// 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);
}