Fix kernel-headers package: build modpost in postinst for DKMS support

[iav]

Summary

Since kernel 6.18, make M=scripts/mod/ no longer works due to kbuild changes (commit 13b25489b6f8). The standard make scripts doesn't build scripts/mod (modpost, mk_elfconfig) which are required for DKMS/external module builds.

Solution: In postinst, temporarily add subdir-y += mod to scripts/Makefile before running make scripts. This makes kbuild descend into scripts/mod/ and build modpost with proper dependencies (empty.o → mk_elfconfig → elfconfig.h → modpost).

This approach:

• Works for both native and cross-compilation scenarios
• Builds modpost natively on the target system (correct architecture)
• Uses standard kbuild mechanisms with proper dependency handling
• No binaries are shipped in the package

Test plan

• Cross-compiled kernel-headers package on x86_64 for arm64
• Installed package on arm64 (helios64) - postinst completed successfully
• Verified modpost/mk_elfconfig are native aarch64 binaries
• DKMS module (bcachefs) built successfully
• Module loaded and works

🤖 Generated with Claude Code

[coderabbitai]

📝 Walkthrough

Walkthrough

Copied host binaries scripts/mod/modpost and scripts/mod/mk_elfconfig into the kernel headers package, added existence checks, and removed the prior in-tree build invocation (make M=scripts/mod/) from the headers packaging path.

Changes

Cohort / File(s)	Summary
Kernel Build Script lib/functions/compilation/kernel-debs.sh	In kernel_package_callback_linux_headers: copy host binaries scripts/mod/modpost and scripts/mod/mk_elfconfig into the headers package with existence checks; update file-type verification to include those binaries; remove the make M=scripts/mod/ invocation and add explanatory comments.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Suggested reviewers

• rpardini
• igorpecovnik

Poem

🐰 I hopped through scripts with a curious stare,
I packed modpost and mk_elfconfig with care,
No in-tree build, just copies so spry,
Headers now carry the tools that comply,
Happy builds hop onward — carrot-powered! 🥕✨

🚥 Pre-merge checks | ✅ 2 | ❌ 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 (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly describes the main change: fixing the kernel-headers package to build modpost in postinst for DKMS support, which aligns with the core objective of including pre-compiled modpost and mk_elfconfig binaries.

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✨ Finishing touches

• 📝 Generate docstrings

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[rpardini]

We can't include binaries in kernel-headers.
We cross-compile during build (on CI, usually on large amd64 machines, targetting arm64) and there would be an architecture mismatch.
Even if that was not the case, there would be a glibc mismatch.