Use developer overrides for assembly of a Fuchsia product

In some cases, you may want to override the configuration of a Fuchsia product.

Use assembly overrides

To start using developer overrides for assembly:

  1. In your Fuchsia checkout, make a //local/BUILD.gn file:

    A //local.BUILD.gn file may look like:

    import("//build/assembly/developer_overrides.gni")

assembly_developer_overrides("my_overrides") {
    kernel = {
        command_line_args = ["foo"]
    }
}

    This is what this BUILD.gn file indicates:

    • The assembly developer override is named my_overrides. You can use any meaningful identifier for this field.
    • The keys kernel, command_line_args are all based on supported values for Platform configuration. For more information, see PlatformConfig.

  2. Once you have made a //local/BUILD.gn file that meets your requirements, you can set your fx set command to use this assembly platform configuration. For example:

    fx set --assembly-override=//local:my_overrides

  3. Build Fuchsia:

    fx build

    You should now be able to build Fuchsia as you normally would.

Additionally, you can also do the following:

Add a package

There may be some cases where you may need to include additional packages to your product's packages. In this case, your //local/BUILD.gn may look like:

import("//build/assembly/developer_overrides.gni")

assembly_developer_overrides("my_custom_base_packages") {
  base_packages = [
    "//some/gn/target/for/a:package",
    "//some/other/target/for/a:package",
    "//third_party/sbase",
  ]
}

Add a shell command

There may be some cases where you may need to include shell components to your product's configuration. In this case, your //local/BUILD.gn may look like:

import("//build/assembly/developer_overrides.gni")

assembly_developer_overrides("my_custom_shell_commands") {
  shell_commands = [
    {
      package_name = "//third_party/sbase"
      components = [ "cp" ]
    },
  ]
}

Use developer_only_options

In some cases, you may want to use some of the developer_only_options. These options can be combined with the general overrides covered in Use assembly overrides.

In this case, your //local/BUILD.gn may look like:

import("//build/assembly/developer_overrides.gni")

assembly_developer_overrides("my_overrides") {
  developer_only_options = {
    all_packages_in_base = true
    netboot_mode = true
  }
}

all_packages_in_base

This option redirects all cache and on_demand package-set packages into the base package set. This feature allows the use of a product image that has cache or universe packages in a context where networking may be unavailable or a package server cannot be run.

The primary use case for this option is to allow debugging tools to be available on-device when networking is non-functional.

netboot_mode

This option creates a ZBI (Zircon Boot Image) that includes the fvm/fxfs image inside a ramdisk, to allow the netbooting of the product.

This is a replacement for the netboot assembly that was previously generated with //build/images/fuchsia:netboot.