Use rdrand as entropy source on supported platforms

Includes squashed commits:
5155d1101eb4fc9d4d797b583bb29f71807bd10b
a9e82f6512662054f64ed2bde590b2bb0831fc9d
674848fe1c43fb88870cf5ba16fca4e2524da793

Cherry-picked from: cb24c8539d1098d1a61605b452ecfa11a693320d

src/bench/bench_bitcoin.cpp

@@ -5,10 +5,12 @@
 #include "bench.h"  // for BenchRunner
 #include "key.h"    // for ECC_Start, ECC_Stop
 #include "util.h"   // for SetupEnvironment, fPrintToDebugLog
+#include "random.h"
 
 int
 main(int argc, char** argv)
 {
+    RandomInit();
     ECC_Start();
     SetupEnvironment();
     fPrintToDebugLog = false; // don't want to write to debug.log file

src/init.cpp

@@ -1188,6 +1188,7 @@ bool AppInitSanityChecks()
     // ********************************************************* Step 4: sanity checks
 
     // Initialize elliptic curve code
+    RandomInit();
     ECC_Start();
     globalVerifyHandle.reset(new ECCVerifyHandle());
 

src/random.cpp

@@ -37,6 +37,10 @@
 #include <sys/sysctl.h>
 #endif
 
+#if defined(__x86_64__) || defined(__amd64__) || defined(__i386__)
+#include <cpuid.h>
+#endif
+
 #include <openssl/err.h>
 #include <openssl/rand.h>
 
@@ -66,6 +70,61 @@ static inline int64_t GetPerformanceCounter()
 #endif
 }
 
+
+#if defined(__x86_64__) || defined(__amd64__) || defined(__i386__)
+static std::atomic<bool> hwrand_initialized{false};
+static bool rdrand_supported = false;
+static constexpr uint32_t CPUID_F1_ECX_RDRAND = 0x40000000;
+static void RDRandInit()
+{
+    uint32_t eax, ebx, ecx, edx;
+    if (__get_cpuid(1, &eax, &ebx, &ecx, &edx) && (ecx & CPUID_F1_ECX_RDRAND)) {
+        LogPrintf("Using RdRand as an additional entropy source\n");
+        rdrand_supported = true;
+    }
+    hwrand_initialized.store(true);
+}
+#else
+static void RDRandInit() {}
+#endif
+
+static bool GetHWRand(unsigned char* ent32) {
+#if defined(__x86_64__) || defined(__amd64__) || defined(__i386__)
+    assert(hwrand_initialized.load(std::memory_order_relaxed));
+    if (rdrand_supported) {
+        uint8_t ok;
+        // Not all assemblers support the rdrand instruction, write it in hex.
+#ifdef __i386__
+        for (int iter = 0; iter < 4; ++iter) {
+            uint32_t r1, r2;
+            __asm__ volatile (".byte 0x0f, 0xc7, 0xf0;" // rdrand %eax
+                              ".byte 0x0f, 0xc7, 0xf2;" // rdrand %edx
+                              "setc %2" :
+                              "=a"(r1), "=d"(r2), "=q"(ok) :: "cc");
+            if (!ok) return false;
+            WriteLE32(ent32 + 8 * iter, r1);
+            WriteLE32(ent32 + 8 * iter + 4, r2);
+        }
+#else
+        uint64_t r1, r2, r3, r4;
+        __asm__ volatile (".byte 0x48, 0x0f, 0xc7, 0xf0, " // rdrand %rax
+                                "0x48, 0x0f, 0xc7, 0xf3, " // rdrand %rbx
+                                "0x48, 0x0f, 0xc7, 0xf1, " // rdrand %rcx
+                                "0x48, 0x0f, 0xc7, 0xf2; " // rdrand %rdx
+                          "setc %4" :
+                          "=a"(r1), "=b"(r2), "=c"(r3), "=d"(r4), "=q"(ok) :: "cc");
+        if (!ok) return false;
+        WriteLE64(ent32, r1);
+        WriteLE64(ent32 + 8, r2);
+        WriteLE64(ent32 + 16, r3);
+        WriteLE64(ent32 + 24, r4);
+#endif
+        return true;
+    }
+#endif
+    return false;
+}
+
 void RandAddSeed()
 {
     // Seed with CPU performance counter
@@ -228,6 +287,11 @@ void GetStrongRandBytes(unsigned char* out, int num)
     GetOSRand(buf);
     hasher.Write(buf, 32);
 
+    // Third source: HW RNG, if available.
+    if (GetHWRand(buf)) {
+        hasher.Write(buf, 32);
+    }
+
     // Produce output
     hasher.Finalize(buf);
     memcpy(out, buf, num);
@@ -345,3 +409,8 @@ FastRandomContext::FastRandomContext(bool fDeterministic) : requires_seed(!fDete
     uint256 seed;
     rng.SetKey(seed.begin(), 32);
 }
+
+void RandomInit()
+{
+    RDRandInit();
+}

src/random.h

@@ -133,4 +133,7 @@ void GetOSRand(unsigned char *ent32);
  */
 bool Random_SanityCheck();
 
+/** Initialize the RNG. */
+void RandomInit();
+
 #endif // BITCOIN_RANDOM_H

src/test/test_bitcoin.cpp

@@ -43,6 +43,7 @@ static const int COINBASE_MATURITY = 60*4; // 4 hours of blocks
 
 BasicTestingSetup::BasicTestingSetup(const std::string& chainName)
 {
+        RandomInit();
         ECC_Start();
         SetupEnvironment();
         SetupNetworking();