00-README.md

Demonstrates that unmounting a filesystem with MNT_DETACH allows open file descriptors on that filesystem to be inherited by new children of processes than already held them open. I believe this is expected behaviour from the definition of the flag in man 2 umount2.

Steps:

$ gcc -o forkme forkme.c
$ mkdir /tmp/real /tmp/bind
$ sudo mount --bind /tmp/real /tmp/bind
$ ./forkme /tmp/bind/foo
[ wait for confirmation that it has opened & written to the file ]
[ in another shell ]
$ sudo umount --lazy /tmp/bind
[ back in the first shell, press any key ]
$ cat /tmp/real/foo

forkme.c

#include <errno.h>
#include <fcntl.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>
#include <inttypes.h>

int
main (int argc,
      char **argv)
{
  if (argc != 2 ||
      strcmp (argv[1], "--help") == 0 ||
      strcmp (argv[1], "-h") == 0)
    {
      fprintf (stderr, "Usage: %s PATH", argv[0]);
      return 1;
    }

  int fd = creat (argv[1], 0644);
  if (fd < 0)
    {
      perror ("open");
      return 2;
    }

  char hello_from_parent[] = "hello from parent\n";
  ssize_t written = write (fd, hello_from_parent, sizeof (hello_from_parent) - 1);
  if (written < 0)
    {
      perror ("write");
      return 2;
    }

  printf ("%ld bytes written to %s\n"
          "unmount its filesystem with --lazy, then press any key to fork a child\n", written, argv[1]);
  char in;
  if (fread (&in, 1, 1, stdin) <= 0)
    {
      perror ("fread");
      return 3;
    }

  pid_t p = fork ();
  if (p == 0)
    {
      char hello_from_child[] = "hello from child\n";
      written = write (fd, hello_from_child, sizeof (hello_from_child) - 1);
      if (written < 0)
        {
          perror ("write in child");
          return 4;
        }
    }
  else if (p < 0)
    {
      perror ("fork");
      return 4;
    }
  else
    {
      int status;
      if (waitpid (p, &status, 0) < 0)
        {
          perror ("waitpid");
          return 5;
        }
    }

  return 0;
}