feat: install LTS kernel for Ubuntu when available

vhdbuilder/packer/pre-install-dependencies.sh

@@ -115,17 +115,29 @@ if [[ ${OS} == ${MARINER_OS_NAME} ]] && [[ "${ENABLE_CGROUPV2,,}" == "true" ]];
   enableCgroupV2forAzureLinux
 fi
 
-if [[ "${UBUNTU_RELEASE}" == "22.04" && "${ENABLE_FIPS,,}" != "true" ]]; then
+if [[ ${UBUNTU_RELEASE//./} -ge 2204 && "${ENABLE_FIPS,,}" != "true" ]]; then
+  LTS_KERNEL="linux-image-azure-lts-${UBUNTU_RELEASE}"
+  LTS_TOOLS="linux-tools-azure-lts-${UBUNTU_RELEASE}"
+  LTS_CLOUD_TOOLS="linux-cloud-tools-azure-lts-${UBUNTU_RELEASE}"
+  LTS_HEADERS="linux-headers-azure-lts-${UBUNTU_RELEASE}"
+  LTS_MODULES="linux-modules-extra-azure-lts-${UBUNTU_RELEASE}"
+
   echo "Logging the currently running kernel: $(uname -r)"
   echo "Before purging kernel, here is a list of kernels/headers installed:"; dpkg -l 'linux-*azure*'
 
-  # Purge all current kernels and dependencies
-  DEBIAN_FRONTEND=noninteractive apt-get remove --purge -y $(dpkg-query -W 'linux-*azure*' | awk '$2 != "" { print $1 }' | paste -s)
-  echo "After purging kernel, dpkg list should be empty"; dpkg -l 'linux-*azure*'
+  if apt-cache show "$LTS_KERNEL" &>/dev/null; then
+      echo "LTS kernel is available for ${UBUNTU_RELEASE}, proceeding with purging current kernel and installing LTS kernel..."
+
+      # Purge all current kernels and dependencies
+      DEBIAN_FRONTEND=noninteractive apt-get remove --purge -y $(dpkg-query -W 'linux-*azure*' | awk '$2 != "" { print $1 }' | paste -s)
+      echo "After purging kernel, dpkg list should be empty"; dpkg -l 'linux-*azure*'
 
-  # Install lts-22.04 kernel
-  DEBIAN_FRONTEND=noninteractive apt-get install -y linux-image-azure-lts-22.04 linux-cloud-tools-azure-lts-22.04 linux-headers-azure-lts-22.04 linux-modules-extra-azure-lts-22.04 linux-tools-azure-lts-22.04
-  echo "After installing new kernel, here is a list of kernels/headers installed"; dpkg -l 'linux-*azure*'
+      # Install LTS kernel
+      DEBIAN_FRONTEND=noninteractive apt-get install -y "$LTS_KERNEL" "$LTS_TOOLS" "$LTS_CLOUD_TOOLS" "$LTS_HEADERS" "$LTS_MODULES"
+      echo "After installing new kernel, here is a list of kernels/headers installed:"; dpkg -l 'linux-*azure*'
+  else
+      echo "LTS kernel for Ubuntu ${UBUNTU_RELEASE} is not available. Skipping purging and subsequent installation."
+  fi
 
   update-grub
 fi

vhdbuilder/packer/test/linux-vhd-content-test.sh

@@ -408,6 +408,37 @@ testFips() {
   echo "$test:Finish"
 }
 
+testLtsKernel() {
+  test="testLtsKernel"
+  echo "$test:Start"
+  os_version=$1
+  os_sku=$2
+  enable_fips=$3
+
+  if [[ "$os_sku" == "Ubuntu" && ${enable_fips,,} != "true" ]]; then
+    echo "OS is Ubuntu and FIPS is not enabled, check LTS kernel version"
+    # Check the Ubuntu version and set the expected kernel version
+    if [[ "$os_version" == "2204" ]]; then
+      expected_kernel="5.15"
+    elif [[ "$os_version" == "2404" ]]; then
+      expected_kernel="6.8"
+    else
+      echo "LTS kernel not installed for: $os_version"
+    fi
+
+    kernel=$(uname -r)
+    echo "Current kernel version: $kernel"
+    if [[ "$kernel" == *"$expected_kernel"* ]]; then
+      echo "Kernel version is as expected ($expected_kernel)."
+    else
+      echo "Kernel version is not as expected. Expected $expected_kernel, found $kernel."
+    fi
+  else
+    echo "OS is not Ubuntu OR OS is Ubuntu and FIPS is true, skip LTS kernel test"
+  fi
+
+}
+
 testCloudInit() {
   test="testCloudInit"
   echo "$test:Start"
@@ -1160,4 +1191,5 @@ testPam $OS_SKU $OS_VERSION
 testUmaskSettings
 testContainerImagePrefetchScript
 testAKSNodeControllerBinary
-testAKSNodeControllerService
+testAKSNodeControllerService
+testLtsKernel $OS_VERSION $OS_SKU $ENABLE_FIPS