Detecting bind mounts on directory subtrees

[pascaltaf]

Hello,

Does anyone know if there's any way to detect a bind mount in a C program ?
I explicit my question :
with Linux you can mount any directory on another with the command :
$ mount -o bind /etc /tmp
or in C
mount("/etc", "/tmp", NULL, MS_BIND, NULL);

/etc and /tmp have not to be special directories, or already mountpoints, you could do it with any other directories.
This seems great but is a real trap in fact.
Once you did it, there seems to be no easy way to determine your destination, (in the example /tmp) is a bind mount.

Let's imagine you have a directory toto, in it only 2 dirs : d1 and d2, d1 has a content of 1 MB and d2 is empty, you mount d1 on d2 and then run :
$ du -sh toto
2M toto

even with the -x option that excludes other filesystems, du doesn't detect the bind.

I have thought of several solutions :
- reading in /proc/mounts or /etc/mtab, but it is not reliable enough and too complicated in my case
- comparing inodes, becuase the mountpoint takes the same inode as the source (try the command stat), but it can be horribly consuming to keep a list of visited inodes and compare any new inode with the whole list,
- performing a mount("dest", "dest", NULL, "MS_REMOUNT", NULL) to test if it returns EINVAL or not, but this clears the mount options if it encounters a mountpoint so it is not a harmless solution

Does a system-call, ioctl or something exist to determine such a thing ?

Thank you

Pascal