guid_creator.cc

// Copyright (c) 2006, Google Inc.
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//
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//
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#include "common/linux/eintr_wrapper.h"
#include "common/linux/guid_creator.h"

#include <assert.h>
#include <fcntl.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <time.h>
#include <unistd.h>

#if defined(HAVE_SYS_RANDOM_H)
#include <sys/random.h>
#endif

//
// GUIDGenerator
//
// This class is used to generate random GUID.
// Currently use random number to generate a GUID since Linux has
// no native GUID generator. This should be OK since we don't expect
// crash to happen very offen.
//
class GUIDGenerator {
 public:
  static uint32_t BytesToUInt32(const uint8_t bytes[]) {
    return ((uint32_t) bytes[0]
            | ((uint32_t) bytes[1] << 8)
            | ((uint32_t) bytes[2] << 16)
            | ((uint32_t) bytes[3] << 24));
  }

  static void UInt32ToBytes(uint8_t bytes[], uint32_t n) {
    bytes[0] = n & 0xff;
    bytes[1] = (n >> 8) & 0xff;
    bytes[2] = (n >> 16) & 0xff;
    bytes[3] = (n >> 24) & 0xff;
  }

  static bool CreateGUID(GUID *guid) {
#if defined(HAVE_ARC4RANDOM) // Android, BSD, ...
    CreateGuidFromArc4Random(guid);
#else // Linux
    bool success = false;

#if defined(HAVE_SYS_RANDOM_H) && defined(HAVE_GETRANDOM)
    success = CreateGUIDFromGetrandom(guid);
#endif // HAVE_SYS_RANDOM_H && HAVE_GETRANDOM
    if (!success) {
      success = CreateGUIDFromDevUrandom(guid);
    }

    if (!success) {
      CreateGUIDFromRand(guid);
      success = true;
    }
#endif

    // Put in the version according to RFC 4122.
    guid->data3 &= 0x0fff;
    guid->data3 |= 0x4000;

    // Put in the variant according to RFC 4122.
    guid->data4[0] &= 0x3f;
    guid->data4[0] |= 0x80;

    return true;
  }

 private:
#ifdef HAVE_ARC4RANDOM
  static void CreateGuidFromArc4Random(GUID *guid) {
    char *buf = reinterpret_cast<char *>(guid);

    for (size_t i = 0; i < sizeof(GUID); i += sizeof(uint32_t)) {
      uint32_t random_data = arc4random();

      memcpy(buf + i, &random_data, sizeof(uint32_t));
    }
  }
#else
  static void InitOnce() {
    pthread_once(&once_control, &InitOnceImpl);
  }

  static void InitOnceImpl() {
    // time(NULL) is a very poor seed, so lacking anything better mix an
    // address into it. We drop the four rightmost bits as they're likely to
    // be 0 on almost all architectures.
    srand(time(NULL) | ((uintptr_t)&once_control >> 4));
  }

  static pthread_once_t once_control;

#if defined(HAVE_SYS_RANDOM_H) && defined(HAVE_GETRANDOM)
  static bool CreateGUIDFromGetrandom(GUID *guid) {
    char *buf = reinterpret_cast<char *>(guid);
    int read_bytes = getrandom(buf, sizeof(GUID), GRND_NONBLOCK);

    return (read_bytes == static_cast<int>(sizeof(GUID)));
  }
#endif // HAVE_SYS_RANDOM_H && HAVE_GETRANDOM

  // Populate the GUID using random bytes read from /dev/urandom, returns false
  // if the GUID wasn't fully populated with random data.
  static bool CreateGUIDFromDevUrandom(GUID *guid) {
    char *buf = reinterpret_cast<char *>(guid);
    int fd = open("/dev/urandom", O_RDONLY | O_CLOEXEC);

    if (fd == -1) {
      return false;
    }

    ssize_t read_bytes = HANDLE_EINTR(read(fd, buf, sizeof(GUID)));
    close(fd);

    return (read_bytes == static_cast<ssize_t>(sizeof(GUID)));
  }

  // Populate the GUID using a stream of random bytes obtained from rand().
  static void CreateGUIDFromRand(GUID *guid) {
    char *buf = reinterpret_cast<char *>(guid);

    InitOnce();

    for (size_t i = 0; i < sizeof(GUID); i++) {
      buf[i] = rand();
    }
  }
#endif
};

#ifndef HAVE_ARC4RANDOM
pthread_once_t GUIDGenerator::once_control = PTHREAD_ONCE_INIT;
#endif

bool CreateGUID(GUID *guid) {
  return GUIDGenerator::CreateGUID(guid);
}

// Parse guid to string.
bool GUIDToString(const GUID *guid, char *buf, int buf_len) {
  // Should allow more space the the max length of GUID.
  assert(buf_len > kGUIDStringLength);
  int num = snprintf(buf, buf_len, kGUIDFormatString,
                     guid->data1, guid->data2, guid->data3,
                     GUIDGenerator::BytesToUInt32(&(guid->data4[0])),
                     GUIDGenerator::BytesToUInt32(&(guid->data4[4])));
  if (num != kGUIDStringLength)
    return false;

  buf[num] = '\0';
  return true;
}