[Devel] Re: [PATCH] namespaces: introduce sys_hijack (v7)

[Serge E. Hallyn]

Quoting Serge E. Hallyn (serue at us.ibm.com):
> This is just a first stab at doing hijack by cgroup files.  I force
> using the 'tasks' file just so that I can (a) predict and check
> the name of the file, (b) make sure it's a cgroup file, and then
> (c) trust that taking __d_cont(dentry parent) gives me a legitimate
> container.
> 
> Seems to work at least.
> 
> Paul, does this look reasonable?  task_from_cgroup_fd() in particular.

The patch I sent does 'hijack a cgroup' by taking a task in the cgroup
and hijacking it.

Paul would like to be able to 'enter a cgroup', even if it is empty.
Hijack takes more than just the nsproxy from the hijacked task, so
this would result in different behavior between hijacking a populated
cgroup and an empty cgroup.  So we might want to introduce a third
type of hijacking, so we have HIJACK_PID, HIJACK_CGROUP, and
HIJACK_EMPTY_CGROUP.

It also then acts like the nsproxy cgroup patchset I sent out months
ago for simply entering namespaces.  In fact this would need to be
restricted to ns cgroups, and ns cgroups would need to grab a reference
to the nsproxy.

So do we want to allow hijacking/entering an empty cgroup?


-serge
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