SilverMetal/linux/build

SilverMetal Linux — reproducible ISO build pipeline

Milestone: Phase 1 / M1.1 — Kicksecure fork builds reproducibly. Exit criterion: two clean builds of the same commit produce a byte-identical SHA256.

This directory holds everything that turns a SilverMetal commit into a SilverMetal Linux ISO. M1.1 ships only the base (un-hardened) Kicksecure derivative. Hardening overlay, kernel swap, AppArmor profiles, etc. land in M1.2+ and must not be added here in the M1.1 PR.

Layout

linux/build/
├── README.md                    (this file)
├── derivative-maker/            git submodule -> Kicksecure/derivative-maker
├── config/
│   ├── silvermetal-base.conf    derivative selection + branding
│   ├── snapshot-pin.env         pinned snapshot.debian.org timestamp
│   └── source-date-epoch.env    optional SOURCE_DATE_EPOCH override
├── docker/
│   └── Dockerfile.builder       pinned debian:bookworm-slim builder image
└── scripts/
    ├── build.sh                 wrapper: container run -> derivative-maker
    ├── verify-reproducibility.sh build twice, compare SHA256
    └── diagnose-divergence.sh   diffoscope on mismatch

How reproducibility is achieved

The same levers any deterministic Debian build relies on, stacked together:

Lever	Where it lives
Pinned snapshot.debian.org mirror	config/snapshot-pin.env
SOURCE_DATE_EPOCH from commit time	scripts/build.sh (auto)
Pinned builder image (by digest)	docker/Dockerfile.builder + BUILDER_IMAGE
Deterministic mksquashfs flags	MKSQUASHFS_OPTIONS in base conf
Pinned upstream toolchain	derivative-maker/ submodule
LC_ALL=C.UTF-8, TZ=UTC	scripts/build.sh

diffoscope is the diagnostic tool used by diagnose-divergence.sh; the gate itself is plain sha256sum.

Reproduce a release locally

Procedure mirrors docs/trust-model.md § Reproducible builds.

Prerequisites: a Linux host (or WSL2) with Docker, ~30 GB free disk, ~8 GB RAM.

# 1. Clone the repo at the release tag.
git clone --recurse-submodules https://git.silverlabs.uk/SilverLABS/SilverMetal.git
cd SilverMetal
git checkout v1.1.0   # whichever release you want to verify

# 2. Build twice and compare. ~60-90 minutes per build.
linux/build/scripts/verify-reproducibility.sh

# 3. Compare against the published release.
sha256sum -c <(curl -fsSL https://git.silverlabs.uk/SilverLABS/SilverMetal/releases/download/v1.1.0/SHA256SUMS)

Mismatch with the published artefact = supply-chain anomaly. Report channel: security@silverlabs.uk.

Build once (no reproducibility check)

linux/build/scripts/build.sh
# Output lands in linux/build/output/<short-sha>/

The wrapper requires BUILDER_IMAGE to be pinned by digest. Local dev that hasn't built and pushed an image yet should override:

BUILDER_IMAGE=docker-registry:5000/silvermetal-builder@sha256:<digest> \
    linux/build/scripts/build.sh

Gitea Actions

The CI workflow (.gitea/workflows/build-iso-linux.yaml) is the authority for "did this commit build reproducibly?". It:

1. Checks out the commit with submodules.
2. Runs build.sh twice in ${GITHUB_WORKSPACE}/build-{a,b}.
3. Fails the run if the two ISO SHA256s differ, and uploads a diffoscope report as an artefact.
4. On a tag push, attaches the verified ISO + SHA256SUMS + BUILD_INFO to a Gitea release.

Self-hosted runner setup

The workflow runs on runs-on: silvermetal-builder, a self-hosted, privileged-capable Gitea Actions runner. Create it before merging the workflow:

1. Provision a Debian 12 VM on the cluster with ≥ 8 vCPU, ≥ 16 GB RAM, ≥ 100 GB disk.
2. Install Docker (apt install docker.io); ensure the runner user can run docker run --privileged.
3. Register act_runner against git.silverlabs.uk with label silvermetal-builder.
4. Pre-pull the builder image so the first reproducibility run isn't a cold start: docker pull docker-registry:5000/silvermetal-builder:latest
5. Cache the apt snapshot in a Docker volume to avoid throttling: docker volume create silvermetal-apt-cache

The runner host name must not leak into ISO content. LC_ALL=C.UTF-8 and a constant TZ in the wrapper guard against that, but spot-check with diagnose-divergence.sh.

Bumping pinned inputs

Each of these is a deliberate, reviewed action — never automate:

• derivative-maker submodule — bump in its own PR, with a verification log showing two clean builds match.
• snapshot-pin.env — same procedure.
• Builder image (Dockerfile.builder digest) — rebuild, push, update BUILDER_IMAGE in build.sh, run reproducibility check, commit all four together.

What this milestone is not

• No hardening overlay (M1.2)
• No SilverBrowser/SilverVPN/SilverSync/SilverChat integration (M1.6–1.9)
• No installer branding (M1.5)
• No update server (M1.10)
• No SBOM publication (M1.11)
• No signing ceremony / MOK / Secure Boot wiring (separate milestone)

If a change to this directory expands its scope into one of those, push back — the M1.1 gate is intentionally narrow.