Re: [RFC][PATCH] rbind across namespaces

Miklos Szeredi
2005-05-24T17:32:43Z

> >>In what sense? readlink of /proc/PID/fd/* will provide a pathname
> >>relative to current's root: useless for any paths not in current's
> >>namespace.
> >
> >
> > Not readlink, but actual dereference of link will give you exactly the
> > vfsmount/dentry the file was opened on. If you want to bind/move
> > whatever on that mount, that's possible, even if it's a "detached
> > tree".
>
> Removing proc_check_root essentially removes any protection that
> namespaces provided in the first place.
>
> Think of it like virtual memory. You can fork off and create your own
> COW instance, and no one can see what you are doing unless they ptrace
> you or explicitly ask you. The mountfd model allows for explicit
> handing off of vfsmounts between namespaces without allowing arbitrary
> access.

Note: I didn't say we should remove proc_check_root(). I said _if_
you remove it, _then_ mounting on foreign namespace will work, which
has actually been confirmed experimentally.

So your follow_link argument doesn't hold. The proc code does the
follow_link in some clever way, that the looked up object will end up
with the same vfsmount/dentry pair as the file.

> Yet even this doesn't allow userspace to define it's own policy for
> inter-namespace manipulation.

OK. Let's keep the kernel policy simple: just allow a process to
access it's _own_ file descriptors in proc. I.e. allow access to
/proc/self/fd/* even if it comes from a foreign namespace.

Then the policy (who passes whom the file descriptors) is entirely up
to userspace (just as with your scheme).

/proc/self/fd/FD doesn't give any extra rights to the process, it just
makes mounting from/to detached mounts, and foreign namespaces
possible without new interfaces.

> Beware that due to the detached-subtrees bit, the locking became a bit
> ugly, requiring a global rw_lock for mntget/mntput. I still haven't
> figured out a better way to keep per-vfsmounts counts and per-subtree
> counts in sync.

Did you measure the effect on performace? Maybe it isn't so bad.

Miklos