NFSv4: OPEN must handle the NFS4ERR_IO return code correctly

decode_op_hdr() cannot distinguish between an XDR decoding error and
the perfectly valid errorcode NFS4ERR_IO. This is normally not a
problem, but for the particular case of OPEN, we need to be able
to increment the NFSv4 open sequence id when the server returns
a valid response.
Reported-by: J Bruce Fields <bfields@fieldses.org>
Link: http://lkml.kernel.org/r/20131204210356.GA19452@fieldses.orgSigned-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: stable@vger.kernel.org

fs/nfs/nfs4xdr.c

@@ -3097,7 +3097,8 @@ static int decode_compound_hdr(struct xdr_stream *xdr, struct compound_hdr *hdr)
 	return -EIO;
 }
 
-static int decode_op_hdr(struct xdr_stream *xdr, enum nfs_opnum4 expected)
+static bool __decode_op_hdr(struct xdr_stream *xdr, enum nfs_opnum4 expected,
+		int *nfs_retval)
 {
 	__be32 *p;
 	uint32_t opnum;
@@ -3107,19 +3108,32 @@ static int decode_op_hdr(struct xdr_stream *xdr, enum nfs_opnum4 expected)
 	if (unlikely(!p))
 		goto out_overflow;
 	opnum = be32_to_cpup(p++);
-	if (opnum != expected) {
+	if (unlikely(opnum != expected))
+		goto out_bad_operation;
+	nfserr = be32_to_cpup(p);
+	if (nfserr == NFS_OK)
+		*nfs_retval = 0;
+	else
+		*nfs_retval = nfs4_stat_to_errno(nfserr);
+	return true;
+out_bad_operation:
 	dprintk("nfs: Server returned operation"
 		" %d but we issued a request for %d\n",
 			opnum, expected);
-		return -EIO;
-	}
-	nfserr = be32_to_cpup(p);
-	if (nfserr != NFS_OK)
-		return nfs4_stat_to_errno(nfserr);
-	return 0;
+	*nfs_retval = -EREMOTEIO;
+	return false;
 out_overflow:
 	print_overflow_msg(__func__, xdr);
-	return -EIO;
+	*nfs_retval = -EIO;
+	return false;
+}
+
+static int decode_op_hdr(struct xdr_stream *xdr, enum nfs_opnum4 expected)
+{
+	int retval;
+
+	__decode_op_hdr(xdr, expected, &retval);
+	return retval;
 }
 
 /* Dummy routine */
@@ -5001,10 +5015,11 @@ static int decode_open(struct xdr_stream *xdr, struct nfs_openres *res)
 	uint32_t savewords, bmlen, i;
 	int status;
 
-	status = decode_op_hdr(xdr, OP_OPEN);
-	if (status != -EIO)
+	if (!__decode_op_hdr(xdr, OP_OPEN, &status))
+		return status;
 	nfs_increment_open_seqid(status, res->seqid);
-	if (!status)
+	if (status)
+		return status;
 	status = decode_stateid(xdr, &res->stateid);
 	if (unlikely(status))
 		return status;