tor/src/common/sandbox.c

/* Copyright (c) 2001 Matej Pfajfar.
 * Copyright (c) 2001-2004, Roger Dingledine.
 * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
 * Copyright (c) 2007-2013, The Tor Project, Inc. */
/* See LICENSE for licensing information */

/**
 * \file sandbox.c
 * \brief Code to enable sandboxing.
 **/

#include "orconfig.h"

#ifndef _LARGEFILE64_SOURCE
/**
 * Temporarily required for O_LARGEFILE flag. Needs to be removed
 * with the libevent fix.
 */
#define _LARGEFILE64_SOURCE
#endif

/** Malloc mprotect limit in bytes. */
#define MALLOC_MP_LIM 1048576

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#include "sandbox.h"
#include "container.h"
#include "torlog.h"
#include "torint.h"
#include "util.h"
#include "tor_queue.h"

#include "ht.h"

#define DEBUGGING_CLOSE

#if defined(USE_LIBSECCOMP)

#define _GNU_SOURCE

#include <sys/mman.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/epoll.h>
#include <sys/prctl.h>
#include <linux/futex.h>
#include <bits/signum.h>

#include <stdarg.h>
#include <seccomp.h>
#include <signal.h>
#include <unistd.h>
#include <fcntl.h>
#include <time.h>
#include <poll.h>

#if defined(HAVE_EXECINFO_H) && defined(HAVE_BACKTRACE) && \
  defined(HAVE_BACKTRACE_SYMBOLS_FD) && defined(HAVE_SIGACTION)
#define USE_BACKTRACE
#define EXPOSE_CLEAN_BACKTRACE
#include "backtrace.h"
#endif

#ifdef USE_BACKTRACE
#include <execinfo.h>
#endif

/**
 * Linux 32 bit definitions
 */
#if defined(__i386__)

#define REG_SYSCALL REG_EAX
#define M_SYSCALL gregs[REG_SYSCALL]

/**
 * Linux 64 bit definitions
 */
#elif defined(__x86_64__)

#define REG_SYSCALL REG_RAX
#define M_SYSCALL gregs[REG_SYSCALL]

#elif defined(__arm__)

#define M_SYSCALL arm_r7

#endif

/**Determines if at least one sandbox is active.*/
static int sandbox_active = 0;
/** Holds the parameter list configuration for the sandbox.*/
static sandbox_cfg_t *filter_dynamic = NULL;

#undef SCMP_CMP
#define SCMP_CMP(a,b,c) ((struct scmp_arg_cmp){(a),(b),(c),0})
#define SCMP_CMP_STR(a,b,c) \
  ((struct scmp_arg_cmp) {(a),(b),(intptr_t)(void*)(c),0})
#define SCMP_CMP4(a,b,c,d) ((struct scmp_arg_cmp){(a),(b),(c),(d)})
/* We use a wrapper here because these masked comparisons seem to be pretty
 * verbose. Also, it's important to cast to scmp_datum_t before negating the
 * mask, since otherwise the negation might get applied to a 32 bit value, and
 * the high bits of the value might get masked out improperly. */
#define SCMP_CMP_MASKED(a,b,c) \
  SCMP_CMP4((a), SCMP_CMP_MASKED_EQ, ~(scmp_datum_t)(b), (c))

/** Variable used for storing all syscall numbers that will be allowed with the
 * stage 1 general Tor sandbox.
 */
static int filter_nopar_gen[] = {
    SCMP_SYS(access),
    SCMP_SYS(brk),
    SCMP_SYS(clock_gettime),
    SCMP_SYS(close),
    SCMP_SYS(clone),
    SCMP_SYS(epoll_create),
    SCMP_SYS(epoll_wait),
    SCMP_SYS(fcntl),
    SCMP_SYS(fstat),
#ifdef __NR_fstat64
    SCMP_SYS(fstat64),
#endif
    SCMP_SYS(getdents64),
    SCMP_SYS(getegid),
#ifdef __NR_getegid32
    SCMP_SYS(getegid32),
#endif
    SCMP_SYS(geteuid),
#ifdef __NR_geteuid32
    SCMP_SYS(geteuid32),
#endif
    SCMP_SYS(getgid),
#ifdef __NR_getgid32
    SCMP_SYS(getgid32),
#endif
#ifdef __NR_getrlimit
    SCMP_SYS(getrlimit),
#endif
    SCMP_SYS(gettimeofday),
    SCMP_SYS(gettid),
    SCMP_SYS(getuid),
#ifdef __NR_getuid32
    SCMP_SYS(getuid32),
#endif
    SCMP_SYS(lseek),
#ifdef __NR__llseek
    SCMP_SYS(_llseek),
#endif
    SCMP_SYS(mkdir),
    SCMP_SYS(mlockall),
#ifdef __NR_mmap
    /* XXXX restrict this in the same ways as mmap2 */
    SCMP_SYS(mmap),
#endif
    SCMP_SYS(munmap),
    SCMP_SYS(read),
    SCMP_SYS(rt_sigreturn),
    SCMP_SYS(sched_getaffinity),
    SCMP_SYS(set_robust_list),
#ifdef __NR_sigreturn
    SCMP_SYS(sigreturn),
#endif
    SCMP_SYS(stat),
    SCMP_SYS(uname),
    SCMP_SYS(write),
    SCMP_SYS(writev),
    SCMP_SYS(exit_group),
    SCMP_SYS(exit),

    SCMP_SYS(madvise),
#ifdef __NR_stat64
    // getaddrinfo uses this..
    SCMP_SYS(stat64),
#endif

    /*
     * These socket syscalls are not required on x86_64 and not supported with
     * some libseccomp versions (eg: 1.0.1)
     */
#if defined(__i386)
    SCMP_SYS(recv),
    SCMP_SYS(send),
#endif

    // socket syscalls
    SCMP_SYS(bind),
    SCMP_SYS(listen),
    SCMP_SYS(connect),
    SCMP_SYS(getsockname),
    SCMP_SYS(recvmsg),
    SCMP_SYS(recvfrom),
    SCMP_SYS(sendto),
    SCMP_SYS(unlink)
};

/* These macros help avoid the error where the number of filters we add on a
 * single rule don't match the arg_cnt param. */
#define seccomp_rule_add_0(ctx,act,call) \
  seccomp_rule_add((ctx),(act),(call),0)
#define seccomp_rule_add_1(ctx,act,call,f1) \
  seccomp_rule_add((ctx),(act),(call),1,(f1))
#define seccomp_rule_add_2(ctx,act,call,f1,f2)  \
  seccomp_rule_add((ctx),(act),(call),2,(f1),(f2))
#define seccomp_rule_add_3(ctx,act,call,f1,f2,f3)       \
  seccomp_rule_add((ctx),(act),(call),3,(f1),(f2),(f3))
#define seccomp_rule_add_4(ctx,act,call,f1,f2,f3,f4)      \
  seccomp_rule_add((ctx),(act),(call),4,(f1),(f2),(f3),(f4))

/**
 * Function responsible for setting up the rt_sigaction syscall for
 * the seccomp filter sandbox.
 */
static int
sb_rt_sigaction(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  unsigned i;
  int rc;
  int param[] = { SIGINT, SIGTERM, SIGPIPE, SIGUSR1, SIGUSR2, SIGHUP, SIGCHLD,
#ifdef SIGXFSZ
      SIGXFSZ
#endif
      };
  (void) filter;

  for (i = 0; i < ARRAY_LENGTH(param); i++) {
    rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(rt_sigaction),
        SCMP_CMP(0, SCMP_CMP_EQ, param[i]));
    if (rc)
      break;
  }

  return rc;
}

#if 0
/**
 * Function responsible for setting up the execve syscall for
 * the seccomp filter sandbox.
 */
static int
sb_execve(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc;
  sandbox_cfg_t *elem = NULL;

  // for each dynamic parameter filters
  for (elem = filter; elem != NULL; elem = elem->next) {
    smp_param_t *param = elem->param;

    if (param != NULL && param->prot == 1 && param->syscall
        == SCMP_SYS(execve)) {
      rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(execve),
               SCMP_CMP_STR(0, SCMP_CMP_EQ, param->value));
      if (rc != 0) {
        log_err(LD_BUG,"(Sandbox) failed to add execve syscall, received "
            "libseccomp error %d", rc);
        return rc;
      }
    }
  }

  return 0;
}
#endif

/**
 * Function responsible for setting up the time syscall for
 * the seccomp filter sandbox.
 */
static int
sb_time(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  (void) filter;
#ifdef __NR_time
  return seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(time),
       SCMP_CMP(0, SCMP_CMP_EQ, 0));
#else
  return 0;
#endif
}

/**
 * Function responsible for setting up the accept4 syscall for
 * the seccomp filter sandbox.
 */
static int
sb_accept4(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void)filter;

#ifdef __i386__
  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socketcall),
      SCMP_CMP(0, SCMP_CMP_EQ, 18));
  if (rc) {
    return rc;
  }
#endif

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(accept4),
                   SCMP_CMP_MASKED(3, SOCK_CLOEXEC|SOCK_NONBLOCK, 0));
  if (rc) {
    return rc;
  }

  return 0;
}

#ifdef __NR_mmap2
/**
 * Function responsible for setting up the mmap2 syscall for
 * the seccomp filter sandbox.
 */
static int
sb_mmap2(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void)filter;

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mmap2),
       SCMP_CMP(2, SCMP_CMP_EQ, PROT_READ),
       SCMP_CMP(3, SCMP_CMP_EQ, MAP_PRIVATE));
  if (rc) {
    return rc;
  }

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mmap2),
       SCMP_CMP(2, SCMP_CMP_EQ, PROT_NONE),
       SCMP_CMP(3, SCMP_CMP_EQ, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE));
  if (rc) {
    return rc;
  }

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mmap2),
       SCMP_CMP(2, SCMP_CMP_EQ, PROT_READ|PROT_WRITE),
       SCMP_CMP(3, SCMP_CMP_EQ, MAP_PRIVATE|MAP_ANONYMOUS));
  if (rc) {
    return rc;
  }

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mmap2),
       SCMP_CMP(2, SCMP_CMP_EQ, PROT_READ|PROT_WRITE),
       SCMP_CMP(3, SCMP_CMP_EQ,MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK));
  if (rc) {
    return rc;
  }

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mmap2),
      SCMP_CMP(2, SCMP_CMP_EQ, PROT_READ|PROT_WRITE),
      SCMP_CMP(3, SCMP_CMP_EQ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE));
  if (rc) {
    return rc;
  }

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mmap2),
      SCMP_CMP(2, SCMP_CMP_EQ, PROT_READ|PROT_WRITE),
      SCMP_CMP(3, SCMP_CMP_EQ, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS));
  if (rc) {
    return rc;
  }

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mmap2),
      SCMP_CMP(2, SCMP_CMP_EQ, PROT_READ|PROT_EXEC),
      SCMP_CMP(3, SCMP_CMP_EQ, MAP_PRIVATE|MAP_DENYWRITE));
  if (rc) {
    return rc;
  }

  return 0;
}
#endif

/**
 * Function responsible for setting up the open syscall for
 * the seccomp filter sandbox.
 */
static int
sb_open(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc;
  sandbox_cfg_t *elem = NULL;

  // for each dynamic parameter filters
  for (elem = filter; elem != NULL; elem = elem->next) {
    smp_param_t *param = elem->param;

    if (param != NULL && param->prot == 1 && param->syscall
        == SCMP_SYS(open)) {
      rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(open),
            SCMP_CMP_STR(0, SCMP_CMP_EQ, param->value));
      if (rc != 0) {
        log_err(LD_BUG,"(Sandbox) failed to add open syscall, received "
            "libseccomp error %d", rc);
        return rc;
      }
    }
  }

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ERRNO(EACCES), SCMP_SYS(open),
                SCMP_CMP_MASKED(1, O_CLOEXEC|O_NONBLOCK|O_NOCTTY, O_RDONLY));
  if (rc != 0) {
    log_err(LD_BUG,"(Sandbox) failed to add open syscall, received libseccomp "
        "error %d", rc);
    return rc;
  }

  return 0;
}

static int
sb__sysctl(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc;
  (void) filter;
  (void) ctx;

  rc = seccomp_rule_add_0(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(_sysctl));
  if (rc != 0) {
    log_err(LD_BUG,"(Sandbox) failed to add _sysctl syscall, "
        "received libseccomp error %d", rc);
    return rc;
  }

  return 0;
}

/**
 * Function responsible for setting up the rename syscall for
 * the seccomp filter sandbox.
 */
static int
sb_rename(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc;
  sandbox_cfg_t *elem = NULL;

  // for each dynamic parameter filters
  for (elem = filter; elem != NULL; elem = elem->next) {
    smp_param_t *param = elem->param;

    if (param != NULL && param->prot == 1 &&
        param->syscall == SCMP_SYS(rename)) {

      rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(rename),
            SCMP_CMP_STR(0, SCMP_CMP_EQ, param->value),
            SCMP_CMP_STR(1, SCMP_CMP_EQ, param->value2));
      if (rc != 0) {
        log_err(LD_BUG,"(Sandbox) failed to add rename syscall, received "
            "libseccomp error %d", rc);
        return rc;
      }
    }
  }

  return 0;
}

/**
 * Function responsible for setting up the openat syscall for
 * the seccomp filter sandbox.
 */
static int
sb_openat(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc;
  sandbox_cfg_t *elem = NULL;

  // for each dynamic parameter filters
  for (elem = filter; elem != NULL; elem = elem->next) {
    smp_param_t *param = elem->param;

    if (param != NULL && param->prot == 1 && param->syscall
        == SCMP_SYS(openat)) {
      rc = seccomp_rule_add_3(ctx, SCMP_ACT_ALLOW, SCMP_SYS(openat),
          SCMP_CMP(0, SCMP_CMP_EQ, AT_FDCWD),
          SCMP_CMP_STR(1, SCMP_CMP_EQ, param->value),
          SCMP_CMP(2, SCMP_CMP_EQ, O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_DIRECTORY|
              O_CLOEXEC));
      if (rc != 0) {
        log_err(LD_BUG,"(Sandbox) failed to add openat syscall, received "
            "libseccomp error %d", rc);
        return rc;
      }
    }
  }

  return 0;
}

/**
 * Function responsible for setting up the socket syscall for
 * the seccomp filter sandbox.
 */
static int
sb_socket(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  int i;
  (void) filter;

#ifdef __i386__
  rc = seccomp_rule_add_0(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socket));
  if (rc)
    return rc;
#endif

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socket),
      SCMP_CMP(0, SCMP_CMP_EQ, PF_FILE),
      SCMP_CMP_MASKED(1, SOCK_CLOEXEC|SOCK_NONBLOCK, SOCK_STREAM));
  if (rc)
    return rc;

  for (i = 0; i < 2; ++i) {
    const int pf = i ? PF_INET : PF_INET6;

    rc = seccomp_rule_add_3(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socket),
      SCMP_CMP(0, SCMP_CMP_EQ, pf),
      SCMP_CMP_MASKED(1, SOCK_CLOEXEC|SOCK_NONBLOCK, SOCK_STREAM),
      SCMP_CMP(2, SCMP_CMP_EQ, IPPROTO_TCP));
    if (rc)
      return rc;

    rc = seccomp_rule_add_3(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socket),
      SCMP_CMP(0, SCMP_CMP_EQ, pf),
      SCMP_CMP_MASKED(1, SOCK_CLOEXEC|SOCK_NONBLOCK, SOCK_DGRAM),
      SCMP_CMP(2, SCMP_CMP_EQ, IPPROTO_IP));
    if (rc)
      return rc;
  }

  rc = seccomp_rule_add_3(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socket),
      SCMP_CMP(0, SCMP_CMP_EQ, PF_NETLINK),
      SCMP_CMP(1, SCMP_CMP_EQ, SOCK_RAW),
      SCMP_CMP(2, SCMP_CMP_EQ, 0));
  if (rc)
    return rc;

  return 0;
}

/**
 * Function responsible for setting up the socketpair syscall for
 * the seccomp filter sandbox.
 */
static int
sb_socketpair(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

#ifdef __i386__
  rc = seccomp_rule_add_0(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socketpair));
  if (rc)
    return rc;
#endif

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(socketpair),
      SCMP_CMP(0, SCMP_CMP_EQ, PF_FILE),
      SCMP_CMP(1, SCMP_CMP_EQ, SOCK_STREAM|SOCK_CLOEXEC));
  if (rc)
    return rc;

  return 0;
}

/**
 * Function responsible for setting up the setsockopt syscall for
 * the seccomp filter sandbox.
 */
static int
sb_setsockopt(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

#ifdef __i386__
  rc = seccomp_rule_add_0(ctx, SCMP_ACT_ALLOW, SCMP_SYS(setsockopt));
  if (rc)
    return rc;
#endif

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(setsockopt),
      SCMP_CMP(1, SCMP_CMP_EQ, SOL_SOCKET),
      SCMP_CMP(2, SCMP_CMP_EQ, SO_REUSEADDR));
  if (rc)
    return rc;

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(setsockopt),
      SCMP_CMP(1, SCMP_CMP_EQ, SOL_SOCKET),
      SCMP_CMP(2, SCMP_CMP_EQ, SO_SNDBUF));
  if (rc)
    return rc;

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(setsockopt),
      SCMP_CMP(1, SCMP_CMP_EQ, SOL_SOCKET),
      SCMP_CMP(2, SCMP_CMP_EQ, SO_RCVBUF));
  if (rc)
    return rc;

#ifdef IP_TRANSPARENT
  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(setsockopt),
      SCMP_CMP(1, SCMP_CMP_EQ, SOL_IP),
      SCMP_CMP(2, SCMP_CMP_EQ, IP_TRANSPARENT));
  if (rc)
    return rc;
#endif

  return 0;
}

/**
 * Function responsible for setting up the getsockopt syscall for
 * the seccomp filter sandbox.
 */
static int
sb_getsockopt(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

#ifdef __i386__
  rc = seccomp_rule_add_0(ctx, SCMP_ACT_ALLOW, SCMP_SYS(getsockopt));
  if (rc)
    return rc;
#endif

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(getsockopt),
      SCMP_CMP(1, SCMP_CMP_EQ, SOL_SOCKET),
      SCMP_CMP(2, SCMP_CMP_EQ, SO_ERROR));
  if (rc)
    return rc;

  return 0;
}

#ifdef __NR_fcntl64
/**
 * Function responsible for setting up the fcntl64 syscall for
 * the seccomp filter sandbox.
 */
static int
sb_fcntl64(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fcntl64),
      SCMP_CMP(1, SCMP_CMP_EQ, F_GETFL));
  if (rc)
    return rc;

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fcntl64),
      SCMP_CMP(1, SCMP_CMP_EQ, F_SETFL),
      SCMP_CMP(2, SCMP_CMP_EQ, O_RDWR|O_NONBLOCK));
  if (rc)
    return rc;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fcntl64),
      SCMP_CMP(1, SCMP_CMP_EQ, F_GETFD));
  if (rc)
    return rc;

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fcntl64),
      SCMP_CMP(1, SCMP_CMP_EQ, F_SETFD),
      SCMP_CMP(2, SCMP_CMP_EQ, FD_CLOEXEC));
  if (rc)
    return rc;

  return 0;
}
#endif

/**
 * Function responsible for setting up the epoll_ctl syscall for
 * the seccomp filter sandbox.
 *
 *  Note: basically allows everything but will keep for now..
 */
static int
sb_epoll_ctl(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(epoll_ctl),
      SCMP_CMP(1, SCMP_CMP_EQ, EPOLL_CTL_ADD));
  if (rc)
    return rc;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(epoll_ctl),
      SCMP_CMP(1, SCMP_CMP_EQ, EPOLL_CTL_MOD));
  if (rc)
    return rc;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(epoll_ctl),
      SCMP_CMP(1, SCMP_CMP_EQ, EPOLL_CTL_DEL));
  if (rc)
    return rc;

  return 0;
}

/**
 * Function responsible for setting up the fcntl64 syscall for
 * the seccomp filter sandbox.
 *
 * NOTE: if multiple filters need to be added, the PR_SECCOMP parameter needs
 * to be whitelisted in this function.
 */
static int
sb_prctl(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(prctl),
      SCMP_CMP(0, SCMP_CMP_EQ, PR_SET_DUMPABLE));
  if (rc)
    return rc;

  return 0;
}

/**
 * Function responsible for setting up the fcntl64 syscall for
 * the seccomp filter sandbox.
 *
 * NOTE: does not NEED to be here.. currently only occurs before filter; will
 * keep just in case for the future.
 */
static int
sb_mprotect(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mprotect),
      SCMP_CMP(2, SCMP_CMP_EQ, PROT_READ));
  if (rc)
    return rc;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mprotect),
      SCMP_CMP(2, SCMP_CMP_EQ, PROT_NONE));
  if (rc)
    return rc;

  return 0;
}

/**
 * Function responsible for setting up the rt_sigprocmask syscall for
 * the seccomp filter sandbox.
 */
static int
sb_rt_sigprocmask(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(rt_sigprocmask),
      SCMP_CMP(0, SCMP_CMP_EQ, SIG_UNBLOCK));
  if (rc)
    return rc;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(rt_sigprocmask),
      SCMP_CMP(0, SCMP_CMP_EQ, SIG_SETMASK));
  if (rc)
    return rc;

  return 0;
}

/**
 * Function responsible for setting up the flock syscall for
 * the seccomp filter sandbox.
 *
 *  NOTE: does not need to be here, occurs before filter is applied.
 */
static int
sb_flock(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(flock),
      SCMP_CMP(1, SCMP_CMP_EQ, LOCK_EX|LOCK_NB));
  if (rc)
    return rc;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(flock),
      SCMP_CMP(1, SCMP_CMP_EQ, LOCK_UN));
  if (rc)
    return rc;

  return 0;
}

/**
 * Function responsible for setting up the futex syscall for
 * the seccomp filter sandbox.
 */
static int
sb_futex(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

  // can remove
  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(futex),
      SCMP_CMP(1, SCMP_CMP_EQ,
          FUTEX_WAIT_BITSET_PRIVATE|FUTEX_CLOCK_REALTIME));
  if (rc)
    return rc;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(futex),
      SCMP_CMP(1, SCMP_CMP_EQ, FUTEX_WAKE_PRIVATE));
  if (rc)
    return rc;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(futex),
      SCMP_CMP(1, SCMP_CMP_EQ, FUTEX_WAIT_PRIVATE));
  if (rc)
    return rc;

  return 0;
}

/**
 * Function responsible for setting up the mremap syscall for
 * the seccomp filter sandbox.
 *
 *  NOTE: so far only occurs before filter is applied.
 */
static int
sb_mremap(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

  rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mremap),
      SCMP_CMP(3, SCMP_CMP_EQ, MREMAP_MAYMOVE));
  if (rc)
    return rc;

  return 0;
}

/**
 * Function responsible for setting up the poll syscall for
 * the seccomp filter sandbox.
 */
static int
sb_poll(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  (void) filter;

  rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(poll),
      SCMP_CMP(1, SCMP_CMP_EQ, 1),
      SCMP_CMP(2, SCMP_CMP_EQ, 10));
  if (rc)
    return rc;

  return 0;
}

#ifdef __NR_stat64
/**
 * Function responsible for setting up the stat64 syscall for
 * the seccomp filter sandbox.
 */
static int
sb_stat64(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
{
  int rc = 0;
  sandbox_cfg_t *elem = NULL;

  // for each dynamic parameter filters
  for (elem = filter; elem != NULL; elem = elem->next) {
    smp_param_t *param = elem->param;

    if (param != NULL && param->prot == 1 && (param->syscall == SCMP_SYS(open)
        || param->syscall == SCMP_SYS(stat64))) {
      rc = seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(stat64),
          SCMP_CMP_STR(0, SCMP_CMP_EQ, param->value));
      if (rc != 0) {
        log_err(LD_BUG,"(Sandbox) failed to add open syscall, received "
            "libseccomp  error %d", rc);
        return rc;
      }
    }
  }

  return 0;
}
#endif

/**
 * Array of function pointers responsible for filtering different syscalls at
 * a parameter level.
 */
static sandbox_filter_func_t filter_func[] = {
    sb_rt_sigaction,
    sb_rt_sigprocmask,
#if 0
    sb_execve,
#endif
    sb_time,
    sb_accept4,
#ifdef __NR_mmap2
    sb_mmap2,
#endif
    sb_open,
    sb_openat,
    sb__sysctl,
    sb_rename,
#ifdef __NR_fcntl64
    sb_fcntl64,
#endif
    sb_epoll_ctl,
    sb_prctl,
    sb_mprotect,
    sb_flock,
    sb_futex,
    sb_mremap,
    sb_poll,
#ifdef __NR_stat64
    sb_stat64,
#endif

    sb_socket,
    sb_setsockopt,
    sb_getsockopt,
    sb_socketpair
};

const char *
sandbox_intern_string(const char *str)
{
  sandbox_cfg_t *elem;

  if (str == NULL)
    return NULL;

  for (elem = filter_dynamic; elem != NULL; elem = elem->next) {
    smp_param_t *param = elem->param;

    if (param->prot) {
      if (!strcmp(str, (char*)(param->value))) {
        return (char*)param->value;
      }
      if (param->value2 && !strcmp(str, (char*)param->value2)) {
        return (char*)param->value2;
      }
    }
  }

  if (sandbox_active)
    log_warn(LD_BUG, "No interned sandbox parameter found for %s", str);
  return str;
}

/** DOCDOC */
static int
prot_strings_helper(strmap_t *locations,
                    char **pr_mem_next_p,
                    size_t *pr_mem_left_p,
                    char **value_p)
{
  char *param_val;
  size_t param_size;
  void *location;

  if (*value_p == 0)
    return 0;

  param_val = (char*) *value_p;
  param_size = strlen(param_val) + 1;
  location = strmap_get(locations, param_val);

  if (location) {
    // We already interned this string.
    tor_free(param_val);
    *value_p = location;
    return 0;
  } else if (*pr_mem_left_p >= param_size) {
    // copy to protected
    location = *pr_mem_next_p;
    memcpy(location, param_val, param_size);

    // re-point el parameter to protected
    tor_free(param_val);
    *value_p = location;

    strmap_set(locations, location, location); /* good real estate advice */

    // move next available protected memory
    *pr_mem_next_p += param_size;
    *pr_mem_left_p -= param_size;
    return 0;
  } else {
    log_err(LD_BUG,"(Sandbox) insufficient protected memory!");
    return -1;
  }
}

/**
 * Protects all the strings in the sandbox's parameter list configuration. It
 * works by calculating the total amount of memory required by the parameter
 * list, allocating the memory using mmap, and protecting it from writes with
 * mprotect().
 */
static int
prot_strings(scmp_filter_ctx ctx, sandbox_cfg_t* cfg)
{
  int ret = 0;
  size_t pr_mem_size = 0, pr_mem_left = 0;
  char *pr_mem_next = NULL, *pr_mem_base;
  sandbox_cfg_t *el = NULL;
  strmap_t *locations = NULL;

  // get total number of bytes required to mmap. (Overestimate.)
  for (el = cfg; el != NULL; el = el->next) {
    pr_mem_size += strlen((char*) el->param->value) + 1;
    if (el->param->value2)
      pr_mem_size += strlen((char*) el->param->value2) + 1;
  }

  // allocate protected memory with MALLOC_MP_LIM canary
  pr_mem_base = (char*) mmap(NULL, MALLOC_MP_LIM + pr_mem_size,
      PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1, 0);
  if (pr_mem_base == MAP_FAILED) {
    log_err(LD_BUG,"(Sandbox) failed allocate protected memory! mmap: %s",
        strerror(errno));
    ret = -1;
    goto out;
  }

  pr_mem_next = pr_mem_base + MALLOC_MP_LIM;
  pr_mem_left = pr_mem_size;

  locations = strmap_new();

  // change el value pointer to protected
  for (el = cfg; el != NULL; el = el->next) {
    if (prot_strings_helper(locations, &pr_mem_next, &pr_mem_left,
                            &el->param->value) < 0) {
      ret = -2;
      goto out;
    }
    if (prot_strings_helper(locations, &pr_mem_next, &pr_mem_left,
                            &el->param->value2) < 0) {
      ret = -2;
      goto out;
    }
    el->param->prot = 1;
  }

  // protecting from writes
  if (mprotect(pr_mem_base, MALLOC_MP_LIM + pr_mem_size, PROT_READ)) {
    log_err(LD_BUG,"(Sandbox) failed to protect memory! mprotect: %s",
        strerror(errno));
    ret = -3;
    goto out;
  }

  /*
   * Setting sandbox restrictions so the string memory cannot be tampered with
   */
  // no mremap of the protected base address
  ret = seccomp_rule_add_1(ctx, SCMP_ACT_KILL, SCMP_SYS(mremap),
      SCMP_CMP(0, SCMP_CMP_EQ, (intptr_t) pr_mem_base));
  if (ret) {
    log_err(LD_BUG,"(Sandbox) mremap protected memory filter fail!");
    return ret;
  }

  // no munmap of the protected base address
  ret = seccomp_rule_add_1(ctx, SCMP_ACT_KILL, SCMP_SYS(munmap),
        SCMP_CMP(0, SCMP_CMP_EQ, (intptr_t) pr_mem_base));
  if (ret) {
    log_err(LD_BUG,"(Sandbox) munmap protected memory filter fail!");
    goto out;
  }

  /*
   * Allow mprotect with PROT_READ|PROT_WRITE because openssl uses it, but
   * never over the memory region used by the protected strings.
   *
   * PROT_READ|PROT_WRITE was originally fully allowed in sb_mprotect(), but
   * had to be removed due to limitation of libseccomp regarding intervals.
   *
   * There is a restriction on how much you can mprotect with R|W up to the
   * size of the canary.
   */
  ret = seccomp_rule_add_3(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mprotect),
      SCMP_CMP(0, SCMP_CMP_LT, (intptr_t) pr_mem_base),
      SCMP_CMP(1, SCMP_CMP_LE, MALLOC_MP_LIM),
      SCMP_CMP(2, SCMP_CMP_EQ, PROT_READ|PROT_WRITE));
  if (ret) {
    log_err(LD_BUG,"(Sandbox) mprotect protected memory filter fail (LT)!");
    goto out;
  }

  ret = seccomp_rule_add_3(ctx, SCMP_ACT_ALLOW, SCMP_SYS(mprotect),
      SCMP_CMP(0, SCMP_CMP_GT, (intptr_t) pr_mem_base + pr_mem_size +
          MALLOC_MP_LIM),
      SCMP_CMP(1, SCMP_CMP_LE, MALLOC_MP_LIM),
      SCMP_CMP(2, SCMP_CMP_EQ, PROT_READ|PROT_WRITE));
  if (ret) {
    log_err(LD_BUG,"(Sandbox) mprotect protected memory filter fail (GT)!");
    goto out;
  }

 out:
  strmap_free(locations, NULL);
  return ret;
}

/**
 * Auxiliary function used in order to allocate a sandbox_cfg_t element and set
 * it's values according the the parameter list. All elements are initialised
 * with the 'prot' field set to false, as the pointer is not protected at this
 * point.
 */
static sandbox_cfg_t*
new_element2(int syscall, char *value, char *value2)
{
  smp_param_t *param = NULL;

  sandbox_cfg_t *elem = tor_malloc_zero(sizeof(sandbox_cfg_t));
  param = elem->param = tor_malloc_zero(sizeof(smp_param_t));

  param->syscall = syscall;
  param->value = value;
  param->value2 = value2;
  param->prot = 0;

  return elem;
}

static sandbox_cfg_t*
new_element(int syscall, char *value)
{
  return new_element2(syscall, value, NULL);
}

#ifdef __NR_stat64
#define SCMP_stat SCMP_SYS(stat64)
#else
#define SCMP_stat SCMP_SYS(stat)
#endif

int
sandbox_cfg_allow_stat_filename(sandbox_cfg_t **cfg, char *file)
{
  sandbox_cfg_t *elem = NULL;

  elem = new_element(SCMP_stat, file);
  if (!elem) {
    log_err(LD_BUG,"(Sandbox) failed to register parameter!");
    return -1;
  }

  elem->next = *cfg;
  *cfg = elem;

  return 0;
}

int
sandbox_cfg_allow_open_filename(sandbox_cfg_t **cfg, char *file)
{
  sandbox_cfg_t *elem = NULL;

  elem = new_element(SCMP_SYS(open), file);
  if (!elem) {
    log_err(LD_BUG,"(Sandbox) failed to register parameter!");
    return -1;
  }

  elem->next = *cfg;
  *cfg = elem;

  return 0;
}

int
sandbox_cfg_allow_rename(sandbox_cfg_t **cfg, char *file1, char *file2)
{
  sandbox_cfg_t *elem = NULL;

  elem = new_element2(SCMP_SYS(rename), file1, file2);

  if (!elem) {
    log_err(LD_BUG,"(Sandbox) failed to register parameter!");
    return -1;
  }

  elem->next = *cfg;
  *cfg = elem;

  return 0;
}

int
sandbox_cfg_allow_openat_filename(sandbox_cfg_t **cfg, char *file)
{
  sandbox_cfg_t *elem = NULL;

  elem = new_element(SCMP_SYS(openat), file);
  if (!elem) {
    log_err(LD_BUG,"(Sandbox) failed to register parameter!");
    return -1;
  }

  elem->next = *cfg;
  *cfg = elem;

  return 0;
}

#if 0
int
sandbox_cfg_allow_execve(sandbox_cfg_t **cfg, const char *com)
{
  sandbox_cfg_t *elem = NULL;

  elem = new_element(SCMP_SYS(execve), com);
  if (!elem) {
    log_err(LD_BUG,"(Sandbox) failed to register parameter!");
    return -1;
  }

  elem->next = *cfg;
  *cfg = elem;

  return 0;
}

#endif

/** Cache entry for getaddrinfo results; used when sandboxing is implemented
 * so that we can consult the cache when the sandbox prevents us from doing
 * getaddrinfo.
 *
 * We support only a limited range of getaddrinfo calls, where servname is null
 * and hints contains only socktype=SOCK_STREAM, family in INET,INET6,UNSPEC.
 */
typedef struct cached_getaddrinfo_item_t {
  HT_ENTRY(cached_getaddrinfo_item_t) node;
  char *name;
  int family;
  /** set if no error; otherwise NULL */
  struct addrinfo *res;
  /** 0 for no error; otherwise an EAI_* value */
  int err;
} cached_getaddrinfo_item_t;

static unsigned
cached_getaddrinfo_item_hash(const cached_getaddrinfo_item_t *item)
{
  return (unsigned)siphash24g(item->name, strlen(item->name)) + item->family;
}

static unsigned
cached_getaddrinfo_items_eq(const cached_getaddrinfo_item_t *a,
                            const cached_getaddrinfo_item_t *b)
{
  return (a->family == b->family) && 0 == strcmp(a->name, b->name);
}

static void
cached_getaddrinfo_item_free(cached_getaddrinfo_item_t *item)
{
  if (item == NULL)
    return;

  tor_free(item->name);
  if (item->res)
    freeaddrinfo(item->res);
  tor_free(item);
}

static HT_HEAD(getaddrinfo_cache, cached_getaddrinfo_item_t)
     getaddrinfo_cache = HT_INITIALIZER();

HT_PROTOTYPE(getaddrinfo_cache, cached_getaddrinfo_item_t, node,
             cached_getaddrinfo_item_hash,
             cached_getaddrinfo_items_eq);
HT_GENERATE(getaddrinfo_cache, cached_getaddrinfo_item_t, node,
            cached_getaddrinfo_item_hash,
            cached_getaddrinfo_items_eq,
            0.6, tor_malloc_, tor_realloc_, tor_free_);

int
sandbox_getaddrinfo(const char *name, const char *servname,
                    const struct addrinfo *hints,
                    struct addrinfo **res)
{
  int err;
  struct cached_getaddrinfo_item_t search, *item;

  if (servname != NULL) {
    log_warn(LD_BUG, "called with non-NULL servname");
    return EAI_NONAME;
  }
  if (name == NULL) {
    log_warn(LD_BUG, "called with NULL name");
    return EAI_NONAME;
  }

  *res = NULL;

  memset(&search, 0, sizeof(search));
  search.name = (char *) name;
  search.family = hints ? hints->ai_family : AF_UNSPEC;
  item = HT_FIND(getaddrinfo_cache, &getaddrinfo_cache, &search);

  if (! sandbox_is_active()) {
    /* If the sandbox is not turned on yet, then getaddrinfo and store the
       result. */

    err = getaddrinfo(name, NULL, hints, res);
    log_info(LD_NET,"(Sandbox) getaddrinfo %s.", err ? "failed" : "succeeded");

    if (! item) {
      item = tor_malloc_zero(sizeof(*item));
      item->name = tor_strdup(name);
      item->family = hints ? hints->ai_family : AF_UNSPEC;
      HT_INSERT(getaddrinfo_cache, &getaddrinfo_cache, item);
    }

    if (item->res) {
      freeaddrinfo(item->res);
      item->res = NULL;
    }
    item->res = *res;
    item->err = err;
    return err;
  }

  /* Otherwise, the sanbox is on.  If we have an item, yield its cached
     result. */
  if (item) {
    *res = item->res;
    return item->err;
  }

  /* getting here means something went wrong */
  log_err(LD_BUG,"(Sandbox) failed to get address %s!", name);
  return EAI_NONAME;
}

int
sandbox_add_addrinfo(const char *name)
{
  struct addrinfo *res;
  struct addrinfo hints;
  int i;
  static const int families[] = { AF_INET, AF_INET6, AF_UNSPEC };

  memset(&hints, 0, sizeof(hints));
  hints.ai_socktype = SOCK_STREAM;
  for (i = 0; i < 3; ++i) {
    hints.ai_family = families[i];

    res = NULL;
    (void) sandbox_getaddrinfo(name, NULL, &hints, &res);
    if (res)
      sandbox_freeaddrinfo(res);
  }

  return 0;
}

void
sandbox_free_getaddrinfo_cache(void)
{
  cached_getaddrinfo_item_t **next, **item;

  for (item = HT_START(getaddrinfo_cache, &getaddrinfo_cache);
       item;
       item = next) {
    next = HT_NEXT_RMV(getaddrinfo_cache, &getaddrinfo_cache, item);
    cached_getaddrinfo_item_free(*item);
  }

  HT_CLEAR(getaddrinfo_cache, &getaddrinfo_cache);
}

/**
 * Function responsible for going through the parameter syscall filters and
 * call each function pointer in the list.
 */
static int
add_param_filter(scmp_filter_ctx ctx, sandbox_cfg_t* cfg)
{
  unsigned i;
  int rc = 0;

  // function pointer
  for (i = 0; i < ARRAY_LENGTH(filter_func); i++) {
    if ((filter_func[i])(ctx, cfg)) {
      log_err(LD_BUG,"(Sandbox) failed to add syscall %d, received libseccomp "
          "error %d", i, rc);
      return rc;
    }
  }

  return 0;
}

/**
 * Function responsible of loading the libseccomp syscall filters which do not
 * have parameter filtering.
 */
static int
add_noparam_filter(scmp_filter_ctx ctx)
{
  unsigned i;
  int rc = 0;

  // add general filters
  for (i = 0; i < ARRAY_LENGTH(filter_nopar_gen); i++) {
    rc = seccomp_rule_add_0(ctx, SCMP_ACT_ALLOW, filter_nopar_gen[i]);
    if (rc != 0) {
      log_err(LD_BUG,"(Sandbox) failed to add syscall index %d (NR=%d), "
          "received libseccomp error %d", i, filter_nopar_gen[i], rc);
      return rc;
    }
  }

  return 0;
}

/**
 * Function responsible for setting up and enabling a global syscall filter.
 * The function is a prototype developed for stage 1 of sandboxing Tor.
 * Returns 0 on success.
 */
static int
install_syscall_filter(sandbox_cfg_t* cfg)
{
  int rc = 0;
  scmp_filter_ctx ctx;

  ctx = seccomp_init(SCMP_ACT_TRAP);
  if (ctx == NULL) {
    log_err(LD_BUG,"(Sandbox) failed to initialise libseccomp context");
    rc = -1;
    goto end;
  }

  // protectign sandbox parameter strings
  if ((rc = prot_strings(ctx, cfg))) {
    goto end;
  }

  // add parameter filters
  if ((rc = add_param_filter(ctx, cfg))) {
    log_err(LD_BUG, "(Sandbox) failed to add param filters!");
    goto end;
  }

  // adding filters with no parameters
  if ((rc = add_noparam_filter(ctx))) {
    log_err(LD_BUG, "(Sandbox) failed to add param filters!");
    goto end;
  }

  // loading the seccomp2 filter
  if ((rc = seccomp_load(ctx))) {
    log_err(LD_BUG, "(Sandbox) failed to load: %d (%s)!", rc,
            strerror(-rc));
    goto end;
  }

  // marking the sandbox as active
  sandbox_active = 1;

 end:
  seccomp_release(ctx);
  return (rc < 0 ? -rc : rc);
}

#include "linux_syscalls.inc"
static const char *
get_syscall_name(int syscall_num)
{
  int i;
  for (i = 0; SYSCALLS_BY_NUMBER[i].syscall_name; ++i) {
    if (SYSCALLS_BY_NUMBER[i].syscall_num == syscall_num)
      return SYSCALLS_BY_NUMBER[i].syscall_name;
  }

  {
     static char syscall_name_buf[64];
     format_dec_number_sigsafe(syscall_num,
                               syscall_name_buf, sizeof(syscall_name_buf));
     return syscall_name_buf;
  }
}

#ifdef USE_BACKTRACE
#define MAX_DEPTH 256
static void *syscall_cb_buf[MAX_DEPTH];
#endif

/**
 * Function called when a SIGSYS is caught by the application. It notifies the
 * user that an error has occurred and either terminates or allows the
 * application to continue execution, based on the DEBUGGING_CLOSE symbol.
 */
static void
sigsys_debugging(int nr, siginfo_t *info, void *void_context)
{
  ucontext_t *ctx = (ucontext_t *) (void_context);
  const char *syscall_name;
  int syscall;
#ifdef USE_BACKTRACE
  int depth;
  int n_fds, i;
  const int *fds = NULL;
#endif

  (void) nr;

  if (info->si_code != SYS_SECCOMP)
    return;

  if (!ctx)
    return;

  syscall = (int) ctx->uc_mcontext.M_SYSCALL;

#ifdef USE_BACKTRACE
  depth = backtrace(syscall_cb_buf, MAX_DEPTH);
  /* Clean up the top stack frame so we get the real function
   * name for the most recently failing function. */
  clean_backtrace(syscall_cb_buf, depth, ctx);
#endif

  syscall_name = get_syscall_name(syscall);

  tor_log_err_sigsafe("(Sandbox) Caught a bad syscall attempt (syscall ",
                      syscall_name,
                      ")\n",
                      NULL);

#ifdef USE_BACKTRACE
  n_fds = tor_log_get_sigsafe_err_fds(&fds);
  for (i=0; i < n_fds; ++i)
    backtrace_symbols_fd(syscall_cb_buf, depth, fds[i]);
#endif

#if defined(DEBUGGING_CLOSE)
  _exit(1);
#endif // DEBUGGING_CLOSE
}

/**
 * Function that adds a handler for SIGSYS, which is the signal thrown
 * when the application is issuing a syscall which is not allowed. The
 * main purpose of this function is to help with debugging by identifying
 * filtered syscalls.
 */
static int
install_sigsys_debugging(void)
{
  struct sigaction act;
  sigset_t mask;

  memset(&act, 0, sizeof(act));
  sigemptyset(&mask);
  sigaddset(&mask, SIGSYS);

  act.sa_sigaction = &sigsys_debugging;
  act.sa_flags = SA_SIGINFO;
  if (sigaction(SIGSYS, &act, NULL) < 0) {
    log_err(LD_BUG,"(Sandbox) Failed to register SIGSYS signal handler");
    return -1;
  }

  if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
    log_err(LD_BUG,"(Sandbox) Failed call to sigprocmask()");
    return -2;
  }

  return 0;
}

/**
 * Function responsible of registering the sandbox_cfg_t list of parameter
 * syscall filters to the existing parameter list. This is used for incipient
 * multiple-sandbox support.
 */
static int
register_cfg(sandbox_cfg_t* cfg)
{
  sandbox_cfg_t *elem = NULL;

  if (filter_dynamic == NULL) {
    filter_dynamic = cfg;
    return 0;
  }

  for (elem = filter_dynamic; elem->next != NULL; elem = elem->next)
    ;

  elem->next = cfg;

  return 0;
}

#endif // USE_LIBSECCOMP

#ifdef USE_LIBSECCOMP
/**
 * Initialises the syscall sandbox filter for any linux architecture, taking
 * into account various available features for different linux flavours.
 */
static int
initialise_libseccomp_sandbox(sandbox_cfg_t* cfg)
{
  if (install_sigsys_debugging())
    return -1;

  if (install_syscall_filter(cfg))
    return -2;

  if (register_cfg(cfg))
    return -3;

  return 0;
}

int
sandbox_is_active(void)
{
  return sandbox_active != 0;
}
#endif // USE_LIBSECCOMP

sandbox_cfg_t*
sandbox_cfg_new(void)
{
  return NULL;
}

int
sandbox_init(sandbox_cfg_t *cfg)
{
#if defined(USE_LIBSECCOMP)
  return initialise_libseccomp_sandbox(cfg);

#elif defined(__linux__)
  (void)cfg;
  log_warn(LD_GENERAL,
           "This version of Tor was built without support for sandboxing. To "
           "build with support for sandboxing on Linux, you must have "
           "libseccomp and its necessary header files (e.g. seccomp.h).");
  return 0;

#else
  (void)cfg;
  log_warn(LD_GENERAL,
           "Currently, sandboxing is only implemented on Linux. The feature "
           "is disabled on your platform.");
  return 0;
#endif
}

#ifndef USE_LIBSECCOMP
int
sandbox_cfg_allow_open_filename(sandbox_cfg_t **cfg, char *file)
{
  (void)cfg; (void)file;
  return 0;
}

int
sandbox_cfg_allow_open_filename_array(sandbox_cfg_t **cfg, ...)
{
  (void)cfg;
  return 0;
}

int
sandbox_cfg_allow_openat_filename(sandbox_cfg_t **cfg, char *file)
{
  (void)cfg; (void)file;
  return 0;
}

int
sandbox_cfg_allow_openat_filename_array(sandbox_cfg_t **cfg, ...)
{
  (void)cfg;
  return 0;
}

#if 0
int
sandbox_cfg_allow_execve(sandbox_cfg_t **cfg, const char *com)
{
  (void)cfg; (void)com;
  return 0;
}

int
sandbox_cfg_allow_execve_array(sandbox_cfg_t **cfg, ...)
{
  (void)cfg;
  return 0;
}
#endif

int
sandbox_cfg_allow_stat_filename(sandbox_cfg_t **cfg, char *file)
{
  (void)cfg; (void)file;
  return 0;
}

int
sandbox_cfg_allow_stat_filename_array(sandbox_cfg_t **cfg, ...)
{
  (void)cfg;
  return 0;
}

int
sandbox_cfg_allow_rename(sandbox_cfg_t **cfg, char *file1, char *file2)
{
  (void)cfg; (void)file1; (void)file2;
  return 0;
}

int
sandbox_is_active(void)
{
  return 0;
}
#endif