fix(kernel-headers): preserve build-time autoconf.h across postinst olddefconfig (#9425)

[iav]

Summary

Fixes #9425.

The linux-headers postinst script runs make olddefconfig before natively
recompiling scripts/ on the target host (necessary for cross-built kernels).
However, olddefconfig re-evaluates toolchain availability on the installation
host and silently disables CONFIG_* options whose tools are absent there —
even though the compiled kernel image was built with those options enabled.

This causes the installed headers package to describe a different kernel than
the one actually running.

Affected files:

• include/generated/autoconf.h — used by the C preprocessor
• include/config/auto.conf + include/config/ marker files — used by kbuild make rules in external modules
• include/generated/rustc_cfg — used by Rust builds

Fix: at packaging time, save a sidecar tarball of the build-time
include/config/ and include/generated/{autoconf.h,rustc_cfg}; restore it
in postinst as the very last step, after all make targets complete.

Completed tests

• Build linux-headers for BOARD=odroidn2 (aarch64, native build) with kernel-rust extension (CONFIG_RUST=y)
• Build linux-headers for BOARD=odroidm1 (aarch64, cross-compiled on x86_64)
• Build linux-headers for BOARD=uefi-x86 (amd64) — exercises the tools/objtool postinst block
• Verify sidecar tarball present in .deb: include/generated/.armbian-build.tar.gz containing include/config/, autoconf.h, rustc_cfg
• Install on target host without rustc in $PATH; postinst completes successfully
• include/generated/autoconf.h: #define CONFIG_RUST 1, CONFIG_RUSTC_VERSION 108500 (not 0)
• include/config/auto.conf: CONFIG_RUST=y
• include/generated/rustc_cfg: --cfg=CONFIG_RUST

[coderabbitai]

📝 Walkthrough

Walkthrough

Preserves build-time kernel configuration during linux-headers packaging by creating an include/generated/.armbian-build.tar.gz sidecar (when include/config/auto.conf exists) and restoring its contents in the package's postinst finish to avoid target-side make olddefconfig altering the original config.

Changes

Cohort / File(s)	Summary
Kernel headers packaging lib/functions/compilation/kernel-debs.sh	Adds creation of include/generated/.armbian-build.tar.gz (captures include/config/, include/generated/autoconf.h, optional include/generated/rustc_cfg) during headers packaging when include/config/auto.conf exists; excludes the tarball from DEBUG binary checks; restores tarball contents in postinst finish and removes the tarball after restoration.

Sequence Diagram(s)

sequenceDiagram
    participant Packager
    participant Package
    participant TargetPostinst
    participant Filesystem

    Packager->>Filesystem: check for include/config/auto.conf
    alt auto.conf exists
        Packager->>Filesystem: create include/generated/.armbian-build.tar.gz (include/config + generated files)
        Packager->>Package: include tarball in linux-headers package
    end
    Note right of Package: Package shipped to target

    Package->>TargetPostinst: postinst runs on install
    TargetPostinst->>Filesystem: run make olddefconfig / prepare scripts/
    TargetPostinst->>Filesystem: if .armbian-build.tar.gz exists, extract to restore saved config
    TargetPostinst->>Filesystem: remove .armbian-build.tar.gz

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 A tarball snug within the tree,
Saved configs hop back home with glee,
Postinst runs but leaves truth intact,
Host-made kernels stay exact —
Hooray for builds preserved! 🥕

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: preserving build-time autoconf.h across postinst olddefconfig, directly addressing issue #9425.
Linked Issues check	✅ Passed	The PR implementation fully addresses all coding requirements: preserves build-time config artifacts in a sidecar tarball, prevents make olddefconfig mutation, and restores artifacts during postinst.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing kernel-headers preservation of build-time configuration; no unrelated modifications detected.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[coderabbitai]

🧹 Nitpick comments (1)

lib/functions/compilation/kernel-debs.sh (1)

495-502: Sidecar tarball will trigger a false positive in the DEBUG binary check

include/generated/.armbian-build.tar.gz is a gzip binary. The grep -v filter at line 500 excludes include/config/ but not include/generated/, so file will report it as an unexpected binary when DEBUG=yes.

♻️ Proposed fix — exclude the sidecar from the binary scan

- find . -type f | grep -v -e "include/config/" -e "\.h$" -e ".c$" -e "Makefile$" -e "Kconfig$" -e "Kbuild$" -e "\.cocci$" | xargs file | grep -v -e "ASCII" -e "script text" -e "empty" -e "Unicode text" -e "symbolic link" -e "CSV text" -e "SAS 7+" || true
+ find . -type f | grep -v -e "include/config/" -e "include/generated/\.armbian-build\.tar\.gz" -e "\.h$" -e ".c$" -e "Makefile$" -e "Kconfig$" -e "Kbuild$" -e "\.cocci$" | xargs file | grep -v -e "ASCII" -e "script text" -e "empty" -e "Unicode text" -e "symbolic link" -e "CSV text" -e "SAS 7+" || true

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@lib/functions/compilation/kernel-debs.sh` around lines 495 - 502, The DEBUG
binary-scan in kernel-debs.sh (inside the if [[ "${DEBUG}" == "yes" ]] block
that cd's into "${headers_target_dir}" and runs the find | grep -v ... | xargs
file pipeline) falsely flags include/generated/.armbian-build.tar.gz; update the
grep -v filter in that pipeline to also exclude the include/generated/ sidecar
(or the specific filename .armbian-build.tar.gz) so the file is ignored by the
binary check, keeping the rest of the exclusion rules intact.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@lib/functions/compilation/kernel-debs.sh`:
- Around line 495-502: The DEBUG binary-scan in kernel-debs.sh (inside the if [[
"${DEBUG}" == "yes" ]] block that cd's into "${headers_target_dir}" and runs the
find | grep -v ... | xargs file pipeline) falsely flags
include/generated/.armbian-build.tar.gz; update the grep -v filter in that
pipeline to also exclude the include/generated/ sidecar (or the specific
filename .armbian-build.tar.gz) so the file is ignored by the binary check,
keeping the rest of the exclusion rules intact.