd/nfs4xdr.c

@@ -1918,8 +1918,13 @@ nfsd4_decode_compound(struct nfsd4_compoundargs *argp)
 
 	if (argp->taglen > NFSD4_MAX_TAGLEN)
 		goto xdr_error;
-	if (argp->opcnt > 100)
-		goto xdr_error;
+	/*
+	 * NFS4ERR_RESOURCE is a more helpful error than GARBAGE_ARGS
+	 * here, so we return success at the xdr level so that
+	 * nfsd4_proc can handle this is an NFS-level error.
+	 */
+	if (argp->opcnt > NFSD_MAX_OPS_PER_COMPOUND)
+		return 0;
 
 	if (argp->opcnt > ARRAY_SIZE(argp->iops)) {
 		argp->ops = kzalloc(argp->opcnt * sizeof(*argp->ops), GFP_KERNEL);