Re: clone3() example code

On 2019-10-25, Christian Brauner [off-list ref] wrote:

#define ptr_to_u64(ptr) ((__u64)((uintptr_t)(ptr)))

int main(int argc, char *argv[])
{
	int pidfd = -1;
	pid_t parent_tid = -1, pid = -1;
	struct clone_args args = {0};

	args.parent_tid = ptr_to_u64(&parent_tid); /* CLONE_PARENT_SETTID */
	args.pidfd = ptr_to_u64(&pidfd); /* CLONE_PIDFD */
	args.flags = CLONE_PIDFD | CLONE_PARENT_SETTID;
	args.exit_signal = SIGCHLD;

	pid = sys_clone3(&args);

I'd suggest that

	struct clone_args args = {
		.flags = CLONE_PIDFD | CLONE_PARENT_SETTID,
		.parent_tid = ptr_to_u64(&parent_tid), /* CLONE_PARENT_SETTID */
		.pidfd = ptr_to_u64(&pidfd),           /* CLONE_PIDFD */
		.exit_signal = SIGCHLD,
	};

or alternatively

	pid = sys_clone3(&(struct clone_args) {
		.flags = CLONE_PIDFD | CLONE_PARENT_SETTID,
		.parent_tid = ptr_to_u64(&parent_tid), /* CLONE_PARENT_SETTID */
		.pidfd = ptr_to_u64(&pidfd),           /* CLONE_PIDFD */
		.exit_signal = SIGCHLD,
	});

are easier to read.

	if (pid < 0) {
		fprintf(stderr, "%s - Failed to create new process\n", strerror(errno));
		exit(EXIT_FAILURE);
	}

	if (pid == 0) {
		printf("Child process with pid %d\n", getpid());
		exit(EXIT_SUCCESS);
	}

	printf("Parent process received child's pid %d as return value\n", pid);
	printf("Parent process received child's pidfd %d\n", *(int *)args.pidfd);
	printf("Parent process received child's pid %d as return argument\n",
	       *(pid_t *)args.parent_tid);

	if (0) {
		if (waitid(P_ALL, pid, NULL, 0) == 0) {
			fprintf(stderr, "Managed to wait on CLONE_NO_WAITALL process with waitid(P_ALL)\n");
			exit(EXIT_FAILURE);
		}
		printf("Child process %d requested CLONE_NO_WAITALL\n", pid);
	} else {
		printf("Child process %d did not request CLONE_NO_WAITALL\n", pid);
	}

	if (wait_for_pid(pid))
		exit(EXIT_FAILURE);

	if (pid != *(pid_t *)args.parent_tid)
		exit(EXIT_FAILURE);

	close(pidfd);

	return 0;
}

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>