diff --git a/mkbootimg/mkbootimg.c b/mkbootimg/mkbootimg.c
index 34a879b..d598f03 100644
--- a/mkbootimg/mkbootimg.c
+++ b/mkbootimg/mkbootimg.c
@@ -114,6 +114,7 @@
     unsigned ramdisk_offset = 0x01000000;
     unsigned second_offset  = 0x00f00000;
     unsigned tags_offset    = 0x00000100;
+    size_t cmdlen;
 
     argc--;
     argv++;
@@ -192,11 +193,19 @@
 
     memcpy(hdr.magic, BOOT_MAGIC, BOOT_MAGIC_SIZE);
 
-    if(strlen(cmdline) > (BOOT_ARGS_SIZE - 1)) {
+    cmdlen = strlen(cmdline);
+    if(cmdlen > (BOOT_ARGS_SIZE + BOOT_EXTRA_ARGS_SIZE - 2)) {
         fprintf(stderr,"error: kernel commandline too large\n");
         return 1;
     }
-    strcpy((char*)hdr.cmdline, cmdline);
+    /* Even if we need to use the supplemental field, ensure we
+     * are still NULL-terminated */
+    strncpy((char *)hdr.cmdline, cmdline, BOOT_ARGS_SIZE - 1);
+    hdr.cmdline[BOOT_ARGS_SIZE - 1] = '\0';
+    if (cmdlen >= (BOOT_ARGS_SIZE - 1)) {
+        cmdline += (BOOT_ARGS_SIZE - 1);
+        strncpy((char *)hdr.extra_cmdline, cmdline, BOOT_EXTRA_ARGS_SIZE);
+    }
 
     kernel_data = load_file(kernel_fn, &hdr.kernel_size);
     if(kernel_data == 0) {