Provide fallback implementation for getentropy()

[arkq]

Commit ba3396f changed getRandomBytes() implementation from c++ std uniform distribution to getentropy(). The getentropy() function first appeared in glibc 2.25. In order to provide backward compatibility for older glibc (e.g. Android SDK <= 19) restore previous getRandomBytes() implementation as a fallback.

Testing

Right now compilation succeeds with NDK r19c: https://github.com/android/ndk/wiki/Unsupported-Downloads#r19c

[anzz1]

Tested working in Linux 3.10 with glibc 2.24 ! 👍

Just need to add the definition for HAVE_GETENTROPY in configure.ac so it gets configured accordingly.

configure.ac

AC_CHECK_FUNCS([strptime],
        [AM_CONDITIONAL([HAVE_STRPTIME], true)],
        [AM_CONDITIONAL([HAVE_STRPTIME], false)])
+AC_CHECK_FUNCS([getentropy],
+        [AM_CONDITIONAL([HAVE_GETENTROPY], true)],
+        [AM_CONDITIONAL([HAVE_GETENTROPY], false)])
AC_CHECK_FUNCS([daemon], [have_daemon=yes])
AM_CONDITIONAL([HAVE_DAEMON], [test "x$have_daemon" = "xyes"])

Submitted PR to the patch fork : arkq#1
edit: PR is now merged in this PR

[anzz1]

While this already works fine as-is, it could be further improved by adding another fallback:
getentropy() -> /dev/urandom -> std::uniform_int_distribution

SimpleRandomizer.cc

#if !HAVE_GETENTROPY
  auto getentropy = [this](void *buffer, size_t length) {
#if HAVE_URANDOM
    FILE *fp = fopen("/dev/urandom", "rb");
    if (fp) {
      size_t r = fread(buffer, length, 1, fp);
      fclose(fp);
      if (r == 1) {
        return 0;
      }
    }
#endif // HAVE_URANDOM
    auto buf = reinterpret_cast<unsigned int*>(buffer);
    auto dis = std::uniform_int_distribution<unsigned int>();
    for (size_t q = length / sizeof(unsigned int); q > 0; --q, ++buf) {
      *buf = dis(gen_);
    }
    const size_t r = length % sizeof(unsigned int);
    auto last = dis(gen_);
    memcpy(buf, &last, r);
    return 0;
  };
#endif // !HAVE_GETENTROPY

configure.ac

+AC_CHECK_FILE("/dev/urandom",
+        [AM_CONDITIONAL([HAVE_URANDOM], true)],
+        [AM_CONDITIONAL([HAVE_URANDOM], false)]