#!/bin/bash
# build-kernel.sh — Build custom Tiny Core kernel with CONFIG_CGROUP_BPF=y
#
# The stock Tiny Core 17.0 kernel (6.18.2-tinycore64) lacks CONFIG_CGROUP_BPF,
# which is required for cgroup v2 device control in runc/containerd.
# This script downloads the TC-patched kernel source, enables CONFIG_CGROUP_BPF,
# and builds vmlinuz + modules.
#
# Output is cached in $CACHE_DIR/custom-kernel/ and reused across builds.
set -euo pipefail

SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
PROJECT_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
CACHE_DIR="${CACHE_DIR:-$PROJECT_ROOT/build/cache}"

# shellcheck source=../config/versions.env
. "$SCRIPT_DIR/../config/versions.env"

KVER="6.18.2-tinycore64"
KERNEL_BASE_URL="https://distro.ibiblio.org/tinycorelinux/${TINYCORE_VERSION%%.*}.x/${TINYCORE_ARCH}/release/src/kernel"
KERNEL_SRC_URL="${KERNEL_BASE_URL}/linux-6.18.2-patched.tar.xz"
KERNEL_CFG_URL="${KERNEL_BASE_URL}/config-${KVER}"

CUSTOM_KERNEL_DIR="$CACHE_DIR/custom-kernel"
CUSTOM_VMLINUZ="$CUSTOM_KERNEL_DIR/vmlinuz"
CUSTOM_MODULES="$CUSTOM_KERNEL_DIR/modules"

mkdir -p "$CACHE_DIR" "$CUSTOM_KERNEL_DIR"

# --- Skip if already built ---
if [ -f "$CUSTOM_VMLINUZ" ] && [ -d "$CUSTOM_MODULES/lib/modules/$KVER" ]; then
    echo "==> Custom kernel already built (cached)"
    echo "    vmlinuz: $CUSTOM_VMLINUZ ($(du -h "$CUSTOM_VMLINUZ" | cut -f1))"
    MOD_COUNT=$(find "$CUSTOM_MODULES/lib/modules/$KVER" -name '*.ko*' | wc -l)
    echo "    Modules: $MOD_COUNT modules in $CUSTOM_MODULES/lib/modules/$KVER"
    exit 0
fi

echo "==> Building custom kernel with CONFIG_CGROUP_BPF=y..."
echo "    Kernel version: $KVER"

# --- Download kernel source ---
KERNEL_SRC_ARCHIVE="$CACHE_DIR/linux-6.18.2-patched.tar.xz"
if [ ! -f "$KERNEL_SRC_ARCHIVE" ]; then
    echo "==> Downloading kernel source (~149 MB)..."
    echo "    URL: $KERNEL_SRC_URL"
    wget -q --show-progress -O "$KERNEL_SRC_ARCHIVE" "$KERNEL_SRC_URL" 2>/dev/null || \
        curl -fSL "$KERNEL_SRC_URL" -o "$KERNEL_SRC_ARCHIVE"
    echo "    Downloaded: $(du -h "$KERNEL_SRC_ARCHIVE" | cut -f1)"
else
    echo "==> Kernel source already cached: $(du -h "$KERNEL_SRC_ARCHIVE" | cut -f1)"
fi

# --- Download stock config ---
KERNEL_CFG="$CACHE_DIR/config-${KVER}"
if [ ! -f "$KERNEL_CFG" ]; then
    echo "==> Downloading stock kernel config..."
    echo "    URL: $KERNEL_CFG_URL"
    wget -q -O "$KERNEL_CFG" "$KERNEL_CFG_URL" 2>/dev/null || \
        curl -fSL "$KERNEL_CFG_URL" -o "$KERNEL_CFG"
else
    echo "==> Stock kernel config already cached"
fi

# --- Extract source ---
# IMPORTANT: Must extract on a case-sensitive filesystem. The kernel source has
# files that differ only by case (e.g., xt_mark.h vs xt_MARK.h). If the cache
# is on macOS (case-insensitive APFS), extraction silently loses files.
# Use /tmp inside the container (ext4, case-sensitive) for the build.
KERNEL_BUILD_DIR="/tmp/kernel-build"
rm -rf "$KERNEL_BUILD_DIR"
mkdir -p "$KERNEL_BUILD_DIR"

echo "==> Extracting kernel source (case-sensitive filesystem)..."
tar -xf "$KERNEL_SRC_ARCHIVE" -C "$KERNEL_BUILD_DIR"

# Find the extracted source directory (could be linux-6.18.2 or linux-6.18.2-patched)
KERNEL_SRC_DIR=$(find "$KERNEL_BUILD_DIR" -maxdepth 1 -type d -name 'linux-*' | head -1)
if [ -z "$KERNEL_SRC_DIR" ]; then
    echo "ERROR: Could not find kernel source directory after extraction"
    ls -la "$KERNEL_BUILD_DIR"/
    exit 1
fi
echo "    Source dir: $(basename "$KERNEL_SRC_DIR")"

cd "$KERNEL_SRC_DIR"

# --- Apply stock config + enable CONFIG_CGROUP_BPF ---
echo "==> Applying stock Tiny Core config..."
cp "$KERNEL_CFG" .config

echo "==> Enabling required kernel configs..."
./scripts/config --enable CONFIG_CGROUP_BPF
./scripts/config --enable CONFIG_DEVTMPFS
./scripts/config --enable CONFIG_DEVTMPFS_MOUNT
./scripts/config --enable CONFIG_MEMCG
./scripts/config --enable CONFIG_CFS_BANDWIDTH

# Security: AppArmor LSM + Audit subsystem
echo "==> Enabling AppArmor + Audit kernel configs..."
./scripts/config --enable CONFIG_AUDIT
./scripts/config --enable CONFIG_AUDITSYSCALL
./scripts/config --enable CONFIG_SECURITY
./scripts/config --enable CONFIG_SECURITYFS
./scripts/config --enable CONFIG_SECURITY_NETWORK
./scripts/config --enable CONFIG_SECURITY_APPARMOR
./scripts/config --set-str CONFIG_LSM "lockdown,yama,apparmor"
./scripts/config --set-str CONFIG_DEFAULT_SECURITY "apparmor"

# --- Strip unnecessary subsystems for smallest footprint ---
# This is a headless K8s edge appliance — no sound, GPU, wireless, etc.
echo "==> Disabling unnecessary subsystems for minimal footprint..."

# Sound subsystem (not needed on headless appliance)
./scripts/config --disable SOUND

# GPU/DRM (serial console only, no display)
./scripts/config --disable DRM

# KVM hypervisor (this IS the guest/bare metal, not a hypervisor)
./scripts/config --disable KVM

# Media/camera/TV/radio (not needed)
./scripts/config --disable MEDIA_SUPPORT

# Wireless networking (wired edge device)
./scripts/config --disable WIRELESS
./scripts/config --disable WLAN
./scripts/config --disable CFG80211

# Bluetooth (not needed)
./scripts/config --disable BT

# NFC (not needed)
./scripts/config --disable NFC

# Infiniband (not needed on edge)
./scripts/config --disable INFINIBAND

# PCMCIA (legacy, not needed)
./scripts/config --disable PCMCIA

# Amateur radio (not needed)
./scripts/config --disable HAMRADIO

# ISDN (not needed)
./scripts/config --disable ISDN

# ATM networking (not needed)
./scripts/config --disable ATM

# Joystick/gamepad (not needed)
./scripts/config --disable INPUT_JOYSTICK
./scripts/config --disable INPUT_TABLET

# FPGA (not needed)
./scripts/config --disable FPGA

# Resolve dependencies (olddefconfig accepts defaults for new options)
make olddefconfig

# Verify CONFIG_CGROUP_BPF is set
if grep -q 'CONFIG_CGROUP_BPF=y' .config; then
    echo "    CONFIG_CGROUP_BPF=y confirmed in .config"
else
    echo "ERROR: CONFIG_CGROUP_BPF not set after olddefconfig"
    grep 'CGROUP_BPF' .config || echo "    (CGROUP_BPF not found in .config)"
    echo ""
    echo "Prerequisites check:"
    grep -E 'CONFIG_BPF=|CONFIG_BPF_SYSCALL=' .config || echo "    BPF not found"
    exit 1
fi

# Show what changed
echo "    Config diff from stock:"
diff "$KERNEL_CFG" .config | grep '^[<>]' | head -20 || echo "    (no differences beyond CGROUP_BPF)"

# --- Build kernel + modules ---
NPROC=$(nproc 2>/dev/null || echo 4)
echo ""
echo "==> Building kernel (${NPROC} parallel jobs)..."
echo "    This may take 15-25 minutes..."

make -j"$NPROC" bzImage modules 2>&1

echo "==> Kernel build complete"

# --- Install to staging ---
echo "==> Installing vmlinuz..."
cp arch/x86/boot/bzImage "$CUSTOM_VMLINUZ"

echo "==> Installing modules (stripped)..."
rm -rf "$CUSTOM_MODULES"
mkdir -p "$CUSTOM_MODULES"
make INSTALL_MOD_STRIP=1 modules_install INSTALL_MOD_PATH="$CUSTOM_MODULES"

# Remove build/source symlinks (they point to the build dir which won't exist in rootfs)
rm -f "$CUSTOM_MODULES/lib/modules/$KVER/build"
rm -f "$CUSTOM_MODULES/lib/modules/$KVER/source"

# Run depmod to generate proper module dependency files
echo "==> Running depmod..."
depmod -a -b "$CUSTOM_MODULES" "$KVER" 2>/dev/null || true

# Save the final config for reference
cp .config "$CUSTOM_KERNEL_DIR/.config"

# --- Clean up build dir (large, ~1.5 GB) ---
echo "==> Cleaning kernel build directory..."
cd /
rm -rf "$KERNEL_BUILD_DIR"

# --- Summary ---
echo ""
echo "==> Custom kernel build complete:"
echo "    vmlinuz:      $CUSTOM_VMLINUZ ($(du -h "$CUSTOM_VMLINUZ" | cut -f1))"
MOD_COUNT=$(find "$CUSTOM_MODULES/lib/modules/$KVER" -name '*.ko*' | wc -l)
echo "    Modules:      $MOD_COUNT modules"
echo "    Modules size: $(du -sh "$CUSTOM_MODULES/lib/modules/$KVER" | cut -f1)"
echo ""