This repo is an implementation the Plan 9 cpu command, both client and server, for Linux. More detail is available in the CPU chapter of the LinuxBoot book. Unlike the Plan 9 command, this version uses the ssh protocol for the underlying transport. It includes features familiar to ssh users, such as support for the ssh config file.
The cpu command lets you log in from a local system to a remote system and see some or all of the files (how much is up to you) from the local system.
This is wonderfully convenient for embedded systems programmers. Because some or all the files can come from your local machine, including binaries, the only thing you need installed on the remote machine is the cpu daemon itself.
Consider the case of running a complex Python program on an embedded system. We will need to either do a full install of some distro on that system, meaning we need USB ports and local storage; or we will need to run the program over the network.
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 work under different distros! Users are left juggling USB stucks and NVME cards, and this fails the first time there are two 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, meaning finding a sysadmin willing to set it up, and keep it working; or, trying to figure out which files 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 transport so that the files your program needs are available via a 9p mount. For example, if I have an embedded 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 command to 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 need not worry about scp'ing it back, or any such thing; it's just there.
The cpu command sets up the various 9p mounts with a default namespace. Users can override this default by setting the CPU_NAMESPACE environment variable. This variable looks like most PATH 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 binaries hosted from an x86 system.
In the example below, we show starting up a bash on an ARM system (solidrun honeycomb) using a cpu command running on an x86 system.
CPU_NAMESPACE=/home:/bin=`pwd`/bin:/lib=`pwd`/lib:/usr=`pwd`/usr cpu honeycomb /bin/bash
Breaking this down, we set up CPU_NAMESPACE so that:
- the remote /home is from our /home
- the remote /bin is from
pwd/bin -- which, in this case, was an unpacked arm64 file system image - the remote /lib is from
pwd/lib - the remote /usr is from
pwd/usr
We can use the path /bin/bash, because /bin/bash on the remote points to pwd/bin/bash on the local
machine.
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 h
HostName honeycomb
Port 23
User root
IdentityFile ~/.ssh/apu2_rsa
Note that the cpu command is itself a 9p server; i.e., your instance of cpu runs your server. The remote cpu 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 docker to 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; and we 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 h
Unable to find image 'ubuntu@sha256:073e060cec31fed4a86fcd45ad6f80b1f135109ac2c0b57272f01909c9626486' locally
docker.io/library/ubuntu@sha256:073e060cec31fed4a86fcd45ad6f80b1f135109ac2c0b57272f01909c9626486: Pulling from library/ubuntu
a9ca93140713: Pull complete
Digest: sha256:073e060cec31fed4a86fcd45ad6f80b1f135109ac2c0b57272f01909c9626486
Status: Downloaded newer image for ubuntu@sha256:073e060cec31fed4a86fcd45ad6f80b1f135109ac2c0b57272f01909c9626486
WARNING: The requested image's platform (linux/arm64/v8) does not match the detected host platform (linux/amd64) and no specific platform was requested
1970/01/01 21:37:32 CPUD:Warning: mounting /tmp/cpu/lib64 on /lib64 failed: no such file or directory
# ls
bbin buildbin env go init lib proc tcz ubin var
bin dev etc 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, but that 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 NAMES
b92a3576229b 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.
The cpu command makes using small embedded systems dramatically easier. There is no need to install a distro, or juggle distros; there is no need to scp files back and forth; just run commands as needed.
The first version of cpu was developed for Plan 9, and is described here.