Tools: hv: update buffer handling in hv_fcopy_daemon

Currently this warning is triggered when compiling hv_fcopy_daemon:

hv_fcopy_daemon.c:216:4: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
    kernel_modver = *(__u32 *)buffer;

Convert the send/receive buffer to a union and pass individual members as
needed. This also gives the correct size for the buffer.
Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/hv/hv_fcopy_daemon.c

@@ -138,14 +138,17 @@ void print_usage(char *argv[])
 
 int main(int argc, char *argv[])
 {
-	int fcopy_fd, len;
+	int fcopy_fd;
 	int error;
 	int daemonize = 1, long_index = 0, opt;
 	int version = FCOPY_CURRENT_VERSION;
-	char *buffer[4096 * 2];
-	struct hv_fcopy_hdr *in_msg;
-	int in_handshake = 1;
+	union {
+		struct hv_fcopy_hdr hdr;
+		struct hv_start_fcopy start;
+		struct hv_do_fcopy copy;
 		__u32 kernel_modver;
+	} buffer = { };
+	int in_handshake = 1;
 
 	static struct option long_options[] = {
 		{"help",	no_argument,	   0,  'h' },
@@ -195,32 +198,31 @@ int main(int argc, char *argv[])
 		 * In this loop we process fcopy messages after the
 		 * handshake is complete.
 		 */
-		len = pread(fcopy_fd, buffer, (4096 * 2), 0);
+		ssize_t len;
+
+		len = pread(fcopy_fd, &buffer, sizeof(buffer), 0);
 		if (len < 0) {
 			syslog(LOG_ERR, "pread failed: %s", strerror(errno));
 			exit(EXIT_FAILURE);
 		}
 
 		if (in_handshake) {
-			if (len != sizeof(kernel_modver)) {
+			if (len != sizeof(buffer.kernel_modver)) {
 				syslog(LOG_ERR, "invalid version negotiation");
 				exit(EXIT_FAILURE);
 			}
-			kernel_modver = *(__u32 *)buffer;
 			in_handshake = 0;
-			syslog(LOG_INFO, "kernel module version: %d",
-			       kernel_modver);
+			syslog(LOG_INFO, "kernel module version: %u",
+			       buffer.kernel_modver);
 			continue;
 		}
 
-		in_msg = (struct hv_fcopy_hdr *)buffer;
-
-		switch (in_msg->operation) {
+		switch (buffer.hdr.operation) {
 		case START_FILE_COPY:
-			error = hv_start_fcopy((struct hv_start_fcopy *)in_msg);
+			error = hv_start_fcopy(&buffer.start);
 			break;
 		case WRITE_TO_FILE:
-			error = hv_copy_data((struct hv_do_fcopy *)in_msg);
+			error = hv_copy_data(&buffer.copy);
 			break;
 		case COMPLETE_FCOPY:
 			error = hv_copy_finished();
@@ -231,7 +233,7 @@ int main(int argc, char *argv[])
 
 		default:
 			syslog(LOG_ERR, "Unknown operation: %d",
-				in_msg->operation);
+				buffer.hdr.operation);
 
 		}