// SPDX-License-Identifier: GPL-2.0 #define _GNU_SOURCE#include <err.h>#include <errno.h>#include <fcntl.h>#include <inttypes.h>#include <limits.h>#include <sched.h>#include <signal.h>#include <stdio.h>#include <stdlib.h>#include <string.h>#include <sys/stat.h>#include <sys/syscall.h>#include <sys/types.h>#include <sys/wait.h>#include <unistd.h> #ifndef CLONE_PIDFD#define CLONE_PIDFD 0x00001000#endif #ifndef __NR_pidfd_send_signal#define __NR_pidfd_send_signal -1#endif static int do_child(void *args){	printf("%d\n", getpid());	_exit(EXIT_SUCCESS);} static pid_t pidfd_clone(int flags, int *pidfd){	size_t stack_size = 1024;	char *stack[1024] = { 0 }; #ifdef __ia64__	return __clone2(do_child, stack, stack_size, flags | SIGCHLD, NULL, pidfd);#else	return clone(do_child, stack + stack_size, flags | SIGCHLD, NULL, pidfd);#endif} static inline int sys_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,					unsigned int flags){	return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);} static int pidfd_metadata_fd(pid_t pid, int pidfd){	int procfd, ret;	char path[100]; 	snprintf(path, sizeof(path), "/proc/%d", pid);	procfd = open(path, O_DIRECTORY | O_RDONLY | O_CLOEXEC);	if (procfd < 0) {		warn("Failed to open %s\n", path);		return -1;	} 	/*	 * Verify that the pid has not been recycled and our /proc/<pid> handle	 * is still valid.	 */	ret = sys_pidfd_send_signal(pidfd, 0, NULL, 0);	if (ret < 0) {		switch (errno) {		case EPERM:			/* Process exists, just not allowed to signal it. */			break;		default:			warn("Failed to signal process\n");			close(procfd);			procfd = -1;		}	} 	return procfd;} int main(int argc, char *argv[]){	int pidfd = -1, ret = EXIT_FAILURE;	char buf[4096] = { 0 };	pid_t pid;	int procfd, statusfd;	ssize_t bytes; 	pid = pidfd_clone(CLONE_PIDFD, &pidfd);	if (pid < 0)		err(ret, "CLONE_PIDFD");	if (pidfd == -1) {		warnx("CLONE_PIDFD is not supported by the kernel");		goto out;	} 	procfd = pidfd_metadata_fd(pid, pidfd);	close(pidfd);	if (procfd < 0)		goto out; 	statusfd = openat(procfd, "status", O_RDONLY | O_CLOEXEC);	close(procfd);	if (statusfd < 0)		goto out; 	bytes = read(statusfd, buf, sizeof(buf));	if (bytes > 0)		bytes = write(STDOUT_FILENO, buf, bytes);	close(statusfd);	ret = EXIT_SUCCESS; out:	(void)wait(NULL); 	exit(ret);}