Re: [RFC][PATCH] rbind across namespaces

[Miklos Szeredi <miklos@szeredi.hu>]

> >>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
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