Creating Bill of Materials

Generating a Bill of Materials for Your Project

Introduction

To generate a Bill of Materials for your project, make sure you have bom installed. See how to here

cd gitrepo/
bom -n 'http://mybom.com/' . 

All of the tool’s options are explained on its page. Keep reading for more information about our tools, SBOMs, and the SPDX standard.

What is a Bill of Materials?

A Software Bill of Materials (often BOM or SBOM for short) is a manifest that lists everything included in a software release. “Everything” can take different meanings: software packages or images, documentation, tarballs, single files. These pieces can be components or source code, variants of the same artifact (eg a binary for different platforms).

An SBOM can also provide visibility on the dependencies of your project. There are many types of dependencies and many reasons that consumers of a project need to know them: security, compliance, compatibility.

Finally, a Bill of Materials allows software creators to express licensing information for their project as a whole, but also for individual pieces and its dependencies. You can release your project under the Apache 2.0 license but have its documentation published under Creative Commons. Then, there are all of your dependencies’ original licenses. A well-written SBOM can express all of them in the same document.

bom: A tool to generate SBOMs for your Project

As part of the effort to produce a bill of materials for Kubernetes, SIG Release developed a set of libraries to produce fully compliant SPDX SBOMs. Our tools support license scanning, image layer analyzers, processing of golang dependencies, and other features. These libraries are available for other projects to automate the production of their own Bills of Materials.

For simpler use cases, all of our SBOM automation is also available in a general-purpose tool called bom. You can find all the options that bom supports in its README.md.

bom supports generating Bills of Materials in SPDX compliant tag-value format. It can process directories, single files, read the contents of container images (both from container tar archives and directly from registries), and tarred sources.

In addition bom will scan your code to find licensing information. Its classifier supports detection of all the SPDX recognized licenses.

SPDX: Software Package Data Exchange

SPDX or Software Package Data Exchange is an open standard to create bills of materials. It has been in the works for 10+ years, coordinated by the SPDX Workgroup, a project of the Linux Foundation.

As of June 2020, the SPDX specification is in version 2.2. The current version allows software authors to include metadata about their project describing its contents, relationships among them, and other components and licensing.

Files and Packages

There are two main building blocks in an SPDX manifest: Files and Packages.

Files are what you would expect: an individual item in a filesystem tree. The data about a file in an SPDX SBOM includes its name, checksums, license, file type, copyright data, and other attributes.

Packages are a non-specific element in SPDX representing anything that can group other elements. An .rpm or .deb package can be an SPDX package, but so can be a container image or a tarball. Packages contain files, but can also contain other files or a mix of both. An image, for example, can be viewed as a package, which contains other packages (its layers), and those, in turn, a set of files.

SPDX metadata about packages is similar to metadata about files but it also includes data about its version, where it came from, who wrote it, and an important one: the package verification code.

To provide a mechanism to ensure the integrity of its contents, the package construct defines a checksum verification code. This is a SHA1 sum derived from concatenating a hash of each item in the package.

Relationships

The most useful feature of SPDX is the ability to express relationships among elements. For example, a Package CONTAINS a File. A SPDX Document DESCRIBES Packages and Files. A File is GENERATED_FROM a source Package and so on.

The SPDX spec defines a rich relationship vocabulary which enables developers to describe very complex interaction among components, source code, the artifacts its build produces but also its dependencies and build tools.

Licensing Information

Independently of source code and artifacts, software licensing is a complex problem in itself. SPDX has been thought from the ground up to express licensing of each element in the document.

The pertinence of a license over a file or package can come from different sources: it can be expressed by the file itself, it can be inherited from a package, enforced by its dependencies, or perhaps it can be inferred by an automated tool. SPDX makes no attempt to make any determination about the licensing of elements but it has many different ways to allow authors to express a license and where the licensing determination came from.

SPDX maintains a large list of open source licenses. All licenses have a tag that represents them in a document. For example, the tag for the Mozilla Public License 2.0 is MPL-2.0, MIT-0 is MIT without attribution. The SPDX licenses are published in a public repository and are available in machine-readable formats such as JSON and XML.

Create your Bill of Materials

There are a couple of factors to take into consideration when drafting your Bill of Materials.

Thinking about your Release

The first thing you need to consider when planning your Bill of Materials is your release structure. What does it look like? What are the main artifacts I want to list in my BOM? Is your source code available?

But the main focus should be the consumers of the BOM. How is your document going to be used? Is it for checking the completeness of your release? Is someone trying to check for vulnerabilities in your dependencies? Perhaps your compliance person needs to check the licenses that interact with your project. Think about these and other use cases and create one or more SPDX documents which are useful for your consumers.

As the name implies, open source software releases include a snapshot of the source code in time: the state of your repo when a git tag was cut, for example. Do you want to include the source code in the same document? When we were testing for the Kubernetes SBOM, the file produced was over 11 MB long, so we decided to split the source data to its own SPDX file.

Generating the SBOM

When you are ready to generate the BOM for your project, make sure everything you want to list in the SPDX document is available. bom can read container images remotely from their registry but everything else has to be available locally.

Namespace

Every SPDX document has to declare its namespace. The namespace is a URI, it must be unique for the document you are generating. The purpose of the namespace is to have an anchor point to reference your release in the SPDX world. Other software components which rely on your project will use the URI to declare they are using your thing.

Simplest Use Case: One Package

In the simplest case, you can feed bom a source and build a single package SBOM. For example, to generate SBOM from your git repository run the following (note the dot at the end):

bom generate -n http://example.com/ .

This command will traverse your repository directory structure, listing everything it finds, scanning license files. If your repository is a Go module, it will process the dependencies. bom will use your .gitignore file and skip any patterns listed in it.

After bom runs, all your source code will be expressed as Files in an SPDX Package. bom will do some determinations to complete the data it needs to produce the document such as generating names for packages and files.

Adding Additional Sources

Generally, an SPDX bill of materials will include more than one package. You can pass bom more sources to add to the document. These can be container images, other directories, container archives, etc. When you add other sources, bom will add them as top-level packages in the document. Some of these will include sub-packages: layers of images, dependencies, etc.

Here is a sample of other command line flags you can pass to bom generate to add more elements to your bill of materials:

ShortLong FlagDescription
–archivelist of archives to add as packages (supports tar, tar.gz)
-d–dirsList of directories to include in the manifest as packages
-f–fileList of files to include
-i–imageList of image references
–image-archivelist of docker archive tarballs to include in the manifest

Example: Generate an SBOM for etcd

Let us say you want to generate a bill of materials for etcd, which is at version v3.4.16 as I write this. If you only want to build an SBOM describing only the source in the repository, do the following:

git clone https://github.com/etcd-io/etcd
cd etcd
bom generate -n https://etcd.io/etcd-v3.4.16.spdx -o etcd-v3.4.16.spdx \
  --dirs=.

This will produce a manifest describing the repo and its golang dependencies in etcd-v3.4.16.spdx.

Now, to make your SBOM more complete, you may want to include a container image. To do that run the same invocation, but this time adding the image with the --image flag:

bom generate -n https://etcd.io/etcd-v3.4.16.spdx -o etcd-v3.4.16.spdx \
  --dirs=.\
  --image=quay.io/coreos/etcd:v3.4.16

This command will fetch the container image from the coreos repo and add it as a package. At this point, your bom will contain two top-level Packages: the directory and the image. If you inspect it, you will see the image’s layers as subpackages too.

Finally, perhaps you want to add a binary distribution file. Download the compressed artifact from Github and add it to the SBOM:

curl -L https://github.com/etcd-io/etcd/releases/download/v3.4.16/etcd-v3.4.16-darwin-amd64.zip \
  -O /tmp/etcd-v3.4.16-darwin-amd64.zip

bom generate -n https://etcd.io/etcd-v3.4.16.spdx -o etcd-v3.4.16.spdx \
  --dirs=. \
  --image=quay.io/coreos/etcd:v3.4.16 \
  --file=/tmp/etcd-v3.4.16-darwin-amd64.zip

The resulting sbom from the last invocation will include at the top level of the SBOM three things: two Packages (the directory and the image) and one File: the binary distribution. Note that when listing the zip file as a single file, bom did not perform any special treatment to it. You can see a copy of the resulting file: etcd-v3.4.16.spdx

Last modified July 2, 2022: Add docs (377737b)