Add dynamic command handshake support

Author: f1shy-dev

CMakeLists.txt

@@ -2,7 +2,7 @@ cmake_minimum_required(VERSION 3.12.0)
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 
-project(ttyd VERSION 1.7.7 LANGUAGES C)
+project(ttyd VERSION 1.7.8 LANGUAGES C)
 
 set(TTYD_VERSION "${PROJECT_VERSION}")
 

src/protocol.c

@@ -110,21 +110,82 @@ static void process_exit_cb(pty_process *process) {
 
 static char **build_args(struct pss_tty *pss) {
   int i, n = 0;
-  char **argv = xmalloc((server->argc + pss->argc + 1) * sizeof(char *));
-
-  for (i = 0; i < server->argc; i++) {
-    argv[n++] = server->argv[i];
+  char **argv;
+  bool use_dynamic = (pss->dynamic_argv != NULL && pss->dynamic_argc > 0);
+
+  // Use dynamic command if available, otherwise use server default
+  if (use_dynamic) {
+    argv = xmalloc((pss->dynamic_argc + 1) * sizeof(char *));
+    for (i = 0; i < pss->dynamic_argc; i++) {
+      argv[n++] = pss->dynamic_argv[i];
+    }
+  } else {
+    argv = xmalloc((server->argc + pss->argc + 1) * sizeof(char *));
+    for (i = 0; i < server->argc; i++) {
+      argv[n++] = server->argv[i];
+    }
   }
 
-  for (i = 0; i < pss->argc; i++) {
-    argv[n++] = pss->args[i];
+  if (!use_dynamic) {
+    for (i = 0; i < pss->argc; i++) {
+      argv[n++] = pss->args[i];
+    }
   }
 
   argv[n] = NULL;
 
   return argv;
 }
 
+static void free_dynamic_command(struct pss_tty *pss);
+
+// Parse dynamic command string into argv array using /bin/sh -c
+// Returns true on success, false on failure (e.g., empty command)
+static bool parse_dynamic_command(struct pss_tty *pss, const char *cmd) {
+  if (cmd == NULL || strlen(cmd) == 0) {
+    return false;
+  }
+
+  // Limit command length to prevent excessive memory allocation
+  size_t cmd_len = strlen(cmd);
+  if (cmd_len > 8192) {
+    lwsl_warn("dynamic command too long (max 8192 chars)\n");
+    return false;
+  }
+
+  free_dynamic_command(pss);
+
+#ifdef _WIN32
+  const char *shell = "cmd.exe";
+  const char *flag = "/C";
+#else
+  const char *shell = "/bin/sh";
+  const char *flag = "-c";
+#endif
+
+  // Use shell -c to execute the command
+  pss->dynamic_argc = 3;
+  pss->dynamic_argv = xmalloc(4 * sizeof(char *));
+  pss->dynamic_argv[0] = strdup(shell);
+  pss->dynamic_argv[1] = strdup(flag);
+  pss->dynamic_argv[2] = strdup(cmd);
+  pss->dynamic_argv[3] = NULL;
+
+  return true;
+}
+
+// Free dynamic command argv
+static void free_dynamic_command(struct pss_tty *pss) {
+  if (pss->dynamic_argv != NULL) {
+    for (int i = 0; i < pss->dynamic_argc; i++) {
+      free(pss->dynamic_argv[i]);
+    }
+    free(pss->dynamic_argv);
+    pss->dynamic_argv = NULL;
+    pss->dynamic_argc = 0;
+  }
+}
+
 static char **build_env(struct pss_tty *pss) {
   int i = 0, n = 2;
   char **envp = xmalloc(n * sizeof(char *));
@@ -231,6 +292,8 @@ int callback_tty(struct lws *wsi, enum lws_callback_reasons reason, void *user,
     case LWS_CALLBACK_ESTABLISHED:
       pss->initialized = false;
       pss->authenticated = false;
+      pss->dynamic_argv = NULL;
+      pss->dynamic_argc = 0;
       pss->wsi = wsi;
       pss->lws_close_status = LWS_CLOSE_STATUS_NOSTATUS;
 
@@ -345,6 +408,19 @@ int callback_tty(struct lws *wsi, enum lws_callback_reasons reason, void *user,
               return -1;
             }
           }
+          // Parse dynamic command if enabled and present
+          if (server->allow_dynamic_cmd) {
+            struct json_object *cmd_obj = NULL;
+            if (json_object_object_get_ex(obj, "command", &cmd_obj)) {
+              const char *cmd = json_object_get_string(cmd_obj);
+              if (cmd != NULL && strlen(cmd) > 0) {
+                if (!parse_dynamic_command(pss, cmd)) {
+                  lwsl_warn("failed to parse dynamic command\n");
+                  // Fall back to server default command
+                }
+              }
+            }
+          }
           json_object_put(obj);
           if (!spawn_process(pss, columns, rows)) return 1;
           break;
@@ -366,9 +442,13 @@ int callback_tty(struct lws *wsi, enum lws_callback_reasons reason, void *user,
       lwsl_notice("WS closed from %s, clients: %d\n", pss->address, server->client_count);
       if (pss->buffer != NULL) free(pss->buffer);
       if (pss->pty_buf != NULL) pty_buf_free(pss->pty_buf);
-      for (int i = 0; i < pss->argc; i++) {
-        free(pss->args[i]);
+      if (pss->args != NULL) {
+        for (int i = 0; i < pss->argc; i++) {
+          free(pss->args[i]);
+        }
+        free(pss->args);
       }
+      free_dynamic_command(pss);
 
       if (pss->process != NULL) {
         ((pty_ctx_t *)pss->process->ctx)->ws_closed = true;

src/server.c

@@ -80,11 +80,12 @@ static const struct option options[] = {{"port", required_argument, NULL, 'p'},
                                         {"once", no_argument, NULL, 'o'},
                                         {"exit-no-conn", no_argument, NULL, 'q'},
                                         {"browser", no_argument, NULL, 'B'},
+                                        {"allow-dynamic-command", no_argument, NULL, 'D'},
                                         {"debug", required_argument, NULL, 'd'},
                                         {"version", no_argument, NULL, 'v'},
                                         {"help", no_argument, NULL, 'h'},
                                         {NULL, 0, 0, 0}};
-static const char *opt_string = "p:i:U:c:H:u:g:s:w:I:b:P:f:6aSC:K:A:Wt:T:Om:oqBd:vh";
+static const char *opt_string = "p:i:U:c:H:u:g:s:w:I:b:P:f:6aSC:K:A:Wt:T:Om:oqBDd:vh";
 
 static void print_help() {
   // clang-format off
@@ -112,6 +113,7 @@ static void print_help() {
           "    -o, --once              Accept only one client and exit on disconnection\n"
           "    -q, --exit-no-conn      Exit on all clients disconnection\n"
           "    -B, --browser           Open terminal with the default system browser\n"
+          "    -D, --allow-dynamic-command  Allow client to specify command via WebSocket handshake\n"
           "    -I, --index             Custom index.html path\n"
           "    -b, --base-path         Expected base path for requests coming from a reverse proxy (eg: /mounted/here, max length: 128)\n"
 #if LWS_LIBRARY_VERSION_NUMBER >= 4000000
@@ -155,6 +157,7 @@ static void print_config() {
   if (server->max_clients > 0) lwsl_notice("  max clients: %d\n", server->max_clients);
   if (server->once) lwsl_notice("  once: true\n");
   if (server->exit_no_conn) lwsl_notice("  exit_no_conn: true\n");
+  if (server->allow_dynamic_cmd) lwsl_notice("  allow dynamic command: true\n");
   if (server->index != NULL) lwsl_notice("  custom index.html: %s\n", server->index);
   if (server->cwd != NULL) lwsl_notice("  working directory: %s\n", server->cwd);
   if (!server->writable) lwsl_warn("The --writable option is not set, will start in readonly mode\n");
@@ -378,6 +381,9 @@ int main(int argc, char **argv) {
       case 'B':
         browser = true;
         break;
+      case 'D':
+        server->allow_dynamic_cmd = true;
+        break;
       case 'p':
         info.port = parse_int("port", optarg);
         if (info.port < 0) {

src/server.h

@@ -46,6 +46,10 @@ struct pss_tty {
   char **args;
   int argc;
 
+  // dynamic command support
+  char **dynamic_argv;     // argv for dynamic command (NULL if using server default)
+  int dynamic_argc;        // argc for dynamic command
+
   struct lws *wsi;
   char *buffer;
   size_t len;
@@ -81,6 +85,7 @@ struct server {
   bool exit_no_conn;       // whether exit on all clients disconnection
   char socket_path[255];   // UNIX domain socket path
   char terminal_type[30];  // terminal type to report
+  bool allow_dynamic_cmd;  // allow client to send command via websocket
 
   uv_loop_t *loop;         // the libuv event loop
 };