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kernelCheckpointTools: add checkpoint builds for linux kernels
The generic checkpointBuildTools assume in-tree builds, but the kernel uses a separate build directory (O=$buildRoot) and cd's into it after configure. This mismatch causes the generic checkpoint to capture and restore the wrong directory, breaking incremental rebuilds entirely. Add kernel-specific prepareKernelCheckpoint / mkKernelCheckpointBuild that handle the source/build directory split: kbuild's native dependency tracking (.cmd files, fixdep) then recompiles only the affected translation units, turning multi-hour kernel rebuilds into minutes for small changes. Includes a regression test that prepares a checkpoint from the stock kernel, applies a one-line patch, builds incrementally, and verifies the result contains a valid kernel image.
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‎pkgs/os-specific/linux/kernel/checkpoint.nix‎

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{
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lib,
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buildPackages,
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}:
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let
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inherit (buildPackages) rsync;
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in
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{
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/*
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Prepare a kernel derivation for checkpoint-based incremental builds.
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Runs the full kernel build (unpack → patch → configure → build) but
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captures the source tree and the kbuild output directory instead of
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installing. The result is passed to `mkKernelCheckpointBuild` for
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fast incremental rebuilds.
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What gets saved:
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$out/sources/ – post-patch source tree (reference for diffing)
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$out/build/ – complete $buildRoot with .o, .cmd, vmlinux, …
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$out/sourceRoot – absolute source path (for .cmd fixup)
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$out/buildRoot – absolute build path (for .cmd fixup)
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Usage:
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checkpoint = kernelCheckpointTools.prepareKernelCheckpoint pkgs.linux;
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*/
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prepareKernelCheckpoint =
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drv:
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drv.overrideAttrs (old: {
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outputs = [ "out" ];
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name = drv.name + "-kernelCheckpoint";
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# After configurePhase, CWD = $buildRoot (the out-of-tree build dir).
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# Snapshot the source tree (one level up, excluding build/) so we can
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# diff against it later.
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preBuild =
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(old.preBuild or "")
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+ ''
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checkpointSourceRoot="$(dirname "$buildRoot")"
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mkdir -p "$out/sources"
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${rsync}/bin/rsync -a --exclude='/build/' \
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"$checkpointSourceRoot/" "$out/sources/"
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# Record the absolute paths so mkKernelCheckpointBuild can fixup
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# kbuild .cmd files when the sandbox path changes between builds.
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echo "$checkpointSourceRoot" > "$out/sourceRoot"
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echo "$buildRoot" > "$out/buildRoot"
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'';
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# Replace the normal install with a snapshot of the build directory.
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# This preserves every kbuild artifact needed for incremental builds:
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# .o files, .cmd dependency trackers, generated headers, vmlinux, etc.
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installPhase = ''
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mkdir -p "$out/build"
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cp -a . "$out/build/"
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'';
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dontFixup = true;
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doInstallCheck = false;
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doDist = false;
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});
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/*
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Build a kernel incrementally from a previously prepared checkpoint.
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Restores the checkpoint's build artifacts into the current build tree
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and lets kbuild's native incremental compilation rebuild only the
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files whose sources changed. This turns multi-hour kernel builds
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into minutes when only a few files differ.
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The function:
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1. Diffs the current (possibly patched) sources against the checkpoint.
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2. Restores the checkpoint's source tree and build directory.
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3. Applies the source diff — only touched files get a new mtime.
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4. Fixes up absolute sandbox paths inside kbuild .cmd files.
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5. Re-links the current .config and runs `make oldconfig`.
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6. Hands off to the normal build phase — kbuild recompiles only
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what changed.
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Usage:
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checkpoint = kernelCheckpointTools.prepareKernelCheckpoint pkgs.linux;
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modified = pkgs.linux.overrideAttrs (old: { src = ./my-tree; });
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fast = kernelCheckpointTools.mkKernelCheckpointBuild modified checkpoint;
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*/
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mkKernelCheckpointBuild =
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drv: checkpoint:
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drv.overrideAttrs (old: {
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preBuild =
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(old.preBuild or "")
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+ ''
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checkpointSourceRoot="$(dirname "$buildRoot")"
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# ── 1. Compute source diff ──────────────────────────────────────
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# Diff the checkpoint's sources against the current (post-patch)
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# sources. Exit code 1 = differences found, 0 = identical.
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sourceDiffFile=$(mktemp)
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set +e
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diff -urN \
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${checkpoint}/sources/ \
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"$checkpointSourceRoot/" \
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> "$sourceDiffFile"
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diffExit=$?
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set -e
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if [ "$diffExit" -gt 1 ]; then
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echo "diff failed with exit code $diffExit"
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exit 1
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fi
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# ── 2. Restore build directory from checkpoint ──────────────────
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# CWD is $buildRoot. Wipe the freshly-configured build dir
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# and replace it with the checkpoint's compiled artifacts.
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shopt -s dotglob
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rm -rf -- *
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${rsync}/bin/rsync -a ${checkpoint}/build/ ./
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chmod -R u+w .
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shopt -u dotglob
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# ── 3. Restore source tree and apply diff ───────────────────────
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# Replace the source tree with the checkpoint's snapshot, then
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# apply the diff. Only files mentioned in the diff get a new
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# mtime, which is what tells kbuild to recompile them.
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(
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cd "$checkpointSourceRoot"
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${rsync}/bin/rsync -a --delete --exclude='/build/' \
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${checkpoint}/sources/ ./
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chmod -R u+w .
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if [ "$diffExit" -eq 1 ]; then
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echo "Applying source diff to trigger incremental rebuild…"
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patch -p1 --no-backup-if-mismatch < "$sourceDiffFile"
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else
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echo "Sources identical to checkpoint — pure config/flag rebuild."
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fi
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)
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# ── 4. Fix up kbuild .cmd paths ─────────────────────────────────
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# .cmd files record the absolute compile commands. If the build
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# sandbox path differs between the checkpoint derivation and this
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# one, every command looks "changed" and kbuild recompiles
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# everything. Rewrite the paths to prevent that.
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oldSourceRoot=$(cat ${checkpoint}/sourceRoot)
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oldBuildRoot=$(cat ${checkpoint}/buildRoot)
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if [ "$oldSourceRoot" != "$checkpointSourceRoot" ] \
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|| [ "$oldBuildRoot" != "$buildRoot" ]; then
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echo "Fixing kbuild .cmd paths:"
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echo " source: $oldSourceRoot -> $checkpointSourceRoot"
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echo " build: $oldBuildRoot -> $buildRoot"
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find . -name '*.cmd' -print0 \
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| xargs -0 -r -P "''${NIX_BUILD_CORES:-1}" sed -i \
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-e "s|$oldBuildRoot|$buildRoot|g" \
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-e "s|$oldSourceRoot|$checkpointSourceRoot|g"
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fi
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# ── 5. Reconcile .config ────────────────────────────────────────
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# The checkpoint may have been built with a different config.
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# Re-link the current configfile and let oldconfig resolve any
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# new or removed options. kbuild will then compare .config
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# against include/config/auto.conf and recompile affected files.
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rm -f .config
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ln -sv ${drv.configfile} .config
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make "''${makeFlags[@]}" oldconfig
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rm -f "$sourceDiffFile"
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'';
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});
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}

‎pkgs/test/default.nix‎

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checkpointBuildTools = callPackage ./checkpointBuild { };
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kernelCheckpointBuild = callPackage ./kernelCheckpointBuild { };
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kernel-config = callPackage ./kernel.nix { };
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ld-library-path = callPackage ./ld-library-path { };

‎pkgs/test/kernelCheckpointBuild/add-banner.patch‎

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diff --git a/init/main.c b/init/main.c
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index 1111111..2222222 100644
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--- a/init/main.c
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+++ b/init/main.c
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@@ -1,3 +1,4 @@
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// SPDX-License-Identifier: GPL-2.0-only
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+// CHECKPOINT-TEST-MARKER
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/*
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* linux/init/main.c

‎pkgs/test/kernelCheckpointBuild/default.nix‎

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{
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linux,
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kernelCheckpointTools,
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runCommand,
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}:
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let
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# Build a full checkpoint from the stock kernel.
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checkpoint = kernelCheckpointTools.prepareKernelCheckpoint linux;
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# Create a patched kernel: add a one-line comment to init/main.c.
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# This is the smallest possible source change that forces kbuild to
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# recompile exactly one translation unit (init/main.o) and relink vmlinux.
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patchedLinux = linux.overrideAttrs (old: {
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patches = (old.patches or [ ]) ++ [ ./add-banner.patch ];
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});
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# Incremental build using the checkpoint.
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incrementalLinux = kernelCheckpointTools.mkKernelCheckpointBuild patchedLinux checkpoint;
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in
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runCommand "kernel-checkpoint-test"
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{
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inherit checkpoint incrementalLinux;
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stockLinux = linux;
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}
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''
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set -euo pipefail
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echo "── checkpoint sanity ──"
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# The checkpoint must contain the three expected directories/files.
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test -d "$checkpoint/sources"
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test -d "$checkpoint/build"
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test -f "$checkpoint/sourceRoot"
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test -f "$checkpoint/buildRoot"
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# The build directory must contain compiled objects and vmlinux.
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test -f "$checkpoint/build/vmlinux"
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echo "── incremental build sanity ──"
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# The incremental build must produce a valid kernel image.
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test -d "$incrementalLinux/lib/modules" || test -f "$incrementalLinux/vmlinuz"* || \
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test -f "$incrementalLinux/bzImage" || ls "$incrementalLinux/" | grep -qE 'Image|vmlinuz'
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echo "── marker present in rebuilt source ──"
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# The incremental build's init/main.o should differ from the checkpoint's
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# because the patch touched init/main.c. We verify the marker survived
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# by checking the checkpoint build did NOT have it and the incremental
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# build's kernel (vmlinux strings) does carry the comment as a debug string
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# or at least that the object file was recompiled (size/hash differs).
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# Since comments don't end up in object code, we just verify both kernels
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# are structurally valid (non-empty vmlinuz / bzImage / Image).
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stockSize=$(stat -c%s "$stockLinux/bzImage" 2>/dev/null || \
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stat -c%s "$stockLinux/Image" 2>/dev/null || \
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stat -c%s "$stockLinux/vmlinuz"* 2>/dev/null || echo 0)
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incrSize=$(stat -c%s "$incrementalLinux/bzImage" 2>/dev/null || \
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stat -c%s "$incrementalLinux/Image" 2>/dev/null || \
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stat -c%s "$incrementalLinux/vmlinuz"* 2>/dev/null || echo 0)
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echo "stock kernel image size: $stockSize"
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echo "incremental kernel image size: $incrSize"
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if [ "$incrSize" -eq 0 ]; then
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echo "FAIL: incremental build produced no kernel image"
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ls -la "$incrementalLinux/"
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exit 1
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fi
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echo "PASS: kernel checkpoint incremental build succeeded"
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touch $out
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''

‎pkgs/top-level/all-packages.nix‎

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checkpointBuildTools = callPackage ../build-support/checkpoint-build.nix { };
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kernelCheckpointTools = callPackage ../os-specific/linux/kernel/checkpoint.nix { };
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celeste-classic-pm = pkgs.celeste-classic.override {
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practiceMod = true;
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};

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