Nomad exec driver does not support rootfs root partition

[th0m]

Nomad version

Nomad v0.10.2

Operating system and Environment details

Debian Stretch
Root partition is a rootfs (ramdisk)

$ df -h /
Filesystem      Size  Used Avail Use% Mounted on
rootfs           63G  4.5G   59G   8% /

Issue

Any job using the exec driver fails to start which I believe because pivot_root does not work when the root partition is a ramdisk, see man 2 pivot_root

The rootfs (initial ramfs) cannot be pivot_root()ed.

I think Docker addressed this issue with the DOCKER_RAMDISK env variable which essentially disables pivot_root. I think what I need here is a way to disable pivot_root, probably as a configuration variable in the exec driver.

Reproduction steps

1. Boot a Ubuntu live cd
2. Run Nomad
3. Plan a job using the exec driver

Job file (if appropriate)

$ cat /etc/nomad-jobs/job-hello_world.hcl 
job "hello_world" {
  datacenters = ["foo"]

    task "server" {
      driver = "exec"

      config {
        command = "/bin/true"
      }
  }
}

Nomad Client logs (if appropriate)

Feb 12 19:49:03 machine nomad-client[258433]:     2020-02-12T19:49:03.332Z [ERROR] client.alloc_runner.task_runner: running driver failed: alloc_id=fca82002-653b-33df-9324-4ba2bc1bbd15 task=server error="failed to launch command with executor: rpc error: code = Unknown desc = container_linux.go:346: starting container process caused "process_linux.go:449: container init caused \"rootfs_linux.go:109: jailing process inside rootfs caused \\\"pivot_root invalid argument\\\"\"""

[tgross]

Hi @th0m! Thanks for opening this issue. I'm really curious to hear what you're doing with Nomad that you're running it from a rootfs!

As far as the feature request goes, that sounds reasonable. It looks like the underlying runc library supports this with the Config.NoPivot root field, so it'd be a matter of adding the config value to the exec stanza and then plumbing it through to runc. We'd love to get this as a contribution if you're interested.

[th0m]

Hi @tgross thanks for the quick reply. About the rootfs, nothing fancy, just running Nomad on mostly stateless machines :)

If that sounds reasonable to you, I'll start working on a PR then!

[tgross]

Great! Looking forward to seeing the PR! 😀

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