// SPDX-License-Identifier: GPL-2.0
#define _GNU_SOURCE
#include <sched.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <sys/prctl.h>
#include <sys/wait.h>

#define NSIO    0xb7
#define NS_GET_USERNS   _IO(NSIO, 0x1)

#define pr_err(fmt, ...) \
                ({ \
                        fprintf(stderr, "%s:%d:" fmt ": %m\n", \
                                __func__, __LINE__, ##__VA_ARGS__); \
                        1; \
                })

int main(int argc, char *argvp[])
{
        int pfd[2], ns, uns, init_uns;
        struct stat st1, st2;
        char path[128];
        pid_t pid;
        char c;

        if (pipe(pfd))
                return 1;

        pid = fork();
        if (pid < 0)
                return pr_err("fork");
        if (pid == 0) {
                prctl(PR_SET_PDEATHSIG, SIGKILL);
                if (unshare(CLONE_NEWUTS | CLONE_NEWUSER))
                        return pr_err("unshare");
                close(pfd[0]);
                close(pfd[1]);
                while (1)
                        sleep(1);
                return 0;
        }
        close(pfd[1]);
        if (read(pfd[0], &c, 1) != 0)
                return pr_err("Unable to read from pipe");
        close(pfd[0]);

        snprintf(path, sizeof(path), "/proc/%d/ns/uts", pid);
        ns = open(path, O_RDONLY);
        if (ns < 0)
                return pr_err("Unable to open %s", path);

        uns = ioctl(ns, NS_GET_USERNS);
        if (uns < 0)
                return pr_err("Unable to get an owning user namespace");

        if (fstat(uns, &st1))
                return pr_err("fstat");

        snprintf(path, sizeof(path), "/proc/%d/ns/user", pid);
        if (stat(path, &st2))
                return pr_err("stat");

        if (st1.st_ino != st2.st_ino)
                return pr_err("NS_GET_USERNS returned a wrong namespace");

        init_uns = ioctl(uns, NS_GET_USERNS);
        if (uns < 0)
                return pr_err("Unable to get an owning user namespace");

        if (ioctl(init_uns, NS_GET_USERNS) >= 0 || errno != EPERM)
                return pr_err("Don't get EPERM");

        if (unshare(CLONE_NEWUSER))
                return pr_err("unshare");

        if (ioctl(ns, NS_GET_USERNS) >= 0 || errno != EPERM)
                return pr_err("Don't get EPERM");
        if (ioctl(init_uns, NS_GET_USERNS) >= 0 || errno != EPERM)
                return pr_err("Don't get EPERM");

        kill(pid, SIGKILL);
        wait(NULL);
        return 0;
}