um: port_user: Allow setting path to port-helper using UML_PORT_HELPER envvar

This is useful when the uml-utilities user-space package has not been
installed by the administrator and an unprivileged user wants to be able to
telnet into a UML instance. The user can install the port-helper binary to
a writable path and set UML_PORT_HELPER. Fallback to using hardcoded path to
port-helper if environment variable is not set.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

Author: crass

arch/um/drivers/port_user.c

@@ -167,10 +167,14 @@ static void port_pre_exec(void *arg)
 int port_connection(int fd, int *socket, int *pid_out)
 {
 	int new, err;
+	char *env;
 	char *argv[] = { "in.telnetd", "-L",
 			 OS_LIB_PATH "/uml/port-helper", NULL };
 	struct port_pre_exec_data data;
 
+	if ((env = getenv("UML_PORT_HELPER")))
+		argv[2] = env;
+
 	new = accept(fd, NULL, 0);
 	if (new < 0)
 		return -errno;