[PATCH 0/20] Pid namespaces

[Pavel Emelyanov <xemul@openvz.org>]

A pid namespace is a "view" of a particular set of tasks on the system.
They work in a similar way to filesystem namespaces.  A file (or a process)
can be accessed in multiple namespaces, but it may have a different name in
each.  In a filesystem, this name might be /etc/passwd in one namespace,
but /chroot/etc/passwd in another.

For processes, a process may have pid 1234 in one namespace, but be pid 1
in another. This allows new pid namespaces to have basically arbitrary
pids, and not have to worry about what pids exist in other namespaces.
This is essential for checkpoint/restart where a restarted process's pid
might collide with an existing process on the system's pid.

In this particular implementation, pid namespaces have a parent-child
relationship, just like processes.  A process in a pid namespace may see
all of the processes in the same namespace, as well as all of the processes
in all of the namespaces which are children of its namespace. Processes may
not, however, see others which are in their parent's namespace, but not in
their own. The same goes for sibling namespaces.

The know issue to be solved in the nearest future is signal handling in 
the namespace boundary. That is, currently the namespace's init is treated 
like an ordinary task that can be killed from within an namespace. Ideally, 
the signal handling by the namespace's init should have two sides: when 
signaling the init from its namespace, the init should look like a real
init task, i.e. receive only those signals, that is explicitly wants to; 
when signaling the init from one of the parent namespaces, init should look 
like an ordinary task, i.e. receive any signal, only taking the general 
permissions into account.

The pid namespace was developed by Pavel Emlyanov and Sukadev Bhattiprolu
and we eventually came to almost the same implementation, which differed
in some details. This set is based on Pavel's patches, but it includes 
comments and patches that from Sukadev.

Many thanks to Oleg, who reviewed the patches, pointed out many BUGs and
made valuable advises on how to make this set cleaner.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
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