agent-ctl

Agent Control tool

Overview

The Kata Containers agent control tool (kata-agent-ctl) is a low-level test tool. It allows basic interaction with the Kata Containers agent, kata-agent, that runs inside the virtual machine.

Unlike the Kata Runtime, which only ever makes sequences of correctly ordered and valid agent API calls, this tool allows users to make arbitrary agent API calls and to control their parameters.

Audience and environment

Warning:

This tool is for advanced users familiar with the low-level agent API calls. Further, it is designed to be run on test and development systems only: since the tool can make arbitrary API calls, it is possible to easily confuse irrevocably other parts of the system or even kill a running container or sandbox.

Full details

For a usage statement, run:

$ cargo run -- --help

To see some examples, run:

$ cargo run -- examples

Code summary

The table below summarises where to look to learn more about both this tool, the agent protocol and the client and server implementations.

Description	File	Example RPC or function	Example summary
Protocol buffers definition of the Kata Containers Agent API protocol	agent.proto	CreateContainer	API to create a Kata container.
Agent Control (client) API calls	src/client.rs	agent_cmd_container_create()	Agent Control tool function that calls the CreateContainer API.
Agent (server) API implementations	rpc.rs	create_container()	Server function that implements the CreateContainers API.

Run the tool

Prerequisites

It is necessary to create an OCI bundle to use the tool. The simplest method is:

$ bundle_dir="bundle"
$ rootfs_dir="$bundle_dir/rootfs"
$ image="busybox"
$ mkdir -p "$rootfs_dir" && (cd "$bundle_dir" && runc spec)
$ sudo docker export $(sudo docker create "$image") | tar -C "$rootfs_dir" -xvf -

Specify API commands to run

The tool allows one or more API commands to be specified using the -c or --cmd command-line options. At their simplest, these are just the name of the API commands, which will make the API command using default values (generally blank or empty) where possible. However, some API calls require some basic value to be specified such as a sandbox ID or container ID. For these calls, the tool will generate a value by default unless told not to.

If the user wishes to, they may specify these values as part of the command using name=value syntax.

In addition to this, it is possible to specify either a complete or partial set of values for the API call using JSON syntax, either directly on the command-line or via a file URI.

The table below summarises the possible ways of specifying an API call to make.

CLI values	API Query
-c 'SomeAPIName' -n	Calls the API using the default values for all request options
-c 'SomeAPIName'	Calls the API specifying some values automatically if possible
-c 'SomeAPIName foo=bar baz="hello world" x=3 y="a cat"'	Calls the API specifying various values in name/value form
-c 'SomeAPIName json://{}' -n	Calls the API specifying empty values via an empty JSON document
-c 'SomeAPIName json://{"foo": true, "bar": "hello world"}' -n	Calls the API specifying some values in JSON syntax
-c 'SomeAPIName file:///foo.json' -n	Calls the API passing the JSON values from the specified file

JSON Example

An example showing how to specify the messages fields for an API call (GetGuestDetails):

$ cargo run -- -l debug connect --server-address "unix://@/tmp/foo.socket" --bundle-dir "$bundle_dir" -c Check -c 'GetGuestDetails json://{"mem_block_size": true, "mem_hotplug_probe": true}'

Note:

For details of the names of the APIs to call and the available fields in each API, see the Code Summary section.

Connect to a real Kata Container

The method used to connect to Kata Containers agent depends on the configured hypervisor. Although by default the Kata Containers agent listens for API calls on a VSOCK socket, the way that socket is exposed to the host depends on the hypervisor.

QEMU

Since QEMU supports VSOCK sockets in the standard way, it is only necessary to establish the VSOCK guest CID value to connect to the agent.

1. Start a Kata Container

2. Establish the VSOCK guest CID number for the virtual machine:

   $ guest_cid=$(sudo ss -H --vsock | awk '{print $6}' | cut -d: -f1)

3. Run the tool to connect to the agent:

   # Default VSOCK port the agent listens on
   $ agent_vsock_port=1024
   
   $ cargo run -- -l debug connect --bundle-dir "${bundle_dir}" --server-address "vsock://${guest_cid}:${agent_vsock_port}" -c Check -c GetGuestDetails

   This examples makes two API calls:

   • It runs Check to see if the agent's RPC server is serving.
   • It then runs GetGuestDetails to establish some details of the environment the agent is running in.

Cloud Hypervisor and Firecracker

Cloud Hypervisor and Firecracker both use "hybrid VSOCK" which uses a local UNIX socket rather than the host kernel to handle communication with the guest. As such, you need to specify the path to the UNIX socket.

Since the UNIX socket path is sandbox-specific, you need to run the kata-runtime env command to determine the socket's "template path". This path includes a {ID} tag that represents the real sandbox ID or name.

Further, since the socket path is below the sandbox directory and since that directory is root owned, it is necessary to run the tool as root when using a Hybrid VSOCKS hypervisor.

Determine socket path template value

Configured hypervisor is Cloud Hypervisor

$ socket_path_template=$(sudo kata-runtime env --json | jq '.Hypervisor.SocketPath')
$ echo "$socket_path_template"
"/run/vc/vm/{ID}/clh.sock"

Configured hypervisor is Firecracker

$ socket_path_template=$(sudo kata-runtime env --json | jq '.Hypervisor.SocketPath')
$ echo "$socket_path_template"
"/run/vc/firecracker/{ID}/root/kata.hvsock"

Note:

Do not rely on the paths shown above: you should run the command yourself as these paths may change.

Once you have determined the template path, build and install the tool to make it easier to run as the root user.

Build and install

# Install for user
$ make install

# Install centrally
$ sudo install -o root -g root -m 0755 ~/.cargo/bin/kata-agent-ctl /usr/local/bin

1. Start a Kata Container

   Create a container called foo.

2. Run the tool

   # Name of container
   $ sandbox_id="foo"
   
   # Create actual socket path
   $ socket_path=$(echo "$socket_path_template" | sed "s/{ID}/${sandbox_id}/g" | tr -d '"')
   
   $ sudo kata-agent-ctl -l debug connect --bundle-dir "${bundle_dir}" --server-address "unix://${socket_path}" --hybrid-vsock -c Check -c GetGuestDetails

   Note: The socket_path_template variable was set in the Determine socket path template value section.

Run the tool and the agent in the same environment

Warnings:

• This method is only for testing and development!
• Only continue if you are using a non-critical system (such as a freshly installed VM environment).

1. Start the agent, specifying a local socket for it to communicate on:

   $ sudo KATA_AGENT_SERVER_ADDR=unix:///tmp/foo.socket target/x86_64-unknown-linux-musl/release/kata-agent

   Note: This example assumes an Intel x86-64 system.

2. Run the tool in the same environment:

   $ cargo run -- -l debug connect --server-address "unix://@/tmp/foo.socket" --bundle-dir "$bundle_dir" -c Check -c GetGuestDetails

   Note:

   The @ in the server address is required - it denotes an abstract socket which the agent requires (see unix(7)).