Move UBSan warnings from a blacklist to the source

webrtc/rtc_base/random.cc

Index: webrtc/rtc_base/random.cc
diff --git a/webrtc/rtc_base/random.cc b/webrtc/rtc_base/random.cc
index c0cf7fea71ac982ac5d653500935d44a9d31cb5b..3dddf53b23effd0f71ea1936ec7e6c80af1631b2 100644
--- a/webrtc/rtc_base/random.cc
+++ b/webrtc/rtc_base/random.cc
@@ -40,11 +40,9 @@ uint32_t Random::Rand(uint32_t low, uint32_t high) {
 
 int32_t Random::Rand(int32_t low, int32_t high) {
   RTC_DCHECK(low <= high);
-  // We rely on subtraction (and addition) to be the same for signed and
-  // unsigned numbers in two-complement representation. Thus, although
-  // high - low might be negative as an int, it is the correct difference
-  // when interpreted as an unsigned.
-  return Rand(high - low) + low;
+  const auto low_i64 = static_cast<int64_t>(low);

[kwiberg-webrtc, 2017/09/04 23:09:50]

This can be

  const int64_t low_i64{low};

[oprypin_webrtc, 2017/09/05 07:27:34]

I used the code you suggested in
https://bugs.chromium.org/p/webrtc/issues/detail?id=8189&desc=3#c7
I checked that with this change it's still the same byte-for-byte, so I agree.
https://godbolt.org/g/upEsVg
Changed.

[kwiberg-webrtc, 2017/09/05 08:37:30]

Excellent. That matches my results from when I wrote that bug comment.

+  return static_cast<int32_t>(Rand(static_cast<uint32_t>(high - low_i64)) +
+                              low_i64);

[kwiberg-webrtc, 2017/09/04 23:09:51]

Both of these static_casts can be proven to always preserve values, right? But
we don't expect the compiler to be able to prove it? In that case, they should
probably be dchecked_casts.

[oprypin_webrtc, 2017/09/05 07:27:34]

high - low_i64 always fits in uint32_t:
(2**31-1) - (-2**31) == (2**32-1)

Minimal result of Rand+low_i64 is 0 + (-2**31) == (-2**31)
Maximal result of Rand+low_i64 is (2**32-1) + (-2**31) == (2**31-1)

So seems like truncations are impossible here.
dchecked_cast will work. Changed.

[kwiberg-webrtc, 2017/09/05 08:37:31]

Thanks for double-checking the math!

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