clipboard: fix a bug when pasting image to LibreOffice

While here, embed correct file size in BMP file header.

Fixes:          neutrinolabs#3102
Sponsored by:   Krämer Pferdesport GmbH & Co KG

(cherry picked from commit 4968a34)

sesman/chansrv/clipboard.c

@@ -183,13 +183,21 @@ x-special/gnome-copied-files
 
 static char g_bmp_image_header[] =
 {
-    /* this is known to work */
-    //0x42, 0x4d, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
-
-    /* THIS IS BEING SENT BY WIN2008 */
-    0x42, 0x4d, 0x16, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x36, 0x00, 0x00, 0x00
+    /*
+     * Bitmap file header
+     * https://en.wikipedia.org/wiki/BMP_file_format#Bitmap_file_header
+     *
+     * NOTE: All of the integer values are stored in little-endian format
+     */
+    0x42, 0x4d,                 /* signature */
+    0x00, 0x00, 0x00, 0x00,     /* file size, to be filled later */
+    0x00, 0x00,                 /* reserved 1 */
+    0x00, 0x00,                 /* reserved 2 */
+    0x36, 0x00, 0x00, 0x00      /* offset to pixel array (14 + 40 bytes) */
 };
 
+#define BMPFILEHEADER_LEN sizeof(g_bmp_image_header)
+
 extern int g_cliprdr_chan_id;   /* in chansrv.c */
 
 extern Display *g_display;      /* in xcommon.c */
@@ -1186,6 +1194,8 @@ clipboard_process_data_response_for_image(struct stream *s,
 {
     XSelectionRequestEvent *lxev;
     int len;
+    char bmp_file_header[BMPFILEHEADER_LEN] = { 0 };
+    uint32_t bmp_size; /* file size stored in bmp file header */
 
     LOG_DEVEL(LOG_LEVEL_DEBUG, "clipboard_process_data_response_for_image: "
               "CLIPRDR_DATA_RESPONSE_FOR_IMAGE");
@@ -1199,20 +1209,35 @@ clipboard_process_data_response_for_image(struct stream *s,
     {
         return 0;
     }
+
     g_free(g_clip_c2s.data);
-    g_clip_c2s.data = (char *) g_malloc(len + 14, 0);
+    g_clip_c2s.data = (char *) g_malloc(len + BMPFILEHEADER_LEN, 0);
     if (g_clip_c2s.data == 0)
     {
         g_clip_c2s.total_bytes = 0;
         return 0;
     }
-    g_clip_c2s.total_bytes = len;
+    g_clip_c2s.total_bytes = len + BMPFILEHEADER_LEN;
     g_clip_c2s.read_bytes_done = g_clip_c2s.total_bytes;
-    g_memcpy(g_clip_c2s.data, g_bmp_image_header, 14);
-    in_uint8a(s, g_clip_c2s.data + 14, len);
+
+    /* assemble bmp header */
+    g_memcpy(bmp_file_header, g_bmp_image_header, BMPFILEHEADER_LEN);
+    bmp_size = (uint32_t) g_clip_c2s.total_bytes;
+#if defined(L_ENDIAN)
+    for (int i = 2, j = 0; i < 6; i++, j += 8)
+#else
+    for (int i = 5, j = 0; i >= 2; i--, j += 8)
+#endif
+    {
+        bmp_file_header[i] = ((bmp_size >> j) & 0xff);
+    }
+
+    g_memcpy(g_clip_c2s.data, bmp_file_header, BMPFILEHEADER_LEN);
+    in_uint8a(s, g_clip_c2s.data + BMPFILEHEADER_LEN, len);
     LOG_DEVEL(LOG_LEVEL_DEBUG, "clipboard_process_data_response_for_image: calling "
               "clipboard_provide_selection_c2s");
     clipboard_provide_selection_c2s(lxev, lxev->target);
+
     return 0;
 }