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SubjectCLONE_NAMESPACE, links for dirs and mount(2) for normal users questions

Long long ago, (March 2000) Alexander Viro replied to Pavel Machek:
>> Am I right that from now on each process can have completely different
>> view of filesystem like in plan9?
>Almost there ;-) And yes, the only thing we lack for proper namespaces is
>the union-directories (clone() bit is trivial).
Are there any patches already?
If not, where should I start to implement them?

Probably related to the first question, what about allowing mount(2)
(as a CONFIG-option) for normal user processes when they
have a) rw access to the device and b) are the owner/have rw-access
to the mountpoint. (There would be at least one security problem:
A normal user could mount a loopback ext2 filesystem with
panic-on-error (man tune2fs) and then corrupt it)

In April, Al Viro wrote:
> 1. We should never have more than one dentry for a writable directory.
> Print it and hang it on the wall. It's a fundamental requirement. There is
> no way to work around it in our VFS. I tried to invent a scheme that would
> allow that for more than a year. And I've done most of namespace-related code
> in our VFS since the moment when Bill Hawes stopped working on it, so I suspect
> that right now I have the best working knowledge of that stuff. There is no
> fscking way to survive multiple dentries for writable directory without major
> lossage. Period.
Do I understand correctly that this means hardlinks to directories
(except . and ..) are fundamentally impossible in Linux?
(I'm thinking about trying to write a garbage collected
filesystem with hardlinks to directories.)

Linux 2.4.0-test11 #1 Mon Nov 20 17:19:26 CET 2000
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