mount / unmount and mountcheckdirs()

[Konstantin Belousov]

On Thu, Sep 14, 2017 at 08:14:36PM -0700, Kirk McKusick wrote:
> > To: Kirk McKusick <mckusick at mckusick.com>
> > Subject: Re: mount / unmount and mountcheckdirs()
> > Cc: freebsd-arch at FreeBSD.org, freebsd-fs <freebsd-fs at FreeBSD.org>
> > From: Andriy Gapon <avg at FreeBSD.org>
> > Date: Thu, 14 Sep 2017 15:45:07 +0300
> > 
> > On 22/05/2016 09:40, Kirk McKusick wrote:
> >> I added the checkdirs functionality in the mount direction only
> >> (I actually did it in 4.4BSD-Lite and it got swept in with commit
> >> 22521). The reason is that when a directory that is not empty is
> >> mounted on, the expectation is that the entries in that directory
> >> should no longer be present; rather they should be replaced by the
> >> entries in the newly mounted directory. Thus all processes sitting
> >> in the mounted on directory should see the newly mounted directory
> >> as if they had come to it using a lookup after the mount had been
> >> done. If a process had proceeded through the mounted on directory
> >> into one of its other entries, then they are left alone until such
> >> time as they chdir back into the mount point directory through ".."
> >> at which time they will be passed up to the mounted directory using
> >> the same mechanism that would put them there if they traversed into
> >> the mount point from above it in the tree. I believe this is the
> >> correct behavior, is not a security threat, and should be left alone.
> > 
> > I almost dropped a ball on this issue, but I am now picking it up again.
> > At the moment I am moving forward with the dounmount change as it seems to be
> > non-contentious and rather simple to do and test.
> > 
> > Regarding the mount part, I am not sure that I completely agree with you.
> > Even if mountcheckdirs() does not cause any problems in the mount path, I still
> > fail to see its usefulness.  Specifically, I still do not see any significant
> > difference between the covered directory and any directory below it.  So, if we
> > leave the lower directories alone, while bother with the covered directory...
> > 
> > The covered directory:
> > - absolute paths work correctly
> > - relative paths with enough ".." (one) can access the actual namespace
> > - other relative paths operate on the shadowed sub-tree of the original
> >   filesystem
> > 
> > 
> > The lower directories:
> > - absolute paths work correctly
> > - relative paths with enough ".." (> 1) can access the actual namespace
> > - other relative paths operate on the shadowed sub-tree of the original
> >   filesystem
> > 
> > The only difference I can think of is that the root of the mounted filesystem
> > cannot be reached with just ".."-s from the covered directory.  But is this
> > difference of any significance?
> > 
> > Mateusz also raised some interesting points.
> > 
> > On the other hand, it seems that illumos and probably Solaris has
> > interesting parallels to the FreeBSD behavior.  It does not allow
> > to mount over a directory that is a current directory for any process
> > ("Device busy"), but does not object against processes in directories
> > below the mount point.
> > 
> > So, probably it's just I who misses something about that scenario :-)
> > 
> >> I was not aware that the functionality had been added at unmount
> >> time, and I do not believe that it should have been done. Normally
> >> an unmount will not succeed if any vnodes are busy (for example, if
> >> any directory in the filesystem is a current directory). The only
> >> way that it can succeed in such a case is if a forcible unmount is
> >> done. The forcible unmount will effectively do a revoke(2) on all
> >> current directory vnodes in the unmounted filesystem. Further attempts
> >> to access them will fail with "." not found errors. The only way to
> >> get a valid current directory is to chdir to an absolute pathname.
> >> Gratuitously fixing this if you happen to be in the former root of
> >> the filesystem is wrong. And as you note can lead to unintensionally
> >> giving an escape path from a prison. So I concur with your removing
> >> this added functionality.
> > -- 
> > Andriy Gapon
> 
> I had to dig back through some *really* old emails to find out what
> triggered the addition of mountcheckdirs(). The problem that it was
> specifically solving was that as part of the startup script a minimal
> root directory was replaced by the real root directory. The shell
> running the startup script needed to be moved to the new mounted-on
> root so that the rest of the script would not fail.
If the mountcheckdirs() code not going away, please add your spelunking
results as a comment before the function. This theme is recurring, and
it would be highly beneficial to not loose the non-trivial reasoning
behind the code existence.

> 
> That disaster of a hack has been replaced with the much more functional
> code that deals with setting up the root and the devfs filesystem on
> /dev. So the need for which it was designed no longer exists. But I
> still believe that it is the correct thing to do. For example, if you
> are using automount code and chdir into your home directory triggering
> an auto-mount, you should just be in your home directory after the
> mount rather than having to do cd ../$USER to get there.
I believe that the current autofs does not allow a process to get into
this situation at all.

In fact, the behavior implemented by mountcheckdirs() is surprising as
well. For instance, I did expected that the system would operate as if
mountcheckdirs() does not exist, and it caused me some head-scratching
when I see it first time.