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This repo is an implementation the Plan 9 cpu command, both client and server, for Linux.More detail is available in theCPU chapter of the LinuxBoot book.Unlike the Plan 9 command, this version uses the ssh protocol for the underlying transport. It includesfeatures familiar to ssh users, such as support for the ssh config file.

The cpu commandlets you log in from a local system to a remote system and see some or all of the files (how much isup to you) from the local system.

This is wonderfully convenient for embedded systems programmers. Because some or all the filescan come from your local machine, including binaries, the only thing you need installedon the remote machine is the cpu daemon itself.

Consider the case of running acomplex Python program on an embedded system.We will need to either do a full install of some distro on that system, meaning weneed USB ports and local storage; or we will need to run the program over thenetwork.

Installing distros can turn into a mess. Some programs only work under specific distros.In some cases, when two programs are needed in a pipeline, it can happen that they only workunder different distros!Users are left juggling USB sticks and NVME cards, and this fails the first time there aretwo programs which need two different distros.

Running over a network is usually done with ssh, but ssh can not supply the programs and files.We would need to either set up a network file system, meaningfinding a sysadmin willing to set it up, and keep it working; or, trying to figure out whichfiles the program needs, and using rsync or scp to get them there. In some cases,the target system might not have enough memory to hold those files!

Cpu looks like ssh, but with an important difference: it also provides a file transportso that the files your program needs are available via a 9p mount. For example, if I have anembedded system named camera, and I need to read the flash with the flashrom command, I simply type:

cpu camera flashrom -r rom.img

Breaking this down: cpu is the cpu command; camera is the host name; flashrom is the commandto run; the options are to do a read (-r) into a file called rom.img.

Where does that file end up? In whatever of my home directories I ran the cpu command from. I neednot worry about scp'ing it back, or any such thing; it's just there.

You can easily build your own docker container to try things out.

docker build -t "${USER}/cpu:latest" .

or if you have installed a version of docker buildx, you can build a multi-arch manifest container and push it to docker hub:

% docker login% docker buildx build --platform "linux/amd64,linux/arm64,linux/arm/v7" --progress plain --pull -t "${USER}/cpu:latest" .

We have created a docker container so you can try cpu client and server:

ghcr.io/u-root/cpu:main

It includes both the cpud (server) and cpu (client) commands. In thecontainer, you only have access to date and cat commands, but that is enough to getthe idea.

You will need keys. You can either use your own SSH keys that you use forother things, for example:

export KEY=~/.ssh/id_rsaexport KEY=~/.ssh/a_special_key_for_this_docker

or generate one and use it.

ssh-keygen -f key -t rsa  -P ""export KEY=`pwd`/key

NOTE! The name KEY is not required. Instead of KEY, youcan use any name you want, as long as you use it in the dockercommand below.

To start the cpud, you need docker installed. Once that is done, you need to createa docker network and start the daemon, with public and private keys.The --mount option allows docker to provide the keys, using a bind mountfor both the private and public key.That is how we avoidstoring keys in the container itself.

docker network create cpud# If you ran docker before and it failed in some way, you may need to remove the# old identity (e.g. docker rm cpud_test)docker run --rm -v $KEY.pub:/key.pub -v $KEY:/key -v /tmp --name cpud_test --privileged=true -t -i -p 17010:17010 ghcr.io/u-root/cpu:main

Then you can try running a command or two by using the embedded cpu client in the docker container.
NOTE: when you run cpu in this way, it does not immediately have access to your host's file system,which means you won't be able to really leverage the back-mount and you'll have to artificially setenvironment variables (like PWD) so that the remote task can execute.

docker exec -it -e PWD=/root cpud_test  /bin/cpu -key /key localhost /bin/date

Remember, this cpu command is running in the container. You need to use the name /key in thecontainer, not $KEY and you'll only be able to run binaries that have been pre-loaded into thecontainer (which in the public container is /bin/cat and /bin/date)

To see the mounts:

docker exec -it -e PWD=/root cpud_test  /bin/cpu -key /key localhost /bin/cat /proc/mounts

You might want to just get a cpu command to let you talk to the docker cpuddirectly:

go install github.com/u-root/cpu/cmds/cpu@latest

And now you can run

cpu -key $KEY localhost date

NOTE: if you are running on OSX, remember that your cpud docker is linux, so if you try the abovecommand you'll see:

$ cpu -key $KEY localhost date2022/10/13 18:52:17 CPUD(as remote):fork/exec /bin/date: exec format error

To deal with that we'll have to play games with the namespace, which we'll cover next.

The cpu command sets up the various 9p mounts with a default namespace. Users can override thisdefault with the -namespace switch. The argument to the switch looks likePATH variables, with comma-separated values, but with one extra option: users can, optionally,specify the local path and the remote path. This is useful when running ARM binarieshosted from an x86 system.

In the example below, we show starting up a bash on an ARM system (solidrun honeycomb) usinga cpu command running on an x86 system.

cpu -namespace /home:/bin=`pwd`/bin:/lib=`pwd`/lib:/usr=`pwd`/usr honeycomb /bin/bash

Breaking this down, we set up the namespace so that:

• the remote /home is from our /home
• the remote /bin is frompwd/bin -- which, in this case, was an unpacked arm64 file system image
• the remote /lib is frompwd/lib
• the remote /usr is frompwd/usr

We can use the path /bin/bash, because /bin/bash on the remote points topwd/bin/bash on the localmachine.

We can use the same trick to cpu to Linux from OSX, but instead of having an Arm tree underpwdwe'll need a Linux binary tree to pick binaries from.

There are many IoT like devices that do not have an ethernet port.Fear not though: TheLinux USB gadgetdriversoffer ethernet via USB!

There aretutorials outthere, and here is the gist:

• enable the Linux kernel options
  • CONFIG_USB_GADGET
  • CONFIG_USB_ETH
  • CONFIG_INET
• add the MAC addresses for your gadget device and the machine you connect to inthe kernelCMDLINE, e.g.,g_ether.dev_addr=12:34:56:78:9a:bc g_ether.host_addr=12:34:56:78:9a:bd
  • fixed MAC addresses allow your laptop to recognize the gadget again
  • when using a CDC gadget (e.g. CDC ethernet+serial), useg_cdc.dev_addr/g_cdc.host_addr

You will also need to configure the network. You can either go with DHCP orfixed addresses. Sincecpud spawns a shell, if your shell allows for it, youcan use a shell rc file.

As mentioned, cpu looks and feels a lot like ssh, to the point of honoring ssh config files.For the honeycomb, for example, the ssh config entry looks like this (we shorten the name to 'h'for convenience):

Host hHostName honeycombPort 17010User rootIdentityFile ~/.ssh/apu2_rsa

Note that the cpu command is itself a 9p server; i.e., your instance of cpu runs your server. The remotecpu server may run as root, but all file accesses happen locally as you. Hence,the cpu command does not grant greater access to the local machine than you already possess.I.e., there is no privilege escalation.

Maintaining file system images is inconvenient.We can use Docker containers on remote hosts instead.We can take a standard Docker container and, with suitable options, use dockerto start the container with cpu as the first program it runs.

That means we can use any Docker image, on any architecture, at any time; andwe can even run more than one at a time, since the namespaces are private.

In this example, we are starting a standard Ubuntu image:

docker run -v /home/rminnich:/home/rminnich -v /home/rminnich/.ssh:/root/.ssh -v /etc/hosts:/etc/hosts --entrypoint /home/rminnich/go/bin/cpu -it ubuntu@sha256:073e060cec31fed4a86fcd45ad6f80b1f135109ac2c0b57272f01909c9626486 hUnable to find image 'ubuntu@sha256:073e060cec31fed4a86fcd45ad6f80b1f135109ac2c0b57272f01909c9626486' locallydocker.io/library/ubuntu@sha256:073e060cec31fed4a86fcd45ad6f80b1f135109ac2c0b57272f01909c9626486: Pulling from library/ubuntua9ca93140713: Pull completeDigest: sha256:073e060cec31fed4a86fcd45ad6f80b1f135109ac2c0b57272f01909c9626486Status: Downloaded newer image for ubuntu@sha256:073e060cec31fed4a86fcd45ad6f80b1f135109ac2c0b57272f01909c9626486WARNING: The requested image's platform (linux/arm64/v8) does not match the detected host platform (linux/amd64) and no specific platform was requested1970/01/01 21:37:32 CPUD:Warning: mounting /tmp/cpu/lib64 on /lib64 failed: no such file or directory# lsbbin  buildbinenv  go    init     lib    proc  tcz  ubin  varbin   devetc  home  key.pub  lib64  sys tmp  usr#

Note that the image was updated and then started. The /lib64 mount fails, because there is no /lib64 directory in the image, butthat is harmless.

On the local host, on which we ran docker, this image will show up in docker ps:

CONTAINER ID   IMAGE     COMMAND                  CREATED         STATUS         PORTS     NAMESb92a3576229b   ubuntu    "/home/rminnich/go/b…"   9 seconds ago   Up 9 seconds             inspiring_mcnulty

Even though the binaries themselves are running on the remote ARM system.

Vsock is a useful transport layer available in Linux, and support by at least QEMU.

We use the mdlayher/vsock package.

In the cpu and cpud, the switch

-net vsock

will enable vsock.

In the host kernel, you needvhost_vsock module:

sudo modprobe vhost_vsock

.

When starting qemu, add

-device vhost-vsock-pci,id=vhost-vsock-pci0,guest-cid=3

to the command line. The '3' is arbitrary; it just needs to be agreed upon on both sides.

When running a cpu command, the host name is the vsock guest-cid you specified in qemu:

cpu -net vsock 3 date

If you want a different port, you can use the same -sp switch you use for other network types.

A variation of the cpu and cpud commands now exist which use dns-sd to autodiscover and register cpu resources. DNS-SD is a multi-cast DNS protocol that will multi-cast out requests for resources and get responses from participating cpud nodes. Meta-data provides information such as architecture, OS, number of cores, free memory, load average, and number of existing cpu clients. When you start a decpud you can specify additional meta-data which might be harder to auto-disocver such as near=storage, near=gateway, near=gpu, or secure=true (if running in a confidential computing domain).

In order to use this functionality, you can use decpu and decpud just as you would cpu and cpud. decpud will enable dns-sd registration by default, and multicast its information in response to requests. In order to use this from decpu, you can just specify decpu without a hostname and, by default, it will find the lowest loaded decpud with the same architecture and OS as the host you run the decpu command from.

You can specify additional constraints by using a dns-sd URI formulation:

  decpu dnssd://?requirement=value\&otherrequirement=\<othervalue\&sort=tenet

Essentially you can provide a set of key/value requirements that must be met in order for a decpud node to be considered. You can override the defaults (arch/os), or you can specify a minimum core count or minimum amount of free memory. Numeric values can use comparison operators (< or >) Finally, you can specify any number of numeric keys to sort based on. If you want to ignore one of the default requirements (e.g. arch, os) then you can set to a *:

  decpu dnssd://?arch=\*\&os=\*

This can be useful if what you are running is a script or an interpretive language like python.If you are running on the shell, make sure you escape any characters the shell may have an interest in (<, >, !, &, *)

In order to use dns-sd you have to be able to send/receive multicast. The easiest way to do this on Linux is to use host networking when you start your docker containers (--network host). This unfortunately does not work on Mac OSX, so you will need to run a relay on your system that tunnels multicast to/from the docker network. The relay client can be run in every container or you can start all dns-sd containers in the same docker network and start a relay container client on that network to communicate for the group.

There is a container in the decent-e fork of the dnssd package:https://github.com/decent-e/dnssd under cmd/relay. Running relay will start the server on the host, and running relay mode=client will start the client inside the docker container. You can also use a pre-build docker image ( docker pull ghcr.io/decent-e/dnssd:main ) to start the client in a docker.

Example: (on mac)

# presumes you have already setup your $KEY appropriately% docker create network cpud% go install github.com/decent-e/dnssd/cmd/relay@latest% export PATH=$GOBIN:$PATH% relay &% docker run -d --network cpud ghcr.io/decent-e/dnssd:main% docker run -d --network cpud -v $KEY:/key -v $KEY.pub:/key.pub -v /tmp:/tmp --privileged --rm --name decpud ghcr.io/decent-e/cpu:decent-e /bin/decpud% docker exec -i -t -e PWD=/ decpud /bindecpu -key /key . /bin/date# you should also be able to see the service from your mac% dns-sd -B _ncpu._tcpBrowsing for _ncpu._tcpDATE: ---Sun 09 Oct 2022---18:35:56.925  ...STARTING...Timestamp     A/R    Flags  if Domain               Service Type         Instance Name18:35:56.925  Add        3  17 local.               _ncpu._tcp.          d3c196958c24-cpud18:35:56.925  Add        2  14 local.               _ncpu._tcp.          d3c196958c24-cpud

The cpu command makes using small embedded systems dramatically easier. There is no need to installa distro, or juggle distros; there is no need to scp files back and forth; just run commandsas needed.

For debugging,tcpdump is very handy. Reada short tutorial to get familiar with it.

• Short Talk "building small stateless network-controlled appliances withcoreboot/linuxboot and u-root’s cpu command"
  • at Open Source Firmware Conference 2019slides /recording
  • at BARC2021slides (PDF)
• Network Managed Processors at IoT World 2021
• "Plan 9 CPU command, in Go, for Linux - the network is the computer -- for real this time" at FOSDEM 2022
• "Drivers From Outer Space at CLT 2022 - Fast, Simple Driver Development"
• Short demo ofAttaching CPUs via USB

The first version of cpu was developed for Plan 9, and is describedhere.