On Mon, Apr 17, 2017 at 8:36 PM, Kirill Tkhai [off-list ref] wrote:
On implementing of nested pid namespaces support in CRIU
(checkpoint-restore in userspace tool) we run into
the situation, that it's impossible to create a task with
specific NSpid effectively. After commit 49f4d8b93ccf
"pidns: Capture the user namespace and filter ns_last_pid"
it is impossible to set ns_last_pid on any pid namespace,
except task's active pid_ns (before the commit it was possible
to write to pid_ns_for_children). Thus, if a restored task
in a container has more than one pid_ns levels, the restorer
code must have a task helper for every pid namespace
of the task's pid_ns hierarhy.
This is a big problem, because of communication with
a helper for every pid_ns in the hierarchy is not cheap
and not performance-good as it implies many helpers wakeups
to create a single task (independently, how you communicate
with the helpers). This patch tries to decide the problem.
It introduces a new pid_ns ns_ioctl(PIDNS_REQ_SET_LAST_PID_VEC),
which allows to write a vector of last pids on pid_ns hierarchy.
The vector is passed as a ":"-delimited string with pids,
written in reverse order. The first number corresponds to
the opened namespace ns_last_pid, the second is to its parent, etc.
So, if you have the pid namespaces hierarchy like:
pid_ns1 (grand father)
|
v
pid_ns2 (father)
|
v
pid_ns3 (child)
and the ns of task's of pid_ns3 is open, then the corresponding
vector will be "last_ns_pid3:last_ns_pid2:last_ns_pid1". This
vector may be short and it may contain less levels, for example,
"last_ns_pid3:last_ns_pid2" or even "last_ns_pid3", in dependence
of which levels you want to populate.
To write in a pid_ns's ns_last_pid we check that the writer task
has CAP_SYS_ADMIN permittions in this pid_ns's user_ns.
One note about struct pidns_ioc_req. It's made extensible and
may expanded in the future. The always existing fields present
at the moment, the future fields and they sizes may be determined
by pidns_ioc_req::req by the future code.
Signed-off-by: Kirill Tkhai <redacted>
Reviewed-by: Cyrill Gorcunov <redacted>