conf: adapt to lxc-user-nic usage

- lxc-user-nic gains the subcommands {create,delete}
- dup2() STDERR_FILENO as well so that we can show helpful messages in our logs
  on failure
- initialize output buffer so that we don't print garbage

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

src/lxc/conf.c

@@ -3277,8 +3277,8 @@ static int unpriv_assign_nic(const char *lxcpath, char *lxcname,
 	pid_t child;
 	int bytes, pipefd[2];
 	char *token, *saveptr = NULL;
-	char buffer[MAX_BUFFER_SIZE];
 	char netdev_link[IFNAMSIZ + 1];
+	char buffer[MAX_BUFFER_SIZE] = {0};
 
 	if (netdev->type != LXC_NET_VETH) {
 		ERROR("nic type %d not support for unprivileged use",
@@ -3308,6 +3308,8 @@ static int unpriv_assign_nic(const char *lxcpath, char *lxcname,
 
 		/* Redirect stdout to write-end of the pipe. */
 		ret = dup2(pipefd[1], STDOUT_FILENO);
+		if (ret >= 0)
+			ret = dup2(pipefd[1], STDERR_FILENO);
 		close(pipefd[1]); /* Close the write-end of the pipe. */
 		if (ret < 0) {
 			SYSERROR("Failed to dup2() to redirect stdout to pipe file descriptor.");
@@ -3351,7 +3353,8 @@ static int unpriv_assign_nic(const char *lxcpath, char *lxcname,
 	buffer[bytes - 1] = '\0';
 
 	if (wait_for_pid(child) != 0) {
-		TRACE("lxc-user-nic failed to configure requested network");
+		ERROR("lxc-user-nic failed to configure requested network: %s",
+		      buffer[0] != '\0' ? buffer : "(null)");
 		close(pipefd[0]);
 		return -1;
 	}

src/lxc/lxc_user_nic.c

@@ -940,7 +940,7 @@ int main(int argc, char *argv[])
 {
 	int fd, n, pid, ret;
 	char *me;
-	char *cnic = NULL, *nicname = NULL, *vethname = NULL;
+	char *cnic = NULL, *nicname = NULL;
 	struct alloted_s *alloted = NULL;
 	struct user_nic_args args;