package main

import (
	"errors"
	"log"
	"os/exec"
	"syscall"
	"unsafe"
)

// Checkout google/kafel in this dir and build libkafel.a.

// #cgo CFLAGS: -I${SRCDIR}/kafel/include
// #cgo LDFLAGS: -L${SRCDIR}/kafel -lkafel
// #include <linux/filter.h>
// #include <linux/seccomp.h>
// #include <sys/prctl.h>
// #include <stdlib.h>
// #include <kafel.h>
import "C"

const killPolicy = `POLICY a { KILL { execve, clone, fork } } USE a DEFAULT ALLOW`
const errorPolicy = `POLICY a { ERRNO(1) { execve, clone, fork } } USE a DEFAULT ALLOW`

func main() {
	prog, err := BuildSeccompBPFFilter(errorPolicy)
	if err != nil {
		log.Fatal(err)
	}

	if err := NoNewPrivs(); err != nil {
		log.Fatal(err)
	}

	if err := InstallSeccomp(prog); err != nil {
		log.Fatal(err)
	}
	log.Println("seccomp filter installed")

	log.Println("attempting to exec")
	out, err := exec.Command("ls", "-la").Output()
	if err != nil {
		log.Fatal("exec failed", err)
	}
	log.Println("cmd output:", string(out))

	log.Println("Done")
}

// Build filter.
func BuildSeccompBPFFilter(policy string) (*C.struct_sock_fprog, error) {
	policyPtr := C.CString(policy)
	defer C.free(unsafe.Pointer(policyPtr))
	prog := C.struct_sock_fprog{}

	if rc := C.kafel_compile_string(policyPtr, &prog); rc != 0 {
		return nil, errors.New("policy compilation failed")
	}

	return &prog, nil
}

// NoNewPrivs will use prctl to stop new privileges using native methods
func NoNewPrivs() error {
	return prctl(C.PR_SET_NO_NEW_PRIVS, 1)
}

// InstallSeccomp will install seccomp using native methods.
func InstallSeccomp(prog *C.struct_sock_fprog) error {
	return seccomp(C.SECCOMP_SET_MODE_FILTER, C.SECCOMP_FILTER_FLAG_TSYNC, unsafe.Pointer(prog))
}

// seccomp syscall wrapper.
func seccomp(op, flags uintptr, uargs unsafe.Pointer) error {
	// This is an amd64 specific syscall number.
	_, _, e := syscall.Syscall(uintptr(317), op, flags, uintptr(uargs))
	if e != 0 {
		return e
	}
	return nil
}

// prctl syscall wrapper.
func prctl(option uintptr, args ...uintptr) error {
	if len(args) > 4 {
		return syscall.E2BIG
	}
	var arg [4]uintptr
	copy(arg[:], args)
	_, _, e := syscall.Syscall6(syscall.SYS_PRCTL, option, arg[0], arg[1], arg[2], arg[3], 0)
	if e != 0 {
		return e
	}
	return nil
}

// Sample output:
// 2017/09/20 20:06:13 seccomp filter installed
// 2017/09/20 20:06:13 attempting to exec
// 2017/09/20 20:06:13 exec failedfork/exec /bin/ls: operation not permitted