Repository: LadybirdBrowser/ladybird
File: AK/Random.cpp (C++)

Code before:
        if (ret == -1 && errno != EINTR) [[unlikely]]
            return Error::from_errno(errno);

        // Per the manual:
        //
        // > On success, getrandom() returns the number of bytes that were
        // > copied to the buffer buf.  This may be less than the number of
        // > bytes requested via buflen if either GRND_RANDOM was specified in
        // > flags and insufficient entropy was present in the random source or
        // > the system call was interrupted by a signal.
        //
        // > The user of getrandom() *must* always check the return value, to
        // > determine whether either an error occurred or fewer bytes than
        // > requested were returned.
        if(ret > 0) [[likely]] {
            size -= ret;
            buf += ret;
        }
    }
#elif defined(AK_OS_WINDOWS)
    if (size > NumericLimits<u32>::max()) [[unlikely]]
        return Error::from_string_literal("The input is too large");

    NTSTATUS result = ::BCryptGenRandom(NULL, buf, size, BCRYPT_USE_SYSTEM_PREFERRED_RNG);
    if (result != STATUS_SUCCESS) [[unlikely]]
        return Error::from_windows_error();
#else
    // There shouldn't be a build target where this can happen.
    //
    // As for MacOS and iOS, they appear to both support arc4random_buf, and AK/Platform.h defines AK_OS_BSD_GENERIC when building for them,
    // so they will be covered by the compile target that uses arc4random_buf.
    //
    // Note that although [SecRandomCopyBytes](https://developer.apple.com/documentation/security/secrandomcopybytes(_:_:_:)) appears to be more commonly used on these platforms,
    // it doesn't seem to be required.
    static_assert(false, "This build target doesn't have a valid CSPRNG interface specified in AK/Random.cpp. This needs to be fixed before you can build for this target.");
#endif
    return {};
}

namespace AK {

void fill_with_random(Bytes bytes)
{
    if (csprng(bytes.data(), bytes.size()).is_error())
        VERIFY_NOT_REACHED();
}

u32 get_random_uniform(u32 max_bounds)
{
    // If we try to divide all 2**32 numbers into groups of "max_bounds" numbers, we may end up
    // with a group around 2**32-1 that is a bit too small. For this reason, the implementation

Review comment (suggestion):
This under the hood calls GetLastError which will return 0 since there is no winapi error. That will cause a verify in Error. You should pass it a dos error, which you can obtain by calling RtlNtStatusToDosError on the status returned.

Code after:
#elif defined(AK_OS_WINDOWS)
    // Documented to always return TRUE
    g_system.ProcessPrng((PBYTE)buf, size);
#else
    static_assert(false, "This build target doesn't have a valid CSPRNG interface specified in AK/Random.cpp.");
#endif
    return {};
}

namespace AK {

void fill_with_random(Bytes bytes)
{
    MUST(csprng(bytes.data(), bytes.size()));
}

u32 get_random_uniform(u32 max_bounds)
{
    // If we try to divide all 2**32 numbers into groups of "max_bounds" numbers, we may end up
    // with a group around 2**32-1 that is a bit too small. For this reason, the implementation
    // `arc4random() % max_bounds` would be insufficient. Here we compute the last number of the
    // last "full group". Note that if max_bounds is a divisor of UINT32_MAX,
    // then we end up with UINT32_MAX:
    u32 const max_usable = UINT32_MAX - (static_cast<u64>(UINT32_MAX) + 1) % max_bounds;
    auto random_value = get_random<u32>();
    for (int i = 0; i < 20 && random_value > max_usable; ++i) {
        // By chance we picked a value from the incomplete group. Note that this group has size at
        // most 2**31-1, so picking this group has a chance of less than 50%.
        // In practice, this means that for the worst possible input, there is still only a
        // once-in-a-million chance to get to iteration 20. In theory we should be able to loop
        // forever. Here we prefer marginally imperfect random numbers over weird runtime behavior.
        random_value = get_random<u32>();
    }
    return random_value % max_bounds;
}

u64 get_random_uniform_64(u64 max_bounds)
{
    // Uses the same algorithm as `get_random_uniform`,
    // by replacing u64 with u128 and u32 with u64.
    u64 const max_usable = UINT64_MAX - static_cast<u64>((static_cast<u128>(UINT64_MAX) + 1) % max_bounds);
    auto random_value = get_random<u64>();
    for (int i = 0; i < 20 && random_value > max_usable; ++i) {
        random_value = get_random<u64>();
    }
    return random_value % max_bounds;
}

XorShift128PlusRNG::XorShift128PlusRNG()
{
    // Splitmix64 is used as xorshift is sensitive to being seeded with all 0s