qga: remove undefined behavior in ga_install_service()

We shouldn't snprintf() from a buffer to the same buffer.
Signed-off-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

qga/service-win32.c

@@ -39,34 +39,36 @@ int ga_install_service(const char *path, const char *logfile)
 {
     SC_HANDLE manager;
     SC_HANDLE service;
-    TCHAR cmdline[MAX_PATH];
+    TCHAR module_fname[MAX_PATH];
+    GString *cmdline;
 
-    if (GetModuleFileName(NULL, cmdline, MAX_PATH) == 0) {
+    if (GetModuleFileName(NULL, module_fname, MAX_PATH) == 0) {
         printf_win_error("No full path to service's executable");
         return EXIT_FAILURE;
     }
 
-    _snprintf(cmdline, MAX_PATH - strlen(cmdline), "%s -d", cmdline);
+    cmdline = g_string_new(module_fname);
+    g_string_append(cmdline, " -d");
 
     if (path) {
-        _snprintf(cmdline, MAX_PATH - strlen(cmdline), "%s -p %s", cmdline, path);
+        g_string_append_printf(cmdline, " -p %s", path);
     }
     if (logfile) {
-        _snprintf(cmdline, MAX_PATH - strlen(cmdline), "%s -l %s -v",
-            cmdline, logfile);
+        g_string_append_printf(cmdline, " -l %s -v", logfile);
     }
 
-    g_debug("service's cmdline: %s", cmdline);
+    g_debug("service's cmdline: %s", cmdline->str);
 
     manager = OpenSCManager(NULL, NULL, SC_MANAGER_ALL_ACCESS);
     if (manager == NULL) {
         printf_win_error("No handle to service control manager");
+        g_string_free(cmdline, TRUE);
         return EXIT_FAILURE;
     }
 
     service = CreateService(manager, QGA_SERVICE_NAME, QGA_SERVICE_DISPLAY_NAME,
         SERVICE_ALL_ACCESS, SERVICE_WIN32_OWN_PROCESS, SERVICE_AUTO_START,
-        SERVICE_ERROR_NORMAL, cmdline, NULL, NULL, NULL, NULL, NULL);
+        SERVICE_ERROR_NORMAL, cmdline->str, NULL, NULL, NULL, NULL, NULL);
 
     if (service) {
         SERVICE_DESCRIPTION desc = { (char *)QGA_SERVICE_DESCRIPTION };
@@ -80,6 +82,7 @@ int ga_install_service(const char *path, const char *logfile)
     CloseServiceHandle(service);
     CloseServiceHandle(manager);
 
+    g_string_free(cmdline, TRUE);
     return (service == NULL);
 }