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SBOM scanning protocol

BuildKit supports automatic creation of SBOMs for builds, attaching them as image attestations.

To scan the filesystem contents, a user can specify an SBOM generator image. When run, this image is passed the rootfs of the build stage as a read-only mount, writes its SBOM scan data to a specified directory.

The SBOM generator image is expected to follow the rules of the BuildKit SBOM generator protocol, defined in this document.

Note

Currently, only SBOMs in the SPDX JSON format are supported.

These SBOMs will be attached to the final image as an in-toto attestation with the https://spdx.dev/Document predicate type.

Implementations

The following SBOM generator images are available:

Parameters

A single run of a generator may specify multiple target filesystems to scan by passing multiple paths - the scanner should scan all of them. Each filesystem target has a name, specified by the final component of the path for that target. A generator may produce any number of scans for the available targets - though ideally it should aim to produce a single scan per target.

These parameters will be passed to the generator image as environment variables by BuildKit:

  • BUILDKIT_SCAN_DESTINATION (required)

    This variable specifies the directory where the scanner should write its SBOM data. Scanners should write their SBOMs to $BUILDKIT_SCAN_DESTINATION/<scan>.spdx.json where <scan> is the name of an arbitrary scan. A scanner may produce multiple scans for a single target - scan names must be unique within a target, but should not be considered significant by producers or consumers.

  • BUILDKIT_SCAN_SOURCE (required)

    This variable specifies the main target, passing the path to the root filesystem of the final build result.

    The scanner should scan this filesystem, and write its SBOM result to $BUILDKIT_SCAN_DESTINATION/$(basename $BUILDKIT_SCAN_SOURCE).spdx.json.

  • BUILDKIT_SCAN_SOURCE_EXTRAS (optional)

    This variable specifies additional targets, passing the path to a directory of other root filesystems. If the variable is not set, is empty, or contains a directory that does not exist, then no extras should be scanned.

    The scanner should iterate through this directory, and write its SBOM scans to $BUILDKIT_SCAN_DESTINATION/<scan>.spdx.json, similar to above.

A scanner must not error if optional parameters are not set.

The scanner should produce SBOM results for all filesystems specified in BUILDKIT_SCAN_SOURCE or BUILDKIT_SCAN_SOURCE_EXTRAS but must not produce SBOM results for any other filesystems.

Further reading

See frontend/attestations/sbom/sbom.go for the code that invokes the user-specified generator.