mount / unmount and mountcheckdirs()

[Edward Napierala]

2017-09-15 11:30 GMT+01:00 Konstantin Belousov <kostikbel at gmail.com>:

> On Fri, Sep 15, 2017 at 11:08:31AM +0100, Edward Napierala wrote:
> > 2017-09-15 10:20 GMT+01:00 Konstantin Belousov <kostikbel at gmail.com>:
> > > I believe that the current autofs does not allow a process to get into
> > > this situation at all.
> > >
> >
> > It does.  For example:
> >
> > [trasz at v2:~]% cd /media/md0
> > [trasz at v2:/media/md0]% mount
> > /dev/ada0s1a on / (ufs, local, noatime, journaled soft-updates)
> > devfs on /dev (devfs, local, multilabel)
> > map -hosts on /net (autofs)
> > map -media on /media (autofs)
> > [trasz at v2:/media/md0]% ls
> > [trasz at v2:/media/md0]% mount
> > /dev/ada0s1a on / (ufs, local, noatime, journaled soft-updates)
> > devfs on /dev (devfs, local, multilabel)
> > map -hosts on /net (autofs)
> > map -media on /media (autofs)
> > /dev/md0 on /media/md0 (ufs, local, noatime, nosuid, automounted)
> >
> > Getting rid of mountcheckdirs() in the unmount path should be fine, I
> think.
> How the example proves that mountcheckdirs() can be removed ?


In the unmount path?  It doesn't; I just don't think autofs would be
affected,
since having a current working directory in a mountpoint prevents the
unmount
(unless it's forced).

In the mount path?  See below.


> How can
> we see which directory content was printed by ls, the covered or mounted ?
>

Well, it would be pretty useless if you got the covered directory.  But ok,
here's
a better example, one that actually shows that:

[trasz at v2:/media/md0]% mount
/dev/ada0s1a on / (ufs, local, noatime, journaled soft-updates)
devfs on /dev (devfs, local, multilabel)
map -hosts on /net (autofs)
map -media on /media (autofs)
[trasz at v2:/media/md0]% ls -al
total 9
drwxr-xr-x  3 root  wheel     512 Sep 10 10:16 .
drwxr-xr-x  3 root  wheel     512 Sep  8 11:42 ..
drwxrwxr-x  2 root  operator  512 Sep 10 10:16 .snap
[trasz at v2:/media/md0]% mount
/dev/ada0s1a on / (ufs, local, noatime, journaled soft-updates)
devfs on /dev (devfs, local, multilabel)
map -hosts on /net (autofs)
map -media on /media (autofs)
/dev/md0 on /media/md0 (ufs, local, noatime, nosuid, automounted)

Notice the .snap/ directory; it's an empty UFS filesystem.