#!/bin/sh -euf
#
# SPDX-License-Identifier: AGPL-3.0-or-later
#
# PASST - Plug A Simple Socket Transport
#  for qemu/UNIX domain socket mode
#
# PASTA - Pack A Subtle Tap Abstraction
#  for network namespace/tap device mode
#
# slirp4netns.sh - Compatibility wrapper for pasta, behaving like slirp4netns(1)
#
# WARNING: Draft quality, not really tested, --enable-sandbox not supported yet
#
# Copyright (c) 2021 Red Hat GmbH
# Author: Stefano Brivio <sbrivio@redhat.com>

PASTA_PID="$(mktemp)"
PASTA_OPTS="-q --ipv4-only -a 10.0.2.0 -n 24 -g 10.0.2.2 -m 1500 --no-ndp --no-dhcpv6 --no-dhcp -P ${PASTA_PID}"
PASTA="$(command -v ./pasta || command -v pasta || :)"

USAGE_RET=1
NOTFOUND_RET=127

# add() - Add single option to $PASTA_OPTS
# $1:	Option name, with or without argument
add() {
	PASTA_OPTS="${PASTA_OPTS} ${1}"
}

# drop() - Drop one option (without argument) from $PASTA_OPTS
# $1:	Option name
drop() {
	old_opts="${PASTA_OPTS}"; PASTA_OPTS=
	for o in ${old_opts}; do [ "${o}" != "${1}" ] && add "${o}"; done
}

# sub() - Substitute option in $PASTA_OPTS, with or without argument
# $1:	Option name
# $2:	Option argument, can be empty
sub() {
	old_opts="${PASTA_OPTS}"; PASTA_OPTS=
	next=0
	for o in ${old_opts}; do
		if [ ${next} -eq 1 ]; then
			next=0; add "${1} ${2}"; shift; shift; continue
		fi

		for r in ${@}; do [ "${o}" = "${r}" ] && next=1 && break; done
		[ "${next}" -eq 0 ] && add "${o}"
	done
}

# xorshift() - pseudorandom permutation of 16-bit group
# $1:	16-bit value to shuffle
xorshift() {
	# Adaptation of Xorshift algorithm from:
	#   Marsaglia, G. (2003). Xorshift RNGs.
	#   Journal of Statistical Software, 8(14), 1 - 6.
	#   doi:http://dx.doi.org/10.18637/jss.v008.i14
	# with triplet (5, 3, 1), suitable for 16-bit ranges.
	n=${1}
	: $((n ^= n << 5))
	: $((n ^= n >> 3))
	: $((n ^= n << 1))
	echo ${n}
}

# opt() - Validate single option from getopts
# $1:	Option type
# $@:	Variable names to assign to
opt() {
	case "${1}" in
	u32)
		if ! printf "%i" "${OPTARG}" >/dev/null 2>&1 || \
		   [ "${OPTARG}" -lt 0 ]; then
			echo "${OPT} must be a non-negative integer"
			usage
		fi
		eval ${2}="${OPTARG}"
		;;
	mtu)
		if ! printf "%i" "${OPTARG}" >/dev/null 2>&1 || \
		   [ "${OPTARG}" -lt 0 ] || [ "${OPTARG}" -ge 65522 ]; then
			echo "MTU must be a positive integer (< 65522)"
			usage
		fi
		eval ${2}="${OPTARG}"
		;;
	str)
		eval ${2}="${OPTARG}"
		;;
	net4)
		addr="${OPTARG%/*}"
		mask="${OPTARG##*/}"

		{ [ -z "${mask}" ] || !printf "%i" "${mask}" >/dev/null 2>&1 \
		  || [ ${mask} -gt 32 ] || ${mask} -le 0 ]; } && usage

		expr "${addr}" :					      \
			'[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*$' \
			>/dev/null
		[ $? -ne 0 ] && usage

		ifs="${IFS}"; IFS='.'
		for q in ${addr}; do [ ${q} -gt 255 ] && usage; done
		IFS="${ifs}"

		eval ${2}="${addr}"
		eval ${3}="${mask}"
		;;
	esac
}

# usage() - Print slirpnetns(1) usage and exit indicating failure
# $1:	Invalid option name, if any
usage() {
	[ ${#} -eq 1 ] && printf "%s: invalid option -- '%s'\n" "${0}" "${1}"
	cat << EOF
Usage: ${0} [OPTION]... PID|PATH TAPNAME
User-mode networking for unprivileged network namespaces.

-c, --configure          bring up the interface
-e, --exit-fd=FD         specify the FD for terminating slirp4netns
-r, --ready-fd=FD        specify the FD to write to when the network is configured
-m, --mtu=MTU            specify MTU (default=1500, max=65521)
-6, --enable-ipv6        enable IPv6 (experimental)
-a, --api-socket=PATH    specify API socket path
--cidr=CIDR              specify network address CIDR (default=10.0.2.0/24)
--disable-host-loopback  prohibit connecting to 127.0.0.1:* on the host namespace
--netns-type=TYPE 	 specify network namespace type ([path|pid], default=pid)
--userns-path=PATH	 specify user namespace path
--enable-sandbox         create a new mount namespace (and drop all caps except CAP_NET_BIND_SERVICE if running as the root)
--enable-seccomp         enable seccomp to limit syscalls (experimental)
-h, --help               show this help and exit
-v, --version            show version and exit
EOF
	exit ${USAGE_RET}
}

# version() - Print version
version() {
	echo "slirp4netns-like wrapper for pasta"
	exit 0
}

# gen_addr6() - Generate pseudorandom IPv6 address, changes every second
gen_addr6() {
	printf "fd00"
	n=$(($(xorshift $(date +%S)) % 65536))
	for i in $(seq 2 8); do
		printf ":%04x" ${n}
		n=$(($(xorshift ${n}) % 65536))
	done
}

# Default options
v6=0
get_pid=0
MTU=1500
A4="10.0.2.0"
M4="255.255.255.0"
no_map_gw=0
EFD=0
RFD=0

[ -z "${PASTA}" ] && echo "pasta command not found" && exit ${NOTFOUND_RET}

while getopts ce:r:m:6a:hv-: OPT 2>/dev/null; do
	if [ "${OPT}" = "-" ]; then
		OPT="${OPTARG%%[= ]*}"
		OPTARG="${OPTARG#${OPT}[= ]}"
	fi
	case "${OPT}" in
	c | configure)		add "--config-net"			      ;;
	e | exit-fd)		opt u32 EFD				      ;;
	r | ready-fd)		opt u32 RFD				      ;;
	m | mtu)		opt mtu MTU && sub -m ${MTU}		      ;;
	6 | enable-ipv6)	V6=1					      ;;
	a | api-socket)		opt str API				      ;;
	cidr)			opt net4 A4 M4 && sub -a ${A4} -n ${M4}	      ;;
	disable-host-loopback)	add "--no-map-gw" && no_map_gw=1	      ;;
	netns-type)		: Autodetected				      ;;
	userns-path)		opt_str USERNS_NAME "${OPTARG}"		      ;;
	enable-sandbox) 	: Not supported yet			      ;;
	enable-seccomp)		: Cannot be disabled			      ;;
	h | help)		USAGE_RET=0 && usage			      ;;
	v | version)		version					      ;;
	??*)			usage "${OPT}"				      ;;
	?)			usage "${OPT}"				      ;;
	esac
done

shift $((OPTIND - 1))
[ ${#} -ne 2 ] && usage
ns_spec="${1}"

ifname="${2}"
add "-I ${ifname}"

if [ ${v6} -eq 1 ]; then
	drop "--ipv4-only"
	add "-a $(gen_addr6) -g fd00::2 -D fd00::3"
fi

${PASTA} ${PASTA_OPTS} ${ns_spec} && \
	[ ${RFD} -ne 0 ] && echo "1" >&${RFD}

trap "kill $(cat ${PASTA_PID}); rm ${PASTA_PID}" INT TERM

cat << EOF
sent tapfd=5 for ${ifname}
received tapfd=5
Starting slirp
* MTU:             ${MTU}
* Network:         ${A4}
* Netmask:         ${M4}
* Gateway:         10.0.2.2
* DNS:             10.0.2.3
* Recommended IP:  10.0.2.100
EOF

if [ ${no_map_gw} -eq 0 ]; then
	echo "WARNING: 127.0.0.1:* on the host is accessible as 10.0.2.2 (set --disable-host-loopback to prohibit connecting to 127.0.0.1:*)"
fi

if [ ${EFD} -ne 0 ]; then
	dd count=1 of=/dev/null 2>/dev/null <&${EFD}
else
	while read a; do :; done
fi

exit 0