Bug 538536

+++ This bug was initially created as a clone of Bug #501848 +++

--- Additional comment from kdudka on 2009-10-21 14:27:51 EDT ---

Reported upstream:
http://lists.gnu.org/archive/html/bug-gnulib/2009-10/msg00207.html

--- Additional comment from kdudka on 2009-10-31 09:13:44 EDT ---

(In reply to comment #42)
> What event triggers the mount?  

From my observation with gdb:
1. calling fstatat() with AT_SYMLINK_NOFOLLOW does NOT trigger the mount.
2. calling fstatat() without AT_SYMLINK_NOFOLLOW triggers the mount, opening a directory as well.

If you are asking which events are guaranteed to trigger the mount and/or which events are guaranteed to NOT trigger the mount, kernel guys might give you a reliable answer.

Jeff, any idea?

--- Additional comment from jlayton on 2009-11-02 14:01:06 EDT ---

Submounts are triggered via the follow_link inode operation, so in some ways these are treated like symlinks...

The short answer is that the mount will be triggered whenever you walk a path in such a way that, if this component were a symlink it would be resolved to its target.

Longer answer:

If the place where you transition into a new filesystem is in the middle of a path, then generally the path will be resolved. If it's the last component of the path, then it depends on whether the LOOKUP_FOLLOW link flag is set in nameidata in the kernel. That varies with the type of operation -- for instance, lstat() won't have that set, but a "normal" stat() generally will.

--- Additional comment from kdudka on 2009-11-03 07:41:30 EDT ---

Minimal example which works reliably on my Fedora 11 installation:

# mount | grep ^/
/dev/sda1 on / type ext3 (rw)
/dev/sda3 on /home type ext4 (rw)

# ls -d /home/test
/home/test

# printf "/ *(fsid=0,crossmnt)\n/home *(crossmnt)\n" \
    > /etc/exports

# service nfs restart
# mkdir /tmp/mnt
# mount -t nfs4 localhost:/ /tmp/mnt \
    && stat --printf "%d\t%i\t%n\n" /tmp/mnt/home \
    && stat --printf "%d\t%i\t%n\n" /tmp/mnt/home/test \
    && stat --printf "%d\t%i\t%n\n" /tmp/mnt/home

29      2       /tmp/mnt/home
30      12      /tmp/mnt/home/test
30      2       /tmp/mnt/home

--- Additional comment from kdudka on 2009-11-03 15:22:57 EDT ---

A patch for gnulib proposed upstream:

http://lists.gnu.org/archive/html/bug-gnulib/2009-11/msg00027.html

--- Additional comment from meyering on 2009-11-04 15:07:51 EDT ---

FYI, I've (re)raised the issue on LKML:

    http://lkml.org/lkml/2009/11/4/451