Send 1 byte over clipboard max to trigger reject

If vmside clipboard is over the maximum limit, sending one byte over
maximum limit should trigger inter-vm clipboard rejection instead of
truncation.

Supplements: QubesOS/qubes-issues#9296

gui-agent/vmside.c

@@ -980,14 +980,23 @@ static void process_xevent_configure(Ghandles * g, XID window,
     send_pixmap_grant_refs(g, window);
 }
 
-static void send_clipboard_data(libvchan_t *vchan, XID window, char *data, uint32_t len)
+static void send_clipboard_data(libvchan_t *vchan, XID window, char *data, uint32_t len, int protocol_version)
 {
     struct msg_hdr hdr;
     hdr.type = MSG_CLIPBOARD_DATA;
     hdr.window = window;
     if (len > MAX_CLIPBOARD_SIZE)
     {
-        len = MAX_CLIPBOARD_SIZE;
+        if (protocol_version >= QUBES_GUID_MIN_CLIPBOARD_4X) {
+            // xside is capable of receiving (up to) 4X of the previous size.
+            // it is also smarter. send one byte over the Maximum limit.
+            // A simple sign for xside to reject it.
+            len = MAX_CLIPBOARD_SIZE + 1;
+        } else {
+            // The dumb case. Truncate the data to the old size. User will lose
+            // some inter-vm clipboard data without being notified.
+            len = 65000;
+        }
     }
     hdr.untrusted_len = len;
     write_struct(vchan, hdr);
@@ -1052,7 +1061,7 @@ static void process_xevent_selection(Ghandles * g, XSelectionEvent * ev)
                 g->utf8_string_atom, g->qprop,
                 g->stub_win, ev->time);
     else
-        send_clipboard_data(g->vchan, g->stub_win, (char *) data, len);
+        send_clipboard_data(g->vchan, g->stub_win, (char *) data, len, g->protocol_version);
     /* even if the clipboard owner does not support UTF8 and we requested
        XA_STRING, it is fine - ascii is legal UTF8 */
     XFree(data);
@@ -2097,7 +2106,7 @@ static void handle_clipboard_req(Ghandles * g, XID winid)
         fprintf(stderr, "clipboard req, owner=0x%x\n",
                 (int) owner);
     if (owner == None) {
-        send_clipboard_data(g->vchan, winid, NULL, 0);
+        send_clipboard_data(g->vchan, winid, NULL, 0, g->protocol_version);
         return;
     }
     XConvertSelection(g->display, Clp, g->targets, g->qprop, g->stub_win, g->time);