// SPDX-FileCopyrightText: 2026 Echolot contributors
// SPDX-License-Identifier: GPL-3.0-or-later

//go:build linux

package dataplane

import (
	"encoding/binary"
	"net"
	"syscall"
)

// Per-packet TTL and TOS/traffic-class arrive as control messages, and only if asked for at
// socket setup. These fill the spec §3.3 observation-block fields that were shipped as the 0xFF
// sentinel until now — received TTL is path-length evidence, the TOS byte is DSCP/ECN survival.

// enableRecvMeta asks the kernel to attach the cmsgs to every received datagram. Both the v4 and
// the v6 option sets are attempted on every socket: a dual-stack socket delivers v4-mapped
// traffic through the v6 fd, and the kernel refuses whichever set does not apply. Errors are
// dropped on purpose — a socket that cannot deliver metadata still serves probes, and the
// sentinel already says "not observed" for it.
func enableRecvMeta(conn *net.UDPConn) {
	raw, err := conn.SyscallConn()
	if err != nil {
		return
	}
	_ = raw.Control(func(fd uintptr) {
		_ = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_RECVTTL, 1)
		_ = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_RECVTOS, 1)
		_ = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IPV6, syscall.IPV6_RECVHOPLIMIT, 1)
		_ = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IPV6, syscall.IPV6_RECVTCLASS, 1)
	})
}

// parseMeta extracts TTL and the TOS byte from one datagram's control messages. Anything absent
// or unparseable keeps the sentinel — reported as unobserved, never guessed.
func parseMeta(oob []byte) pktMeta {
	m := pktMeta{TTL: metaUnavailable, TOS: metaUnavailable}
	if len(oob) == 0 {
		return m
	}
	cmsgs, err := syscall.ParseSocketControlMessage(oob)
	if err != nil {
		return m
	}
	for _, c := range cmsgs {
		switch {
		case c.Header.Level == syscall.IPPROTO_IP && c.Header.Type == syscall.IP_TTL,
			c.Header.Level == syscall.IPPROTO_IPV6 && c.Header.Type == syscall.IPV6_HOPLIMIT:
			m.TTL = cmsgValue(c.Data)
		case c.Header.Level == syscall.IPPROTO_IP && c.Header.Type == syscall.IP_TOS,
			c.Header.Level == syscall.IPPROTO_IPV6 && c.Header.Type == syscall.IPV6_TCLASS:
			m.TOS = cmsgValue(c.Data)
		}
	}
	return m
}

// cmsgValue reads a cmsg the kernel encodes either as a native-endian int (IP_TTL,
// IPV6_HOPLIMIT, IPV6_TCLASS) or as a single byte (IP_TOS). Both fit a byte by definition.
func cmsgValue(data []byte) uint8 {
	switch {
	case len(data) >= 4:
		return uint8(binary.NativeEndian.Uint32(data))
	case len(data) >= 1:
		return data[0]
	}
	return metaUnavailable
}