#!/usr/bin/env bash

set -o errexit
set -o nounset

SYSTEMD_UNIT_DIR="/etc/systemd/system"
SYSTEMD_UNIT="sys-fs-bpf.mount"
SYSTEMD_UNIT_PATH="$SYSTEMD_UNIT_DIR/$SYSTEMD_UNIT"
MOUNT_POINT="/sys/fs/bpf"
FS_TYPE="bpf"

MOUNT_BPF_FS_DEBUG=${MOUNT_BPF_FS_DEBUG:-false}
function debug() {
  if [ "$MOUNT_BPF_FS_DEBUG" = "true" ]; then
    echo >&2 "DEBUG:" "$@"
  fi
}

if [ $(mount --types "$FS_TYPE" | wc -l) -gt 0 ]; then
  debug "$FS_TYPE filesystem already mounted!"
  exit 0
elif mount | awk '{print $3}' | grep "$MOUNT_POINT"; then
  debug "mount point at $MOUNT_POINT already exists!"
  exit 0
elif [ -f "$SYSTEMD_UNIT_PATH" ]; then
  debug "systemd unit at $SYSTEMD_UNIT_PATH already exists!"
  exit 0
fi

mkdir -p "$SYSTEMD_UNIT_DIR"
cat > "$SYSTEMD_UNIT_PATH" << EOL
[Unit]
Description=BPF mounts
Documentation=https://docs.kernel.org/bpf/index.html
DefaultDependencies=no
Before=local-fs.target umount.target
After=swap.target

[Mount]
What=bpffs
Where=$MOUNT_POINT
Type=bpf
Options=rw,nosuid,nodev,noexec,relatime,mode=700

[Install]
WantedBy=multi-user.target
EOL

systemctl enable "$SYSTEMD_UNIT"
systemctl start "$SYSTEMD_UNIT"