Monitoring mount point changes via /proc/mounts

Question

According proc manual, one can monitor for mount point changes in linux system by opening "/proc/mounts", and adding the file descriptor to read fd_set in select() call.

Following piece of code works on Ubuntu 9.04, and not in Ubuntu 10.04 (with 2.6.32 linux kernel):

int mfd = open("/proc/mounts", O_RDONLY, 0);

fd_set rfds;
struct timeval tv;
int rv;

FD_ZERO(&rfds);
FD_SET(mfd, &rfds);
tv.tv_sec = 5;
tv.tv_usec = 0;

int changes = 0;
while ((rv = select(mfd+1, &rfds, NULL, NULL, &tv)) >= 0) {
    if (FD_ISSET(mfd, &rfds)) {
        fprintf(stdout, "Mount points changed. %d.\n", changes++);
    }

    FD_ZERO(&rfds);
    FD_SET(mfd, &rfds);
    tv.tv_sec = 5;
    tv.tv_usec = 0;

    if (changes > 10) {
        exit(EXIT_FAILURE);
    }
}

Compilable snippet.

The file descriptor is always readable in one machine, and hence it keeps popping up in the select call. Even there are no changes in mounts.

Am I missing something here?

Thanks in advance for any help!

man 5 proc:

/proc/[pid]/mounts (since Linux 2.4.19)

This is a list of all the file systems currently mounted in the process's mount namespace. The format of this file is documented in fstab(5). Since kernel version 2.6.15, this file is pollable: after opening the file for reading, a change in this file (i.e., a file system mount or unmount) causes select(2) to mark the file descriptor as readable, and poll(2) and epoll_wait(2) mark the file as having an error condition.

Answer 1

There was a bugfix in linux kernel describing that behavior:

SUSv3 says "Regular files shall always poll TRUE for reading and writing". see http://www.opengroup.org/onlinepubs/009695399/functions/poll.html

So, you have to use poll with POLLPRI | POLLERR flags. Something like this:

     int mfd = open("/proc/mounts", O_RDONLY, 0);
     struct pollfd pfd;
     int rv;

     int changes = 0;
     pfd.fd = mfd;
     pfd.events = POLLERR | POLLPRI;
     pfd.revents = 0;
     while ((rv = poll(&pfd, 1, 5)) >= 0) {
          if (pfd.revents & POLLERR) {
               fprintf(stdout, "Mount points changed. %d.\n", changes++);
          }

          pfd.revents = 0;
          if (changes > 10) {
               exit(EXIT_FAILURE);
          }
     }

Answer 2

The documentation that you pointed to is incorrect. To wait on mount changes using select(), the /proc/mounts or /proc/pid/mounts file descriptor should set be in exceptfds, not readfds. Just swap the 2nd and 4th arguments in your program. File descriptors associated with regular files are required by POSIX to always be readable.