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Ebuild FAQ

This document aims to answer common questions about building packages used in developing ChromiumOS.

The CrOS SDK chroot environment uses the Portage package management system. Portage, from the Gentoo Linux distribution, consists of two main components: the tree of ebuild overlays and emerge. The ebuild system is a tree of bash scripts (ebuilds) that contain build instructions responsible for building and installing packages. Emerge is the user interface to interacting with ebuilds. For questions about overlays, see the Overlay FAQ.

Portage via the CrOS SDK chroot enables the creation of different cross-compilation environments which are used to build system images for Chromebooks.

Good Gentoo Documentation:

How does the cross compiling setup work?

The chroot is used for cross-compiling. Portage, the chroot’s package management system, is designed to compile packages from source and install the resulting binaries into a root directory. Unlike Debian, where build rules are found only in source code, Portage keeps build instructions in dedicated files called ebuilds.

In our chroot, Portage installs packages into multiple directories:

To cross-compile, the SDK requires special tools which are installed via the virtual/target-chromium-os-sdk package. When additional tools are needed, this package can be upgraded and re-merged (see Adding a package to the SDK). The cros build-packages command ensures that re-merging will add required tools automatically. Notice that since we are updating the chroot/host, we use the standard emerge command that installs packages at the root and uses the host's configuration:

sudo emerge -a virtual/target-chromium-os-sdk

Each target board requires a distinct sysroot. First, we ensure the host has the necessary toolchain installed and then we set up sysroot wrappers for the toolchain binaries that set the sysroot location during compilation. We use the toolchain headers and libraries to bootstrap the board target sysroot in /build/<board_name>. When we set up a sysroot for a board, we select an overlay corresponding to that board and link it in to the sysroot. This allows us to customize or add packages & configuration on a per-board basis. The board overlay can also include a make.conf that overrides some default settings such as compiler CFLAGS.

When cross-compiling, you also need packages that correspond to the target, such as header files and libraries. All build-time dependencies for a target are installed in to that target's sysroot. These are only used for compilation and will not be placed into images that are built via cros build-image.

Once a board is set up, we can start building packages for the board target. We use the board target specific version of emerge to do this.

How does the image build work?

Building a full bootable image is a multi-step process. For a given target, the first step is to build binary packages for all packages needed by ChromiumOS using cros build-packages. If you look into the details of cros build-packages, you will see that it works by installing the virtual/target-os package into a target. For example, to prepare to build an x86-generic image:

emerge-amd64-generic -a virtual/target-os

We've set a portage option to build binary packages as a side-effect of building anything from source. If you build all of the packages needed by the virtual/target-os ebuild then we will have a binary package for each of these.

Once cros build-packages finishes, we'll run cros build-image. If we look into the details of cros build-image, you will see that it works by setting up a loopback-mounted ext3 file system, prepping it for booting, and then installing virtual/target-os into it with an option to force using binary packages only. That way we only install the packages and their run-time dependencies and shed all of the build dependencies:

emerge-<board_name> --root="$ROOT_FS_DIR" --root-deps=rdeps --usepkgonly

We use the --root-deps=rdeps option to only install run-time dependencies into the root file system. This discards the build-only dependencies and gives us a clean file system.

*** note SIDE NOTE: It turns out that for most developers, the above is a bit of a lie. Most developers don't actually build all binary packages from source when they do a build. By default, the build system "cheats" and downloads a binary package from the BINHOST if it can find one. See What does cros build-packages actually do.

Where do the important files live?

Here are a few important directories:

Type Directory
main set of ebuilds src/third_party/portage-stable/
ChromiumOS portage overlay src/third_party/chromiumos-overlay/
ChromiumOS ebuilds src/third_party/chromiumos-overlay/chromeos-base/
target profile (per-package unmask, USE flags, etc) src/third_party/chromiumos-overlay/profiles/targets/chromeos/
host and per-target configs src/third_party/chromiumos-overlay/chromeos/config/
crossdev autoconf configs (in chroot) /usr/share/crossdev/include/site/
board sysroot (in chroot) /build/${BOARD}

What are USE flags?

USE flags are boolean flags that can be used to provide conditional logic to packages. They can be used to turn on or off certain parts of a package build (e.g., USE=perl may enable optional perl bindings), or even allow a package to be conditionally included in the build via dependencies that express USE constraints.

USE flags may be enabled globally for a certain board, conditionally for certain packages via profiles, or even enabled by setting the USE environment variable. To see the set of global USE flags for a board:

$ cros query boards -f "name == 'BOARDNAME'" -o "{use_flags}"

How can I make use of USE flags in my ebuild?

The USE variable is filtered down to only flags declared in IUSE (with the exception of IUSE_IMPLICIT flags), which allows us to make guarantees about which flags will affect your build, and only rebuild your package when required.

First, add the flag you need to depend upon to IUSE. Then, you can use it in a number of ways:

  1. To express a conditional dependency in your ebuild:

        ssl? ( >=dev-libs/openssl-1.1.1:= )
  2. To conditionally execute logic in your ebuild:

    src_configure () {
        if use ssl; then
            : do something
        : ...

How can I create new USE flags?

There's no central location to define USE flags. If a flag is listed in IUSE, it may be set in a profile or in the environment, and the flag will be set for the ebuild.

To set the flag for a board, you're likely going to want to add it to the appropriate make.defaults file in the profile for that board. For example:

# src/overlays/overlay-amd64-generic/profiles/base/make.defaults
# ...

# Enable SSL support.
USE="${USE} ssl"

Your next question is probably: how do I find the right make.defaults to add my flags to? Profiles are structured hierarchically, and you may wish to add the flag to a general profile, or to a profile for a very specific board.

To list all profiles visible to a board:

$ cros query boards -t -f "name == 'BOARDNAME'"

A common pattern is to enable a flag for all ChromeOS boards, and only disable the flag on certain boards which should not have the feature. The file src/third_party/chromiumos-overlay/profiles/targets/chromeos/make.defaults will apply to all ChromeOS boards, and you can then disable the flag for certain boards:

# src/overlays/overlay-amd64-generic/profiles/base/make.defaults
# ...

# Disable SSL support.
USE="${USE} -ssl"


Note: BDEPEND was introduced in EAPI=7, so ebuilds using older versions must be updated before it is available.

Note: All BDEPEND packages must be explicitly enumerated in the virtual/target-sdk package. Using BDEPEND alone is not sufficient. See How does the cross compiling setup work? for more info.

The different dependencies fall into two categories; build-time and run-time. DEPEND and BDEPEND are build-time dependencies. If I have a build-time dependency on a package, it means that the package needs to be installed before I can build. RDEPEND is a run-time dependency. If I have a run-time dependency on a package, it means I need that package in order to run, but do not need it to build.

The DEPEND and BDEPEND build-time dependencies are most distinct when cross-compiling (we are always cross-compiling when running a ChromiumOS device build). We use CBUILD and CHOST to differentiate the targets.

There are implications for what is and isn't installed when building from binpkgs (a.k.a. prebuilts). Build-time dependencies ARE NOT installed for a package that uses a prebuilt. Run-time dependencies ARE installed for a package that uses a prebuilt.

These dependency types are also what is used to determine what actually makes it on an image. Build-time dependencies ARE NOT included in an image. Run-time dependencies ARE included in an image.

To put it all together:

As a more concrete example, if you're building an ARM board:

And from the perspective of a package that is only run on devices:

Packages CAN be in any/all [BR]DEPEND at the same time. It is especially common that we have both a DEPEND and an RDEPEND on a package. A good example of this is when we depend on a .so file. We need the shared library's header file at build time and its .so file at runtime.


The following examples use A --DEPEND-> B to mean A DEPENDs on B (i.e. package B is in package A's DEPEND field in its ebuild), and A --DEPEND--> B --DEPEND--> C to mean A DEPENDs on B and B DEPENDs on C.

Example 1: Which RDEPENDs end up in an image?

cros build-packages A && cros build-image:

Location Packages
Sysroot A B C D E
Image A D E

C does NOT make it to the Image because it's an RDEPEND of B, which is only a DEPEND of A, i.e. the RDEPEND graph of A does not include B, so cannot include C.

D DOES make it to the Image because it's a direct RDEPEND of A, so would naturally be in the RDEPEND graph of A.

E DOES make it to the Image because it's an RDEPEND of D, which is an RDEPEND of A, i.e. the RDEPEND graph of A DOES transitively include E.

G does NOT make it to the Image because it's an RDEPEND of F, which is only a BDEPEND of E, i.e. the RDEPEND graph of A does not include F, so cannot include G.

G does NOT make it to the Sysroot because it's an RDEPEND of F, which is a BDEPEND of E, so since F is only ever needed in the SDK, so is G.

Example 2: When is run-time?

cros build-packages A && cros build-image:

Location Packages
SDK (amd64) B C
Sysroot (arm) A B C D
Image (arm) A C D

B is installed to the SDK because A BDEPENDS on B, so B must be compiled for and installed to the SDK. Since B RDEPENDs on C, C must also be built for and installed to the SDK so that it's available to amd64 B when it's run as a part of A's build steps.

B is installed to the Sysroot because D DEPENDs on B. Since B RDEPENDs on C, C must also be built for and installed to the Sysroot so it's available to ARM B when it's included as a part of D's build steps.

C is installed to the Image because A RDEPENDs on C, so C must be on the Image so that it's available to A when A actually runs on the device.

Since B was only ever in DEPENDs, it's no longer needed and does not get installed in the Image.

How do I specify dependency versions, and what are slots?

Version and slot operators work together to specify what dependencies you want and your package's behavior when they change. A very basic description of the most common case for first-party packages can be found in this section.

Please refer to your specific dependencies' ebuilds to determine if this case matches your needs, and see the provided references for more in depth information. The "see also" links are related FAQs that may provide some additional context or use cases.


Most commonly, some sort of version must be specified to get the correct dependency. A package's revision (e.g. -r123) is a component of the version, but typically won't be a component that needs to be included. However, many cros-workon packages simply use the default 0.0.1, in which case it is more likely you'll need to use the revision for constraints.

See Version Dependencies for the available operators. They are largely comparison operators that do what you would expect, but the semantics of = vs ~ and blockers (! and !!) in particular should be reviewed if you think you need to use them.

Note: Constraints on a package's version are not "collapsed". DEPEND=">=foo/bar-1.0.0 <foo/bar-2.0.0" are two constraints that mean it needs a version of bar >=1.0.0, and a version of foo <2.0.0. It does NOT mean it needs a version of foo, X, such that 1.0.0 <= X < 2.0.0, but such a version WOULD satisfy both constraints. For example, it could be satisfied by installing foo/bar-1.2.3 (1.0.0 <= 1.2.3 < 2.0.0), but it could alternatively be satisfied by installing foo/bar-0.0.1 (0.0.1 < 2.0.0) AND foo/bar-2.3.4 (1.0.0 <= 2.3.4). See the version dependencies reference for how to handle ranged dependencies.


Start with Slotting for a description of what slots and subslots are.

cros-workon packages use SLOT="0/${PVR}" by default, meaning the subslot changes on every uprev. For most cros-workon packages, this will happen automatically any time there is a change to the code or ebuild for the package. If your package needs to be rebuilt when a dependency changes, you probably want the := slot operator, e.g. DEPEND=">=foo/bar-1.2.3:=". When the slot or subslot of the installed foo/bar changes, your package would be rebuilt.

See the slot dependencies reference for more information and additional slot operators.

How do I see and trim dependencies?

The easiest way to inspect dependencies is to use the --emptytree and --pretend options for emerge. These options tell you what packages would be installed if you were installing from scratch and you didn't have any of its dependencies yet. The --verbose option will also show the set of USE flags that can be turned on and off for the packages to hopefully trim dependencies:

emerge-amd64-generic --pretend --emptytree --verbose vim

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild   R   ] sys-apps/sed-4.2 to /build/x86-mario/ USE="-acl -nls -static" 862 kB
[ebuild   R   ] sys-libs/ncurses-5.7-r3 to /build/x86-mario/ USE="cxx minimal unicode -ada -debug -doc -gpm -profile -trace" 2,388 kB
[ebuild   R   ] app-editors/vim-core-7.2.303 to /build/x86-mario/ USE="-acl -bash-completion -livecd -nls" 9,475 kBS

To get an idea of the dependency tree and why some packages would be built do emerge --pretend --emptytree --tree like so:

emerge-amd64-generic --pretend --emptytree --tree vim

app-editors/vim-7.2.303: (merge) needs
app-editors/vim-core-7.2.303: (merge) needs
app-vim/gentoo-syntax-20090720: (merge) needs
sys-apps/sed-4.2: (merge) needs
    no dependencies
sys-libs/ncurses-5.7-r3: (merge) needs
    no dependencies

How do I search for a package that I might want to install?

If you wanted to look for busybox, you can:

emerge --search busybox

How do I see a list of all packages installed in a root?

equery-<board_name> list '*'

How do I find out the on-disk package size?

For this, we are primarily interested in getting the size of the package and its runtime dependencies.

# ignores files in package that match <file mask regex>.
qsize -i<file mask regex> package


# Calculate size of package excluding debug symbols and development header files.
qsize -i '/usr/lib/debug/' -i '/usr/include/' shill
# For specific boards, first build the package for the board and then measure it.
# (does the same thing as `ROOT=/board/<board> qsize <rest of command>`)
qsize-<board> -i '/usr/lib/debug/' -i '/usr/include/' shill

For an accurate estimate, the above requires manually excluding files that would normally be removed due to the INSTALL_MASK (defined in chromite/lib/ You can also use the more direct method of replicating what cros build-image does by installing the package into an empty root.

Example: Say you want to find out how much space update_engine and its runtime dependencies would take on an empty image.

export INSTALL_MASK="<DEFAULT_INSTALL_MASK from chromite/lib/>"
mkdir /tmp/foo
emerge-<board> --root=/tmp/foo --root-deps=rdeps --usepkgonly update_engine
du -sh /tmp/foo

How do I figure out which package a file belongs to?

equery-<board-name> belongs
equery belongs /bin/bash

How do I write my own ebuild?

An ebuild file is the recipe to build a package. Since it is written in shell script it is very flexible, but there is a recommended format. Start with one of our existing chromium ebuilds as an example. The ebuild should be named project-version.ebuild and it should live in a directory under an appropriate category. For chromiumos packages we've been using the chromeos-base category. The man page can be extremely helpful when writing an ebuild:

man 5 ebuild

The board setup process creates an ebuild-<board_name> wrapper that you can use within the chroot just as you do with emerge-<boardname>

ebuild-arm-generic openssh-5.2_p1-r3.ebuild compile

If you are writing a new package for that does not exist upstream then you will want to upload the files for the package to the localmirror. See the download mirror section.

How do I write unit tests for my ebuild?

See the Best practices for writing ChromeOS unit tests page.

How do I run unit tests?

See the ChromiumOS Unit Testing page.

How do I store transient artifacts in an ebuild?

Most builds won't need this, but some builds need to store artifacts that are not related to the functionality of the package and don't need to be installed on users' machines. For example, you may wish to collect data about the resource usage of your build. You can store such artifacts in the directory named by the environment variable CROS_ARTIFACTS_TMP_DIR. This directory is created before the clean phase of a build. In the postinst phase, all artifacts in this directory will be moved to "${SYSROOT}/var/lib/chromeos/package-artifacts/${CATEGORY}/${PF}".

How do I check that my package is present on the download mirror?

When we download packages for use in ChromiumOS, we only download packages that are present on the upstream gentoo mirror or on our localmirror. If your package is not present on either download mirror, then you need to upload it manually to localmirror.

Once you have uploaded your package to the mirror, you can test that it's working by running the following commands:

rm -f /var/lib/portage/distfiles-target/*
emerge-arm-generic -F app-laptop/laptop-mode-tools

>>> Fetching (1 of 1) app-laptop/laptop-mode-tools-1.57-r2 from chromiumos for /build/x86-generic/
>>> Downloading ''
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (22) The requested URL returned error: 404
No digest file available and download failed.

>>> Downloading ''
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   97k  100   97k    0     0   565k      0 --:--:-- --:--:-- --:--:--  647k

How do I uprev an ebuild?

An ebuild file is the recipe to build a package. When building a package, portage searches all active overlays, and selects the highest version ebuild that is unmasked for the given architecture. Once it determines which ebuild to build, it compares its version number against the most recently emerged version (if any) to determine if the package must be upgraded or downgraded.

When you are working on a cros-workon package (one with a 9999 ebuild), you only need to modify the 9999 ebuild - the Commit Queue will auto-uprev the version of the ebuild by copying the 9999 ebuild to an ebuild with the new bumped revision. Or if your cros-workon ebuild sets CROS_WORKON_MANUAL_UPREV=1, you can uprev manually using the cros_mark_as_stable script.

But, when you are working on a "non cros-workon" package (i.e a package for whose ebuild doesn't inherit cros-workon, chromiumos doesn't host a git tree, and autouprev isn't supported) you must manually increase the version number in the ebuild filename.

When upgrading to a new version of the package (i.e. grabbing a newer version from upstream), then increase the ebuild version number (foo-M.m.ebuild).

When changing the ebuild contents (i.e. applying a custom patch or fixing cross-compile issues), then increase the ebuild revision number (foo-M.m-rN.ebuild).

When uprevving ebuilds, just rename the symlink (by convention, named as *-r#.ebuild), or create a new one, if it doesn't exist.

$ ls -l
chromeos-bsp-board-0.0.1-r1.ebuild -> chromeos-bsp-board-0.0.1.ebuild
$ git mv chromeos-bsp-board-0.0.1-r1.ebuild chromeos-bsp-board-0.0.1-r2.ebuild

If updating packages versions (say from PV=0.0.1 to PV=0.1.0 above), rename the base file (i.e., chromeos-bsp-board-0.0.1.ebuild).


For cros-workon packages, a script can be used to tell the Commit Queue which version to uprev the stable ebuild version to. The script should be placed under the files/ directory of the ebuild and looks like the following:

# Copyright <copyright_year> The ChromiumOS Authors
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.
# This echo statement sets the package base version (without its -r value).
# If it is necessary to add a new blocker or version dependency on this ebuild
# at the same time as revving the ebuild to a known version value, editing this
# version can be useful.

echo 0.0.2

Here, 0.0.2 is the version you want to set the package to. If you need to set a blocker, it would look like the following:


The version can also be generated from the package sources, whose directory is passed as parameter to the script, e.g.:

exec sed -e 's/devel/pre/g' -e 's/-/_/g' $1/VERSION

The Commit Queue will automatically add and increment -rN part of the version as required.

How do I build a debug package?

There are a few things that are important to get a debug package:

Putting these together:

USE="cros-debug" CFLAGS="-g -O0" FEATURES="nostrip noclean -splitdebug" \
    emerge-amd64-generic -a <package-name>

How do I use the dev server?

See Using the dev server.

How do I install a single package?

Use cros deploy.

How do I install a single package into a local disk image?

Mount the image from file or USB stick using:


Emerge into the mounted system:

emerge-amd64-generic -k --root=/tmp/m package-name

The package I want to install has been "masked". How do I fix that?

When importing a package with emerge-<board_name>, you may get an error message about "masked packages". For instance, the response to

emerge-amd64-generic flashrom

may contain the following:

!!! All ebuilds that could satisfy "sys-apps/flashrom" have been masked.

!!! One of the following masked packages is required to complete your request:

-   sys-apps/flashrom-0.9.0 (masked by: ~x86 keyword)

To unmask a package, edit the KEYWORDS field of the ebuild directly and add "amd64 arm x86". If you see existing entries like "~amd64", simply remove the tilde (~).

How do I build a package without it deleting the build directory (eg, to see a kernel .config file)?

Prepend FEATURES=noclean to emerge:

FEATURES="noclean" emerge-amd64-generic -a kernel
ls -l

How do I modify a portage package?

Within ChromiumOS the portage repository is considered read-only. This means that if the portage package has a problem then we must create an overlay for it in chromiumos-overlay. A common reason for needing to do this is that the package will not cross compile (for ARM or x86). Here is how to do that:

Note: You must run emerge-${BOARD} inside the chroot, but the repo and git commands should be done outside the chroot.

  1. Try emerging the package to see if it builds. We assume that it doesn't (or perhaps builds, but doesn't work properly) which is why you are here.

    (cros-chroot) $ emerge-arm-generic nfs-utils
     * IMPORTANT: 1 news items need reading for repository 'gentoo'.
     * Use eselect news to read news items.
    Calculating dependencies... done!
    >>> Emerging (1 of 1) net-fs/nfs-utils-1.1.4-r1 for /build/arm-generic/
     * nfs-utils-1.1.4.tar.bz2 RMD160 SHA1 SHA256 size ;-) ...               [ ok ]
     * CPV:  net-fs/nfs-utils-1.1.4-r1
     * REPO: gentoo
     * USE:  arm elibc_glibc kernel_linux tcpd userland_GNU
    >>> Unpacking source...
    >>> Unpacking nfs-utils-1.1.4.tar.bz2 to /build/arm-generic/tmp/portage/net-fs/nfs-utils-1.1.4-r1/work
     * Applying nfs-utils-1.1.4-rpcgen-ioctl.patch ...                        [ ok ]
     * Applying nfs-utils-1.1.4-ascii-man.patch ...                           [ ok ]
     * Applying nfs-utils-1.1.4-mtab-sym.patch ...                            [ ok ]
     * Applying nfs-utils-1.1.4-no-exec.patch ...                             [ ok ]
    >>> Source unpacked in /build/arm-generic/tmp/portage/net-fs/nfs-utils-1.1.4-r1/work
    >>> Compiling source in /build/arm-generic/tmp/portage/net-fs/nfs-utils-1.1.4-r1/work/nfs-utils-1.1.4 ...
     * econf: updating nfs-utils-1.1.4/config.guess with /usr/share/gnuconfig/config.guess
     * econf: updating nfs-utils-1.1.4/config.sub with /usr/share/gnuconfig/config.sub
    ./configure --prefix=/usr --build=x86_64-pc-linux-gnu --host=armv7a-cros-linux-gnueabi --mandir=/usr/share/man --infodir=/usr/share/info --datadir=/usr/share --sysconfdir=/etc --localstatedir=/var/lib --mandir=/usr/share/man --with-statedir=/var/lib/nfs --disable-rquotad --enable-nfsv3 --enable-secure-statd --with-tcp-wrappers --enable-nfsv4 --disable-gss
    configure: loading site script /usr/share/
    Making all in locktest
    make[2]: Entering directory `/build/arm-generic/tmp/portage/net-fs/nfs-utils-1.1.4-r1/work/nfs-utils-1.1.4/tools/locktest'
    gcc -DHAVE_CONFIG_H -I. -I../../support/include  -D_GNU_SOURCE -D_GNU_SOURCE  -O2 -pipe -I/build/arm-generic/usr/include/ -I/build/arm-generic/include/ -ggdb -march=armv7-a -mtune=cortex-a8 -mfpu=vfpv3-d16 -mfloat-abi=softfp -MT testlk-testlk.o -MD -MP -MF .deps/testlk-testlk.Tpo -c -o testlk-testlk.o `test -f 'testlk.c' || echo './'`testlk.c
    cc1: error: unrecognized command line option "-mfpu=vfpv3-d16"
    cc1: error: unrecognized command line option "-mfloat-abi=softfp"
    testlk.c:1: error: bad value (armv7-a) for -march= switch
    testlk.c:1: error: bad value (cortex-a8) for -mtune= switch
    make[2]: *** [testlk-testlk.o] Error 1
    make[2]: Leaving directory `/build/arm-generic/tmp/portage/net-fs/nfs-utils-1.1.4-r1/work/nfs-utils-1.1.4/tools/locktest'
    make[1]: *** [all-recursive] Error 1
    make[1]: Leaving directory `/build/arm-generic/tmp/portage/net-fs/nfs-utils-1.1.4-r1/work/nfs-utils-1.1.4/tools'
    make: *** [all-recursive] Error 1
     * ERROR: net-fs/nfs-utils-1.1.4-r1 failed:
     *   Failed to compile
     * Call stack:
     *, line  54:  Called src_compile
     *   environment, line 2648:  Called die
     * The specific snippet of code:
     *       emake || die "Failed to compile"
     * If you need support, post the output of 'emerge --info =net-fs/nfs-utils-1.1.4-r1',
     * the complete build log and the output of 'emerge -pqv =net-fs/nfs-utils-1.1.4-r1'.
     * The complete build log is located at '/build/arm-generic/tmp/portage/net-fs/nfs-utils-1.1.4-r1/temp/build.log'.
     * The ebuild environment file is located at '/build/arm-generic/tmp/portage/net-fs/nfs-utils-1.1.4-r1/temp/environment'.
     * S: '/build/arm-generic/tmp/portage/net-fs/nfs-utils-1.1.4-r1/work/nfs-utils-1.1.4'
    >>> Failed to emerge net-fs/nfs-utils-1.1.4-r1 for /build/arm-generic/, Log file:
  2. Make sure you have the latest version in portage from gentoo. We don't want old versions since it makes for more work when we update

    $ cd ...src/third_party/portage-stable
    $ git checkout remotes/cros/gentoo
    $ ls net-fs/nfs-utils/
    ChangeLog     nfs-utils-1.1.4-r1.ebuild  nfs-utils-1.2.0.ebuild
    Manifest      nfs-utils-1.1.5.ebuild     nfs-utils-1.2.1.ebuild
    files         nfs-utils-1.1.6-r1.ebuild
    metadata.xml  nfs-utils-1.1.6.ebuild
  3. Use repo start to start your modification to chromiumos-overlay

    cd ...src/third_party/chromiumos-overlay/
    repo start add-nfs-utils-to-overlay .
  4. Copy this package into the corresponding place in chromiumos-overlay

    mkdir net-fs
    cp -r ../portage-stable/net-fs/nfs-utils net-fs/.
    # we want to use the latest ebuild, so remove the others
    cd net-fs/nfs-utils
    rm nfs-utils-1.1* nfs-utils-1.2.0*
  5. Upload this change to the review server and go through the process to get it committed

    git commit
    # Imported nfs-utils from portage
    # TEST=emerge, although it doesn't actually build
    git cl upload
    # ... wait for review
    # git push
  6. Use repo start to start your modification to your new overlay package

    cd ...src/third_party/chromiumos-overlay/
    repo start fix-nfs-utils .
  7. Add in a patch and modify the ebuild to use it

    # find or create a patch, put it in the files directory
    # modify the ebuild, e.g. a cross-compile.patch
    src_prepare() {
        epatch "${FILESDIR}"/${PN}-1.1.4-mtab-sym.patch
        epatch "${FILESDIR}"/${PN}-1.1.4-no-exec.patch
        # here is the new one
        epatch "${FILESDIR}"/${PN}-1.1.4-cross-compile.patch
  8. Check that it builds OK now

    (cros-chroot) $ emerge-arm-generic nfs-utils
  9. Upload this change to the review server and go through the process to get it committed

    git commit
    # Modifications to nfs-utils to make it build
    # TEST=emerge-arm-generic
    git cl upload
    # ... wait for review
    # git push
  10. Relax you are done

How do I handle file collisions?

Sometimes when using build_packages or setup_board you might get an error like this:

gtk-doc-am-1.13-r2: >>> Installing (1 of 1) dev-util/gtk-doc-am-1.13-r2
gtk-doc-am-1.13-r2: * This package will overwrite one or more files that may
belong to other
gtk-doc-am-1.13-r2: * packages (see list below).
gtk-doc-am-1.13-r2: *
gtk-doc-am-1.13-r2: * Detected file collision(s):
gtk-doc-am-1.13-r2: *
gtk-doc-am-1.13-r2: * /usr/bin/gtkdoc-rebase
gtk-doc-am-1.13-r2: *
gtk-doc-am-1.13-r2: * Searching all installed packages for file collisions...
gtk-doc-am-1.13-r2: *
gtk-doc-am-1.13-r2: * Press Ctrl-C to Stop
gtk-doc-am-1.13-r2: *
gtk-doc-am-1.13-r2: * dev-util/gtk-doc-1.11
gtk-doc-am-1.13-r2: * /usr/bin/gtkdoc-rebase
gtk-doc-am-1.13-r2: *
gtk-doc-am-1.13-r2: * Package 'dev-util/gtk-doc-am-1.13-r2' NOT merged due to
gtk-doc-am-1.13-r2: * collisions. If necessary, refer to your elog messages for
the whole
gtk-doc-am-1.13-r2: * content of the above message.

This tends to happen when files move between packages, or if the upstream Gentoo packages in your chroot become inconsistent.

Moving files between packages

Sometimes you have one package (let's call it "pkgsrc" for short) providing a file but want to change things so that a different package ("pkgdst") now provides that file. If you were to try to upgrade "pkgdst" and have it install the file but the old "pkgsrc" still installs that file, we get into a bad state.

To solve this, we leverage package blockers. At the simplest level, this is an ebuild dependency syntax that allows you to say "do not allow pkg A to be installed whilst pkg B is installed". But if you combine the blocker syntax with versions, portage is smart enough to detect that it needs to handle these blocking packages specially and do an automatic upgrade for you.

Let's use a real world example. The package sys-fs/e2fsprogs-libs used to install the libuuid library. But the upstream developers decided to move it to the sys-apps/util-linux package. Specifically, the e2fsprogs-libs-1.41.7 release was the last one to include the library while util-linux-2.16 was the first one to include it.

So in the util-linux-2.16 ebuild, we declare that at runtime, it cannot be installed simultaneously with e2fsprogs-libs-1.41.7 or older. Since util-linux does not need e2fsprogs-libs to build libuuid, we don't have to declare the blocker in DEPEND.

# It's helpful to future readers if you document why the blocker is needed. For
# example:
# File /x/y/z moved from e2fsprogs-libs.

    ...other runtime dependency stuff...

Since e2fsprogs-libs doesn't actually need libuuid itself in order to build, there is no dependency in that ebuild on a newer util-linux version. However, let's assume it did. That would mean in the e2fsprogs-libs-1.41.8 ebuild, we would write:


This is enough information for portage to be able to automatically resolve this blocker for you. It will make sure that when upgrading util-linux and e2fsprogs-libs, it will first upgrade util-linux, then ignore file collisions that it hits with e2fsprogs-libs (since util-linux is taking ownership of those files), then upgrade e2fsprogs-libs. If there was a bug and e2fsprogs-libs still tried to install libuuid, portage would throw an error because util-linux now owns those files.

See also the Gentoo devmanual for more information about blockers.

Handling 9999 ebuilds

If the file you are moving comes from a cros-workon package (where you only modify the 9999 ebuild), the uprev is handled automatically. This means there isn't anything to safely block against. Instead, you should add/update and bump the version. See the description for more details.

Testing of upstream packages

For example, you had tried to cros build-packages or emerge after pulling in some new/updated upstream packages.

To recover from mixing of upstream packages, you can:

To find out which package provides the offending file and unmerge it, try:

(cros) equery belongs /usr/bin/gtkdoc-rebase
*   Searching for /usr/bin/gtkdoc-rebase ...
dev-util/gtk-doc-am-1.18 (/usr/bin/gtkdoc-rebase)

(cros) emerge --unmerge gtkdoc

To deal with moving files between packages, you'll want to utilize blockers.

What does cros build-packages actually do?

emerge-arm-generic virtual/target-os

Depending on the options you give, other packages may be emerged also (note that many of these options are the default):

Each of the packages is defined by an 'ebuild' which defines what needs to be done to build that package. The ebuild includes the package's dependencies - both build dependencies (things needed to build the package on the host) and run-time dependencies (things needed at run-time on the target to run any installed software). You can find most of the ebuild packages in the src/third_party/chromiumos-overlay directory. For example, src/third_party/chromiumos-overlay/virtual/target-os/target-os-1.ebuild is the base ebuild containing all the packages in ChromeOS. This ebuild lists all other top-level ChromiumOS packages as explicit dependencies. Thus, all of ChromiumOS is built when building this one package.

A small but important detail is that normally cros build-packages calls emerge, a script which builds a number of packages in parallel, taking advantage of multi-core machines. This is why you will see a display showing how many packages are left to build, and how many are currently in progress.

If you are using the minilayout then you will not have downloaded all the source for ChromeOS. So when an ebuild is emerged it may need to download some source. For example, see the ebuild in:


It has some SRC_URI lines describing where the source package can be downloaded from. The file used in case of tar may be something like:

This file is cached for you after the first download, in, for example, chroot/var/lib/portage/distfiles-target/tar-1.23.tar.bz2. It contains a tarball of the source code. The next time you emerge, portage will untar it from this location. You may also be interested to know that emerge may apply some patches to the source before it starts building - you can find these in a files subdirectory of app-arch/, in the case of tar. If you look at the ebuild you can even see where it patches these in, using epatch.

If you want to try this out, type:

(cros) emerge-arm-generic app-arch/tar

You should see it download some source, build it and install it into your target build in chroot/build/arm-generic.

So does cros build-packages build absolutely everything from source? Wouldn't that take forever? Actually, no. It cheats. There are ChromiumOS build servers constantly building binary packages for many common platforms. These pre-built packages are available and normally these are used instead. There is a --usepkg (default true) option for cros build-packages that adds some flags to the emerge command to make it use binary packages. To see what happens in this case, type:

emerge-amd64-generic --getbinpkg --usepkg --with-bdeps y app-arch/tar

This time you will probably see it download and then simply install the package as before. The C compiler will not be touched. Again the file is cached, this time in something like chroot/build/arm-generic/packages/app-arch/tar-1.23-r4.tbz2. Next time you run the emerge --usepkg it will not need to download anything.

What is a virtual package and how do they work?

A virtual package is used in portage when any of several different packages can perform the same function. A classic example is "virtual/editor". There are dozens of editors out there (including the most fabulous one of them all: ~vim~~ emacs nano ed), all of which work just fine. It's a matter of preference which one to install. If we want to write our own package that depends on an editor being installed, we don't really care which editor is picked. In order to specify this, we specify a runtime dependency on virtual/editor.

Depending on a virtual package

Depending on a virtual package is pretty easy: just add a dependency (either DEPEND or RDEPEND) on the virtual, like:


Adding a virtual

Depending on a virtual is very easy, but what about if you want to add a new virtual? First: create an ebuild inside the virtual category with a -0 version like mything-0.ebuild. This virtual doesn't do too much except to specify dependencies on all of the possible implementations of the virtual.

It's easiest to look at two samples.

First, we'll look at virtual/editor. The important things to notice are that this ebuild doesn't do anything itself--it just specifies dependencies. Specifically, it says that any of the editors listed are OK by specifying this as the RDEPEND (note the || at the beginning):


In this case the ebuild doesn't provide any particular way to choose which editor is chosen--it just specifies all of the options. As long as one of those options is installed, virtual/editor will be happy. If none of those are installed and you try to build virtual/editor (or you depend on it), portage will pick one of the editors and install it (by trying them in order).

A second example is ChromiumOS's virtual/linux-sources ebuild. You can see the important bits here:


kernel_next? ( sys-kernel/chromeos-kernel-next )
!kernel_next? ( sys-kernel/chromeos-kernel )

You can see that the kernel ebuild specifies a way to choose what implementation via USE flags: you can use this to choose between kernel and kernel-next.

Virtuals and central management

You should definitely pay attention to the fact that virtuals are centrally-managed. Said another way: if you need to add another implementation of a virtual (like that nifty new vimacs editor you wrote), you need to go to the virtual itself and add it to the list.

There is one common (at least in ChromiumOS) case where it's not be possible to have things centrally managed. This happens when you've got an implementation of a virtual that's in your private overlay. We'll imagine a virtual called virtual/chromeos-firmware. There might be several public implementations of chromeos-firmware, like:


The virtual/chromeos-firmware ebuild would list all of those as options. ...but what happens when you've got a new project called sisyphus and you've got a private overlay for it (because you don't want to make all the details public yet). How do you make this work?

You can do this by overriding the virtual in your private overlay. Your overlay will have a copy of the virtual/chromeos-firmware ebuild. However, in your copy the RDEPEND will just be sys-boot/chromeos-firmware-sisyphus. Now you can put the chromeos-firmware-sisyphus ebuild in your overlay and you're all set!

Overriding virtual policies

When portage searches for packages to install, it merges all of the packages in all the overlays. If the same ebuild (category/package-name) is found in multiple places, it comes down to comparing versions such that the highest version wins (and if the same version is found, then overlays will "whiteout" other overlays based on search order). But that's a lot to keep straight and is fairly fragile. If the ebuild in the main tree is revbumped to a version higher than is in an overlay, then bad things could start happening.

The policy we have in place for versioning of our virtuals is as follows:

overlay version (pv)
chromiumos-overlay 1
chromeos-overlay 1.3
project-<project name> 1.5
project-<project name>-private 1.7
chipset-<chip> 1.8
chipset-<chip>-private 1.8.5
baseboard-<baseboard> 1.9
baseboard-<baseboard>-private 1.9.5
overlay-<board> 2
overlay-variant-<board> 2.5
overlay-<board>-private 3
overlay-variant-<board>-private 3.5
overlay-<board>-<special>-private 4

Old-style virtuals

The above description is for new-style virtuals. There's also old-style virtuals. You shouldn't ever add one of those, but you might still run into them.

Declaring an old-style virtual is easy, but inflexible. In the profiles/ subdir, you will find files named virtuals. It is a simple text file with one virtual per line:

# Add virtual packages for this profile
virtual/chromeos-bsp chromeos-base/chromeos-bsp-null
virtual/chromeos-bsp-dev chromeos-base/chromeos-bsp-dev-null

The first element is the name of the virtual while the second element is the package that provides that virtual. This file format has no other options/extensions available to it.

When does a dependency cause a rebuild?

Portage's dependency system can be tricky to understand, especially when it comes to what will cause a package to be rebuilt.

Important: package manager's dependencies are NOT like make dependencies. If package B depends on package A, it does NOT mean B is re-built when A is updated. It only means when B is pulled in, A is also pulled in.

Be sure you understand the different dependency types before proceeding.

To think about how everything works, we'll pretend we have three ebuilds:

OK, so what happens when we make changes and rebuild things. Here is what happens:

The above is a little weird, but can be understood by remembering that portage is primarily concerned with speed and correctness. ...but portage is not super concerned with making sure you have the latest version of every last dependency (as long as you have some version it is happy). Specifically:

Note that the above examples use a static library and dynamic library as an example. Hopefully it makes sense that we can map other problems to the same concepts. For instance, if we had an ebuild that took a whole bunch of other build outputs and created a .zip file out of them, that would be just like the static library case.

ChromeOS-specific notes

People working on ChromeOS probably find portage's philosophy (that getting the newest version isn't important) a bit frustrating. If an engineer syncs down new source code and tries to build a new boot image, the engineer would hope that the image has all of the newest versions of all of the packages.

We'll take chromeos-bootimage as an example. This build wants to take the binary output of several other ebuilds (the BCT, the firmware, the flattened device tree, etc) and concatenate them together (with some extra processing) to make a single binary image. Technically, chromeos-bootimage should only DEPEND on all of the other components. However, that means that if a new version of the BCT is checked in and then we do a build, portage will not decide to re-build chromeos-bootimage. Ick.

As a hack, ChromeOS packages often say that they DEPEND and RDEPEND in cases like chromeos-bootimage. Now if you build all packages, portage will be sure to rebuild chromeos-bootimage if the BCT ever changes. This is an awkward way to do things but is the current workaround. The fabled ABIDEPEND feature of portage (doesn't exist yet) is supposed to fix this and make it so we don't need the hack anymore.

Where to look for more information

The Gentoo Development Guide provides plenty of detailed information about Portage, the Gentoo build system. This package management specification is a good writeup describing Gentoo ebuild system.

Instructions for building ChromiumOS can be found here.