Fix RPC calls: ATTRIBUTE buf not null but length 0

Let's add a support for cases when the buffer != NULL but the
length is 0. According to Oasis, buffer = NULL and length = 0
means a query of the length so the subsequent calls with a
buffer != NULL should fill then buffer when length is long enough.
If not, according to Oasis, one should get a CKR_BUFFER_TOO_SMALL.

See the previous commit for IN_ULONG_BUFFER(). This patch is
follow a similar design pattern.

Fix: issue p11-glue#257

p11-kit/rpc-message.c

@@ -241,14 +241,20 @@ p11_rpc_message_write_attribute_buffer (p11_rpc_message *msg,
 	/* Write the number of items */
 	p11_rpc_buffer_add_uint32 (msg->output, num);
 
+	/* Is arr not null ? */
+	p11_rpc_buffer_add_byte (msg->output, arr ? 0 : 1);
+
 	for (i = 0; i < num; ++i) {
 		attr = &(arr[i]);
 
 		/* The attribute type */
 		p11_rpc_buffer_add_uint32 (msg->output, attr->type);
 
+		/* rpc_server.c:value_is_null, is the value buffer not NULL ? */
+		p11_rpc_buffer_add_byte (msg->output, attr->pValue ? 0 : 1);
+
 		/* And the attribute buffer length */
-		p11_rpc_buffer_add_uint32 (msg->output, attr->pValue ? attr->ulValueLen : 0);
+		p11_rpc_buffer_add_uint32 (msg->output, attr->ulValueLen);
 	}
 
 	return !p11_buffer_failed (msg->output);

p11-kit/rpc-server.c

@@ -237,6 +237,7 @@ proto_read_attribute_buffer (p11_rpc_message *msg,
 {
 	CK_ATTRIBUTE_PTR attrs;
 	uint32_t n_attrs, i;
+	uint8_t attrs_is_null;
 	uint32_t value;
 
 	assert (msg != NULL);
@@ -251,32 +252,47 @@ proto_read_attribute_buffer (p11_rpc_message *msg,
 	if (!p11_rpc_buffer_get_uint32 (msg->input, &msg->parsed, &n_attrs))
 		return PARSE_ERROR;
 
+	/* Is attrs pointer null or not ? */
+	if (!p11_rpc_buffer_get_byte (msg->input, &msg->parsed, &attrs_is_null))
+		return PARSE_ERROR;
+
+	if (attrs_is_null) {
+		*result = NULL;
+		*n_result = n_attrs;
+		return CKR_OK;
+	}
+
 	/* Allocate memory for the attribute structures */
 	attrs = p11_rpc_message_alloc_extra (msg, n_attrs * sizeof (CK_ATTRIBUTE));
 	if (attrs == NULL)
 		return CKR_DEVICE_MEMORY;
 
 	/* Now go through and fill in each one */
 	for (i = 0; i < n_attrs; ++i) {
+		uint8_t value_is_null;
 
 		/* The attribute type */
 		if (!p11_rpc_buffer_get_uint32 (msg->input, &msg->parsed, &value))
 			return PARSE_ERROR;
 
 		attrs[i].type = value;
 
+		/* Is the value buffer not NULL ?  */
+		if (!p11_rpc_buffer_get_byte (msg->input, &msg->parsed, &value_is_null))
+			return PARSE_ERROR;
+
 		/* The number of bytes to allocate */
 		if (!p11_rpc_buffer_get_uint32 (msg->input, &msg->parsed, &value))
 			return PARSE_ERROR;
 
-		if (value == 0) {
+		attrs[i].ulValueLen = value;
+
+		if (value_is_null) {
 			attrs[i].pValue = NULL;
-			attrs[i].ulValueLen = 0;
 		} else {
 			attrs[i].pValue = p11_rpc_message_alloc_extra (msg, value);
 			if (!attrs[i].pValue)
 				return CKR_DEVICE_MEMORY;
-			attrs[i].ulValueLen = value;
 		}
 	}