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Linux Capabilities Support in Mesos Containerizer

This document describes the linux/capabilities isolator. The isolator adds support for controlling Linux Capabilities of containers launched using the MesosContainerizer

The Linux capabilities isolator allows operators to control which privileged operations Mesos tasks may perform. Operators can specify which capabilities to allow for containers executing on an agent; containers on the other hand can expose which capabilities they need.

See the protobuf definition of CapabilityInfo::Capability for the list of currently exposed capabilities.

Agent setup

The Linux capabilities isolator is loaded when linux/capabilities is present in the agent’s --isolation flag. This isolator requires the CAP_SETPCAP capability so agent processes typically need to be started as root.

The --effective_capabilities flag specifies a set of capabilities that are always granted to tasks. If the running kernel (Linux 4.3 or later) supports ambient capabilities, these capabilities will be added to the effective capability set of the task when it is launched. Otherwise they must be re-acquired by arranging for the task to execute a file with the relevant file-based capabilities enabled.

In the absence of capabilities specified by the scheduler, an empty list for --effective_capabilities signifies that all capabilities will be explicitly dropped. If the --effective_capabilities flag is not present, the task will be launched with the default capabilities of the task user but the ambient capabilities will not be set.

The --bounding_capabilities flag specifies an upper bound on the the capabilities a task is allowed to acquire or be granted. Schedulers are not allowed to launch tasks with capabilities outside the set specified by the --bounding_capabilities flag, but may specify effective or bounding capabilities that are within this set.

An empty list for --bounding_capabilities signifies that no capabilities are allowed, while an absent --bounding_capabilities flag signifies that all capabilities are allowed.

A possible agent startup invocation could be

sudo mesos-agent --master=<master ip> --ip=<agent ip>
  --isolation=linux/capabilities[,other isolation flags]

Task setup

In order for a Mesos task to acquire specific effective capabilities or limit its bounding capabilities it should declare the required capabilities in the LinuxInfo element of its ContainerInfo.

A Mesos task can only request capabilities which are allowed according to the --bounding_capabilities flag of the agent; a task requesting other capabilities will be rejected. When the --bounding_capabilities flag is not present, all capability requests will be granted.

If the optional effective_capabilities field is not set, the value of the --effective_capabilities flag will be used to populate the task capabilities. If the optional bounding_capabilities field is not set, the value of the --bounding_capabilities flag will be used to populate the task capabilities. In both case, if an empty list of capabilities is given, the Mesos task will drop all capabilities in the corresponding set.