mount / unmount and mountcheckdirs()

[Edward Napierala]

2017-09-15 12:00 GMT+01:00 Konstantin Belousov <kostikbel at gmail.com>:

> On Fri, Sep 15, 2017 at 11:49:17AM +0100, Edward Napierala wrote:
> > 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.
> This is exactly what is being discussed.
>
> > 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.
> And the .snap is from the /dev/md0 volume, right ?
>

Yes.


> If yes, then this demonstration indeed proves that autofs behaves as
> intended on mount.
>

Exactly.  And, from what I understand, that would break if mountcheckdirs()
got removed from the mount path.