overlay.d: add 35coreos-iptables

From `overlay.d/README.md`:

Contains systemd service and script for remaining on iptables-nft after
the migration to nft.

Split out because (1) it will roll out to next first, and (2) it can
more easily be deleted after the barrier release.

For more details, see:
coreos/fedora-coreos-tracker#676
coreos#1324

overlay.d/35coreos-iptables/usr/lib/dracut/modules.d/35coreos-iptables/coreos-enable-iptables-legacy.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=CoreOS Enable iptables-legacy
+ConditionPathExists=/etc/initrd-release
+DefaultDependencies=false
+ConditionPathExists=/sysroot/etc/coreos/iptables-legacy.stamp
+
+# On first boot, allow Ignition config to install stamp file.
+After=ignition-files.service
+
+# On subsequent boots, just make sure the deployment is accessible.
+After=ostree-prepare-root.service
+
+Before=initrd.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/usr/sbin/coreos-enable-iptables-legacy

overlay.d/35coreos-iptables/usr/lib/dracut/modules.d/35coreos-iptables/coreos-enable-iptables-legacy.sh

@@ -0,0 +1,82 @@
+#!/bin/bash
+set -euo pipefail
+
+declare -A SYMLINKS=(
+    [ip6tables]=ip6tables-legacy
+    [ip6tables-restore]=ip6tables-legacy-restore
+    [ip6tables-save]=ip6tables-legacy-save
+    [iptables]=iptables-legacy
+    [iptables-restore]=iptables-legacy-restore
+    [iptables-save]=iptables-legacy-save
+)
+
+STAMP=/sysroot/etc/coreos/iptables-legacy.stamp
+IGNITION_RESULT=/sysroot/etc/.ignition-result.json
+
+# sanity-check the stamp file is present
+if [ ! -e "${STAMP}" ]; then
+    echo "File ${STAMP} not found; exiting."
+    exit 0
+fi
+
+# We only want to run once.
+rm "${STAMP}"
+
+# Ignore firstboot. We don't want the stamp file to be a long-term
+# provisioning-time API for moving to iptables-legacy, so explicitly check for
+# this and don't support it. We use the Ignition report file because it's less
+# hacky than parsing the kernel commandline for `ignition.firstboot`.
+if [ -e "${IGNITION_RESULT}" ]; then
+    ignition_boot=$(jq -r .provisioningBootID "${IGNITION_RESULT}")
+    if [ "$(cat /proc/sys/kernel/random/boot_id)" = "${ignition_boot}" ]; then
+        echo "First boot detected; exiting."
+        exit 0
+    fi
+fi
+
+# if legacy doesn't exist on the host anymore, do nothing
+for legacy in "${SYMLINKS[@]}"; do
+    path=/sysroot/usr/sbin/$legacy
+    if [ ! -e "$path" ]; then
+        echo "Executable $path no longer present; exiting."
+        exit 0
+    fi
+done
+
+symlink_is_default() {
+    local symlinkpath=$1; shift
+    # check that the deployment is still using the symlink (i.e. the user didn't
+    # do something funky), and that the OSTree default is still symlink-based
+    # (i.e. that we didn't change strategy and forgot to update this script)
+    if [ ! -L "/sysroot/$symlinkpath" ] || [ ! -L "/sysroot/usr/$symlinkpath" ]; then
+        return 1
+    fi
+    # compare symlink targets between deployment and OSTree default
+    if [ "$(readlink "/sysroot/$symlinkpath")" != "$(readlink "/sysroot/usr/$symlinkpath")" ]; then
+        return 1
+    fi
+    # it's the default
+    return 0
+}
+
+# If there are any modifications to the symlinks, do nothing. This is basically
+# like `ostree admin config-diff` but more focused and lighter/safer than doing
+# a bwrap call and grepping output.
+for symlink in "${!SYMLINKS[@]}"; do
+    symlinkpath=/etc/alternatives/$symlink
+    if ! symlink_is_default "$symlinkpath"; then
+        echo "Symlink $symlinkpath is not default; exiting without modifying."
+        exit 0
+    fi
+done
+
+# Update symlinks for legacy backend!
+for symlink in "${!SYMLINKS[@]}"; do
+    target=${SYMLINKS[$symlink]}
+    symlink=/etc/alternatives/$symlink
+    ln -vsf "/usr/sbin/$target" "/sysroot/$symlink"
+    # symlink labels don't matter, but relabel to appease unlabeled_t scanners
+    coreos-relabel "$symlink"
+done
+
+echo "Updated /sysroot to use iptables-legacy."

overlay.d/35coreos-iptables/usr/lib/dracut/modules.d/35coreos-iptables/module-setup.sh

@@ -0,0 +1,17 @@
+install_and_enable_unit() {
+    unit="$1"; shift
+    target="$1"; shift
+    inst_simple "$moddir/$unit" "$systemdsystemunitdir/$unit"
+    # note we `|| exit 1` here so we error out if e.g. the units are missing
+    # see https://github.com/coreos/fedora-coreos-config/issues/799
+    systemctl -q --root="$initdir" add-requires "$target" "$unit" || exit 1
+}
+
+install() {
+    inst_simple readlink
+
+    inst_simple "$moddir/coreos-enable-iptables-legacy.sh" \
+        "/usr/sbin/coreos-enable-iptables-legacy"
+    install_and_enable_unit "coreos-enable-iptables-legacy.service" \
+        "initrd.target"
+}

overlay.d/README.md

@@ -46,3 +46,16 @@ Add static chrony configuration for NTP servers provided on platforms
 such as `azure`, `aws`, `gcp`. The chrony config for these NTP servers
 should override other chrony configuration (e.g. DHCP-provided)
 configuration.
+
+35coreos-iptables
+-----------------
+
+Contains systemd service and script for remaining on iptables-nft after
+the migration to nft.
+
+Split out because (1) it will roll out to next first, and (2) it can
+more easily be deleted after the barrier release.
+
+For more details, see:
+https://github.com/coreos/fedora-coreos-tracker/issues/676
+https://github.com/coreos/fedora-coreos-config/pull/1324