Mount tmpfs at /run in a process's mount namespace

mountrun.c

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sched.h>
#include <unistd.h>
#include <errno.h>

#define pr_perror(fmt, ...) fprintf(stderr, "mountrun: " fmt ": %m\n", ##__VA_ARGS__)

#define BUFLEN 1024

int main(int argc, char *argv[])
{
        if (argc < 2) {
                fprintf(stderr, "Usage mountrun <pid>");
                return EXIT_FAILURE;
        }

        int target_pid = atoi(argv[1]);
        printf("Mounting run in mountnamespace of process: %d\n", target_pid);


        char process_mnt_ns_fd[BUFLEN];
        snprintf(process_mnt_ns_fd, BUFLEN - 1, "/proc/%d/ns/mnt", target_pid);
        int fd = open(process_mnt_ns_fd, O_RDONLY);
        if (-1 == fd) {
                pr_perror("Failed to open mnt namespace fd %s", process_mnt_ns_fd);
                exit(1);
        }

        // Join the mount namespace of the target process
        if (setns(fd, 0) == -1) {
                pr_perror("Failed to setns to %s", process_mnt_ns_fd);
                exit(1);
        }

        // Switch to the root directory
        if (chdir("/") == -1) {
                pr_perror("Failed to chdir");
                exit(1);
        }

        // Create the /run directory
        if (mkdir("/run", 0755) == -1) {
                if (errno != EEXIST) {
                        pr_perror("Failed to mkdir");
                        exit(1);
                }
        }

        // Mount tmpfs at /run for systemd
        if (mount("tmpfs", "/run", "tmpfs", MS_NODEV|MS_NOSUID|MS_NOEXEC, "mode=755,size=65536k") == -1) {
                pr_perror("Failed to mount tmpfs at /run");
                exit(1);
        }

        return EXIT_SUCCESS;
}