GUACAMOLE-2002: Warn on clipboard truncation instead of silently dropping data

src/protocols/rdp/channels/cliprdr.c

@@ -403,8 +403,19 @@ static UINT guac_rdp_cliprdr_format_data_request(CliprdrClientContext* cliprdr,
      * requested */
     BYTE* start = (BYTE*) output;
     guac_iconv_read* local_reader = settings->normalize_clipboard ? GUAC_READ_UTF8_NORMALIZED : GUAC_READ_UTF8;
-    guac_iconv(local_reader, &input, clipboard->clipboard->length,
-            remote_writer, &output, output_buf_size);
+
+    /* A zero return indicates the output buffer filled before all input was
+     * consumed (truncation). Charset expansion (UTF-8 -> UTF-16, CRLF
+     * translation, or multi-byte characters such as emoji) can outgrow the
+     * configured clipboard buffer, so warn the operator rather than silently
+     * sending truncated data. */
+    if (!guac_iconv(local_reader, &input, clipboard->clipboard->length,
+            remote_writer, &output, output_buf_size)) {
+        guac_client_log(client, GUAC_LOG_WARNING, "CLIPRDR: Outbound clipboard "
+                "data was truncated to fit the %d-byte clipboard buffer. "
+                "Increase \"clipboard-buffer-size\" to transfer larger "
+                "clipboard content.", output_buf_size);
+    }
 
     CLIPRDR_FORMAT_DATA_RESPONSE data_response = {
         .requestedFormatData = (BYTE*) start,
@@ -513,11 +524,17 @@ static UINT guac_rdp_cliprdr_format_data_response(CliprdrClientContext* cliprdr,
     data_len = format_data_response->dataLen;
     #endif
 
-    /* Convert, store, and forward the clipboard data received from RDP
-     * server */
-    if (guac_iconv(remote_reader, &input, data_len,
-            GUAC_WRITE_UTF8, &output, output_buf_size)) {
-        int length = strnlen(received_data, output_buf_size);
+    /* Convert the clipboard data received from the RDP server. A zero return
+     * indicates the output buffer filled before all input was consumed
+     * (truncation) due to charset expansion. */
+    int converted = guac_iconv(remote_reader, &input, data_len,
+            GUAC_WRITE_UTF8, &output, output_buf_size);
+
+    /* Store and forward whatever was converted, even if truncated. Partial
+     * clipboard text is still useful, so we deliver it and warn the operator
+     * rather than dropping the paste entirely. */
+    int length = strnlen(received_data, output_buf_size);
+    if (length > 0) {
         guac_common_clipboard_reset(clipboard->clipboard, "text/plain");
         guac_common_clipboard_append(clipboard->clipboard, received_data, length);
         guac_common_clipboard_send(clipboard->clipboard, client);
@@ -530,6 +547,13 @@ static UINT guac_rdp_cliprdr_format_data_response(CliprdrClientContext* cliprdr,
                     clipboard->clipboard->length);
     }
 
+    if (!converted) {
+        guac_client_log(client, GUAC_LOG_WARNING, "CLIPRDR: Inbound clipboard "
+                "data was truncated to fit the %d-byte clipboard buffer. "
+                "Increase \"clipboard-buffer-size\" to receive larger "
+                "clipboard content.", output_buf_size);
+    }
+
     guac_mem_free(received_data);
 
     return CHANNEL_RC_OK;

src/protocols/vnc/clipboard.c

@@ -191,9 +191,16 @@ int guac_vnc_clipboard_end_handler(guac_user* user, guac_stream* stream) {
     char* output = output_data;
     guac_iconv_write* writer = vnc_client->clipboard_writer;
 
-    /* Convert clipboard contents */
-    guac_iconv(GUAC_READ_UTF8, &input, clipboard->length,
-               writer, &output, output_buf_size);
+    /* Convert clipboard contents. A zero return indicates the output buffer
+     * filled before all input was consumed (truncation) due to charset
+     * expansion; warn rather than silently sending truncated data. */
+    if (!guac_iconv(GUAC_READ_UTF8, &input, clipboard->length,
+               writer, &output, output_buf_size)) {
+        guac_client_log(client, GUAC_LOG_WARNING, "VNC: Outbound clipboard "
+                "data was truncated to fit the %d-byte clipboard buffer. "
+                "Increase \"clipboard-buffer-size\" to transfer larger "
+                "clipboard content.", output_buf_size);
+    }
 
     SendClientCutText(rfb_client, output_data, output - output_data);
 
@@ -224,9 +231,16 @@ void guac_vnc_cut_text(rfbClient* client, const char* text, int textlen) {
     char* output = received_data;
     guac_iconv_read* reader = vnc_client->clipboard_reader;
 
-    /* Convert clipboard contents */
-    guac_iconv(reader, &input, textlen,
-               GUAC_WRITE_UTF8, &output, output_buf_size);
+    /* Convert clipboard contents. A zero return indicates the output buffer
+     * filled before all input was consumed (truncation) due to charset
+     * expansion; warn rather than silently delivering truncated data. */
+    if (!guac_iconv(reader, &input, textlen,
+               GUAC_WRITE_UTF8, &output, output_buf_size)) {
+        guac_client_log(gc, GUAC_LOG_WARNING, "VNC: Inbound clipboard data "
+                "was truncated to fit the %d-byte clipboard buffer. Increase "
+                "\"clipboard-buffer-size\" to receive larger clipboard "
+                "content.", output_buf_size);
+    }
 
     /* Send converted data */
     guac_common_clipboard_reset(vnc_client->clipboard, "text/plain");