ci: add multi-kernel build matrix (6.8, 6.11, 6.12, 6.14)

- Add .github/scripts/fetch-kernel-headers.sh to download mainline
  kernel headers from kernel.ubuntu.com PPA
- Rewrite ci.yml into three jobs:
  - build: matrix across 4 kernel versions (headers-only compilation)
  - lint: checkpatch + sparse on native kernel
  - test: insmod + test suite on runner's native kernel
- Build matrix uses fail-fast: false so all versions are tested
  even if one fails

Author: sdimitro

.github/scripts/fetch-kernel-headers.sh

@@ -0,0 +1,65 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# fetch-kernel-headers.sh - Download mainline kernel headers from Ubuntu PPA
+#
+# Downloads and extracts the linux-headers .deb packages for a given
+# mainline kernel version from kernel.ubuntu.com. Used by CI to build
+# the module against multiple kernel versions.
+#
+# Usage: ./fetch-kernel-headers.sh <version> <outdir>
+#   version  - kernel version, e.g. "6.8" or "6.12"
+#   outdir   - directory to extract headers into
+#
+# Prints the KDIR path on success, exits non-zero on failure.
+
+set -euo pipefail
+
+VERSION="${1:?Usage: $0 <version> <outdir>}"
+OUTDIR="${2:?Usage: $0 <version> <outdir>}"
+BASE_URL="https://kernel.ubuntu.com/mainline/v${VERSION}/amd64"
+
+mkdir -p "${OUTDIR}/debs"
+
+echo "Fetching package list from ${BASE_URL}/ ..." >&2
+
+# Get the directory listing and extract header .deb filenames
+INDEX=$(curl -sL "${BASE_URL}/")
+if [ -z "$INDEX" ]; then
+    echo "ERROR: Failed to fetch index from ${BASE_URL}/" >&2
+    exit 1
+fi
+
+# Extract header .deb filenames (portable grep -oE, no -P)
+HEADER_DEBS=$(echo "$INDEX" | grep -oE 'linux-headers[^"<>]*\.deb' | sort -u)
+
+if [ -z "$HEADER_DEBS" ]; then
+    echo "ERROR: No linux-headers .deb files found at ${BASE_URL}/" >&2
+    exit 1
+fi
+
+echo "  Found $(echo "$HEADER_DEBS" | wc -l) header packages" >&2
+
+# Download each header .deb
+for deb in $HEADER_DEBS; do
+    echo "  Downloading ${deb} ..." >&2
+    curl -sL "${BASE_URL}/${deb}" -o "${OUTDIR}/debs/${deb}"
+done
+
+# Extract all .debs into the output directory
+for deb in "${OUTDIR}/debs/"*.deb; do
+    dpkg -x "$deb" "${OUTDIR}"
+done
+
+# Find the arch-specific headers directory (the one with Makefile)
+KDIR=$(find "${OUTDIR}/usr/src" -maxdepth 1 -name "linux-headers-*-generic" -type d | head -1)
+
+if [ -z "$KDIR" ] || [ ! -f "${KDIR}/Makefile" ]; then
+    echo "ERROR: Could not find extracted kernel headers directory" >&2
+    echo "  Contents of ${OUTDIR}/usr/src/:" >&2
+    ls -la "${OUTDIR}/usr/src/" >&2 || true
+    exit 1
+fi
+
+echo "  Headers extracted to ${KDIR}" >&2
+echo "$KDIR"

.github/workflows/ci.yml

@@ -8,18 +8,34 @@ on:
 
 jobs:
   build:
-    name: Build & Lint
+    name: Build (kernel ${{ matrix.kernel }})
     runs-on: ubuntu-24.04
+    strategy:
+      fail-fast: false
+      matrix:
+        kernel: ["6.8", "6.11", "6.12", "6.14"]
     steps:
       - uses: actions/checkout@v4
 
-      - name: Install build dependencies
+      - name: Fetch kernel ${{ matrix.kernel }} headers
         run: |
-          sudo apt-get update
-          sudo apt-get install -y linux-headers-$(uname -r) sparse
+          KDIR=$(bash .github/scripts/fetch-kernel-headers.sh \
+                   "${{ matrix.kernel }}" "/tmp/kernel-${{ matrix.kernel }}")
+          echo "KDIR=${KDIR}" >> "$GITHUB_ENV"
 
       - name: Compile module
-        run: make
+        run: make KDIR="${KDIR}"
+
+  lint:
+    name: Lint & Static Analysis
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y linux-headers-$(uname -r) sparse
 
       - name: Run checkpatch.pl
         run: make checkpatch
@@ -30,7 +46,7 @@ jobs:
   test:
     name: Module Tests
     runs-on: ubuntu-24.04
-    needs: build
+    needs: [build, lint]
     steps:
       - uses: actions/checkout@v4