//go:build linux

package instancectl

import (
	"fmt"
	"net"
	"os"
	"strconv"
	"strings"
)

type AuthenticatedPeer struct {
	UID                                uint32
	PID                                int
	ProcessStart, BootID, ControllerID string
}

func procStart(pid int) (string, error) {
	b, err := os.ReadFile(fmt.Sprintf("/proc/%d/stat", pid))
	if err != nil {
		return "", err
	}
	i := strings.LastIndexByte(string(b), ')')
	if i < 0 {
		return "", fmt.Errorf("malformed proc stat")
	}
	f := strings.Fields(string(b[i+1:]))
	if len(f) <= 19 {
		return "", fmt.Errorf("malformed proc stat")
	}
	if _, err := strconv.ParseUint(f[19], 10, 64); err != nil {
		return "", err
	}
	return f[19], nil
}

// AuthenticatePeerTuple binds the accepted Unix peer to kernel identity and
// rereads starttime to reject PID reuse during collection.
func AuthenticatePeerTuple(conn net.Conn, expectedUID uint32, expectedPID int, controllerID string) (AuthenticatedPeer, error) {
	if conn == nil || expectedPID <= 0 || controllerID == "" {
		return AuthenticatedPeer{}, fmt.Errorf("invalid peer expectation")
	}
	u, err := peerCred(conn)
	if err != nil {
		return AuthenticatedPeer{}, err
	}
	if u.Uid != expectedUID || int(u.Pid) != expectedPID {
		return AuthenticatedPeer{}, fmt.Errorf("peer identity mismatch")
	}
	start, err := procStart(expectedPID)
	if err != nil {
		return AuthenticatedPeer{}, err
	}
	boot, err := os.ReadFile("/proc/sys/kernel/random/boot_id")
	if err != nil {
		return AuthenticatedPeer{}, err
	}
	start2, err := procStart(expectedPID)
	if err != nil || start2 != start {
		return AuthenticatedPeer{}, fmt.Errorf("peer process changed during authentication")
	}
	return AuthenticatedPeer{UID: u.Uid, PID: int(u.Pid), ProcessStart: start, BootID: strings.TrimSpace(string(boot)), ControllerID: controllerID}, nil
}