linux-kernel - Re: clone3() example code

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Message-ID: <20191025094956.hvr44v2lbfxf7dfs@yavin.dot.cyphar.com>
Date:   Fri, 25 Oct 2019 20:49:56 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        Christian Brauner <christian@...uner.io>,
        Linux API <linux-api@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: clone3() example code

On 2019-10-25, Christian Brauner <christian.brauner@...ntu.com> 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/>

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