I have compiled my project and it runs from the command line ok, however, in the larger scope of the project this executable is executed via os.Command("runme").Output()... and that's running inside an
OpenBSD chroot.

Is your project closed source? Do you have a reproducible snippet (a smaller one makes investigation easier, but it doesn't have to be) that can be used to reproduce this issue?

Here is a sample....

hello.go

import "fmt"

func main() {
        fmt.Println("Hello, 世界")
}

check.go

package main

import (
        "fmt"
        "os/exec"
)

func main() {
        out, err := exec.Command("./hello").Output()
        if err == nil {
                fmt.Printf("OUT: %s\n", out)
        } else {
                fmt.Printf("ERR: %s\n", err)
        }
}

To build:

go build check.go
go build hello.go

test to make certain it runs...

./check

produces

OUT: Hello, 世界

Then.... try to run in chroot

sudo chroot -g myid -u myid . ./check

and this produces....

ERR: open /dev/null: no such file or directory

so the problem is that the Output() is trying to open the NULL device and that device does not exist in OpenBSD6.6 chroot.

One more note. I created a dev/null in my project folder

mkdir dev
cd dev
ls -l /dev/null
sudo mknod -m 666 null c 2 2

then tried to run it again

sudo chroot -g zurvita -u zurvita . ./check

this time I got

ERR: open /dev/null: device not configured

LASTLY... it is possible that nodev in the /etc/fstab is causing this last error.... still researching as it is not convenient to reboot right now.

You should set stdin, stdout, and stderr of your command.
The spec of exec.Cmd's Stdin field says:

	// Stdin specifies the process's standard input.
	//
	// If Stdin is nil, the process reads from the null device (os.DevNull).
	Stdin io.Reader

Same for Stdout and Stderr.
You could use, e.g., a bytes.Buffer for all of them.

Any OpenBSD system should have a file /dev/null. I don't understand why your system does not have that. I suggest asking the OpenBSD community.

This appears to be working as intended. Within a chroot either

1. mknod /dev/null -- the ENXIO ("device not configured") error is indeed from the nodev mount option and removing that should fix things
2. or, as @randall77 mentions, use empty buffers to avoid opening /dev/null

package main

import (
        "io"
        "os/exec"
        "syscall"
        "testing/iotest"
)

func main() {
        syscall.Chroot("/my/dir")
        cmd := exec.Command("/my/cmd")
        cmd.Stdin = iotest.ErrReader(io.EOF)
        cmd.Stdout = io.Discard
        cmd.Stderr = io.Discard
        cmd.Run()
}

Any OpenBSD system should have a file /dev/null. I don't understand why your system does not have that. I suggest asking the OpenBSD community.

@ianlancetaylor this is specifically within chroot where OpenBSD won't automatically create /dev/null

Thanks. It sounds like this is not a problem in the Go standard library, so closing.