Process interface / requirements :: gokrazy

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# Process interface / requirements

## Process supervision

You can find the corresponding code in func gokrazy.supervise.

gokrazy’s init process (pid 1) supervises all the binaries the user specified via `gokr-packer` flags.

More specifically, gokrazy:

1. Starts your binary using Go’s `os/exec.Command` API.
   - The `stdout` and `stderr` file descriptors are hooked up to a ring buffer and can be viewed via gokrazy’s web interface.
   - Extra command-line flags or environment variables can be specified using per-package configuration.
2. When your binary’s process exits, gokrazy restarts it!
   - If the process exits with status code `0` (or `125`), gokrazy will stop supervision. Exiting immediately with status code `0` when the `GOKRAZY_FIRST_START=1` environment variable is set means “don’t start the program on boot”

## Environment variables

gokrazy sets the `HOME` environment variable to `HOME=/perm/home/<cmd>`, where `<cmd>` is the name of your binary. For example, `tailscale.com/cmd/tailscaled` is started with `HOME=/perm/home/tailscaled`.

When your binary is first started, gokrazy sets the `GOKRAZY_FIRST_START=1` environment variable.

## Privilege dropping / security

An easy way to implement privilege dropping in Go is to re-execute the process with syscall.SysProcAttr fields set. For example, this is how you would drop privileges to user `nobody` (uid/gid 65534):

    // mustDropPrivileges re-executes the program in a child process,
    // dropping root privileges to user nobody.
    func mustDropPrivileges() {
    	if os.Getenv("NTP_PRIVILEGES_DROPPED") == "1" {
    		return
    	}

    	cmd := exec.Command(os.Args[0])
    	cmd.Env = append(os.Environ(), "NTP_PRIVILEGES_DROPPED=1")
    	cmd.Stdout = os.Stdout
    	cmd.Stderr = os.Stderr
    	cmd.SysProcAttr = &syscall.SysProcAttr{
    		Credential: &syscall.Credential{
    			Uid: 65534,
    			Gid: 65534,
    		},
    	}
    	log.Fatal(cmd.Run())
    }

Examples:

- `github.com/gokrazy/gokrazy/cmd/ntp` is a rather involved example which retains the CAP_SYS_TIME capability in the child process
- `github.com/gokrazy/rsync` uses Linux mount namespaces and constructs a file system with read-only bind mounts of the configured rsync modules