Re: Maximum mountpoints + chrooted login

[Alexander Viro]

On Wed, 27 Jun 2001, Magnus Naeslund(f) wrote:

> I was thinking of doing a chrooted login for some ssh accounts.
> The plan is this:

[snip CLONE_NAMESPACE-by-hands]
 
> Does this seem like a bad idea?
> (then please tell me why :))

Mostly because there's a better way to do that. Yes, such scheme would
work (that + massive pending fs/super.c cleanups was the main reason why
I didn't go for proper solution in 2.4.0-test*). However, instead of
crufting up kinda-sorta namespaces one could use the real thing. Relevant
cleanups of superblock handling will go in in 2.5.very_early and the
rest of patch (namespace proper) takes about 10Kb.

You can simply say clone(CLONE_NAMESPACE,NULL) and you get an independent
set of mounts to play with. mount/umount whatever you want before dropping
the root priveleges. All children of that process will share its namespace.
When the last one goes away everything will be garbage-collected - no
need to umount anything on logout.

> One problem could be the _massive_ mounts, 3*online_users.
> Are there any limits/drawbacks doing it like this?

With the mntcache in - not really. It fixes the main performance problem.
Memory cost is sizeof(struct vfsmount)*total amount of mountpoints. I.e.
about 100 bytes per mountpoint. That's it.