qrexec-client: handle failed service exec

Otherwise, qrexec-client just hangs when it fails to spawn a
process, or connect to a socket service.

This corresponds to the fix for qrexec-agent in commit
95fa4bb.

daemon/qrexec-client.c

@@ -252,20 +252,37 @@ static void wait_for_session_maybe(char *cmdline)
     destroy_qrexec_parsed_command(cmd);
 }
 
-static void prepare_local_fds(char *cmdline, struct buffer *stdin_buffer)
+static int prepare_local_fds(char *cmdline, struct buffer *stdin_buffer)
 {
     if (stdin_buffer == NULL)
         abort();
     if (!cmdline) {
         local_stdin_fd = 1;
         local_stdout_fd = 0;
-        return;
+        return 0;
     }
     signal(SIGCHLD, sigchld_handler);
-    execute_qubes_rpc_command(cmdline, &local_pid, &local_stdin_fd, &local_stdout_fd,
+    return execute_qubes_rpc_command(cmdline, &local_pid, &local_stdin_fd, &local_stdout_fd,
             NULL, false, stdin_buffer);
 }
 
+// See also qrexec-agent/qrexec-agent-data.c
+static _Noreturn void handle_failed_exec(libvchan_t *data_vchan)
+{
+    int exit_code = 127;
+    struct msg_header hdr = {
+        .type = MSG_DATA_STDOUT,
+        .len = 0,
+    };
+
+    LOG(ERROR, "failed to spawn process");
+    libvchan_send(data_vchan, &hdr, sizeof(hdr));
+    send_exit_code(data_vchan, exit_code);
+    libvchan_close(data_vchan);
+
+    exit(exit_code);
+}
+
 /* ask the daemon to allocate vchan port */
 static void negotiate_connection_params(int s, int other_domid, unsigned type,
         void *cmdline_param, int cmdline_size,
@@ -491,6 +508,7 @@ int main(int argc, char **argv)
     int connection_timeout = 5;
     struct service_params svc_params;
     int data_protocol_version;
+    int prepare_ret;
 
     setup_logging("qrexec-client");
 
@@ -572,7 +590,7 @@ int main(int argc, char **argv)
         struct buffer stdin_buffer;
         buffer_init(&stdin_buffer);
         wait_for_session_maybe(remote_cmdline);
-        prepare_local_fds(remote_cmdline, &stdin_buffer);
+        prepare_ret = prepare_local_fds(remote_cmdline, &stdin_buffer);
         if (connect_existing) {
             void (*old_handler)(int);
 
@@ -595,6 +613,8 @@ int main(int argc, char **argv)
         data_protocol_version = handle_agent_handshake(data_vchan, connect_existing);
         if (data_protocol_version < 0)
             exit(1);
+        if (prepare_ret < 0)
+            handle_failed_exec(data_vchan);
         select_loop(data_vchan, data_protocol_version, &stdin_buffer);
     } else {
         msg_type = just_exec ? MSG_JUST_EXEC : MSG_EXEC_CMDLINE;
@@ -615,7 +635,7 @@ int main(int argc, char **argv)
         set_remote_domain(domname);
         struct buffer stdin_buffer;
         buffer_init(&stdin_buffer);
-        prepare_local_fds(local_cmdline, &stdin_buffer);
+        prepare_ret = prepare_local_fds(local_cmdline, &stdin_buffer);
         if (connect_existing) {
             s = connect_unix_socket(src_domain_name);
             send_service_connect(s, request_id, data_domain, data_port);
@@ -642,6 +662,8 @@ int main(int argc, char **argv)
             data_protocol_version = handle_agent_handshake(data_vchan, 0);
             if (data_protocol_version < 0)
                 exit(1);
+            if (prepare_ret < 0)
+                handle_failed_exec(data_vchan);
             select_loop(data_vchan, data_protocol_version, &stdin_buffer);
         }
     }

qrexec/tests/socket/daemon.py

@@ -519,6 +519,18 @@ def test_run_dom0_service_exec(self):
         self.client.wait()
         self.assertEqual(self.client.returncode, 0)
 
+    def test_run_dom0_service_failed(self):
+        # qubes.Service does not exist
+        cmd = 'QUBESRPC qubes.Service+arg src_domain name src_domain'
+        source = self.connect_service_request(cmd)
+
+        self.assertEqual(source.recv_all_messages(), [
+            (qrexec.MSG_DATA_STDOUT, b''),
+            (qrexec.MSG_DATA_EXIT_CODE, struct.pack('<L', 127)),
+        ])
+        self.client.wait()
+        self.assertEqual(self.client.returncode, 127)
+
     def test_run_dom0_service_wait_for_session(self):
         log = os.path.join(self.tempdir, 'wait-for-session.log')
         util.make_executable_service(self.tempdir, 'rpc', 'qubes.WaitForSession', '''\