Messages in this thread | | | Subject | Re: [RFC][PATCH] rbind across namespaces | From | Miklos Szeredi <> | Date | Tue, 24 May 2005 19:31:51 +0200 |
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> >>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.
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