[Support,Windows] Tolerate failure of CryptGenRandom

[simon_tatham]

Summary

In Unix/Process.inc, we seed a random number generator from
/dev/urandom if possible, but if not, we're happy to fall back to
ordinary pseudorandom strategies, like the current time and PID.

The corresponding function on Windows calls CryptGenRandom, but it
doesn't have a fallback if that strategy fails. But CryptGenRandom
can fail, if a cryptography provider isn't properly initialized, or
occasionally (by our observation) simply intermittently.

If it's reasonable on Unix to implement traditional pseudorandom-number
seeding as a fallback, then it's surely reasonable to do the same on
Windows. So this patch adds a last-ditch use of ordinary rand(), using
much the same strategy as the Unix fallback code.

[hans]

Seems reasonable to me.

llvm/lib/Support/Windows/Process.inc

455

Why do you need the static_cast<void>?

[simon_tatham]

llvm/lib/Support/Windows/Process.inc

455

I must admit that I copied the idiom from the corresponding function in unix/Process.inc without really thinking about what the cast was doing there.

But now I look, it was added on purpose in rL261270, apparently in the expectation that at some point it would be useful for suppress a warning.

llvm/lib/Support/Windows/Process.inc

 }
 
 const char *Process::ResetColor() {
   if (UseANSI) return "\033[0m";
   SetConsoleTextAttribute(GetStdHandle(STD_OUTPUT_HANDLE), defaultColors());
   return 0;
 }
 
+static unsigned GetRandomNumberSeed() {
+  // Generate a random number seed from the millisecond-resolution Windows
+  // system clock and the current process id.
+  FILETIME Time;
+  GetSystemTimeAsFileTime(&Time);
+  DWORD Pid = GetCurrentProcessId();
+  return hash_combine(Time.dwHighDateTime, Time.dwLowDateTime, Pid);
+}
+
+static unsigned GetPseudoRandomNumber() {
+  // Arrange to call srand once when this function is first used, and
+  // otherwise (if GetRandomNumber always succeeds in using
+  // CryptGenRandom) don't bother at all.
+  static int x = (static_cast<void>(::srand(GetRandomNumberSeed())), 0);
+  (void)x;
+  return ::rand();
+}
+
 unsigned Process::GetRandomNumber() {
+  // Try to use CryptGenRandom.
   HCRYPTPROV HCPC;
-  if (!::CryptAcquireContextW(&HCPC, NULL, NULL, PROV_RSA_FULL,
-                              CRYPT_VERIFYCONTEXT))
-    ReportLastErrorFatal("Could not acquire a cryptographic context");
-
+  if (::CryptAcquireContextW(&HCPC, NULL, NULL, PROV_RSA_FULL,
+                             CRYPT_VERIFYCONTEXT)) {
     ScopedCryptContext CryptoProvider(HCPC);
     unsigned Ret;
-  if (!::CryptGenRandom(CryptoProvider, sizeof(Ret),
+    if (::CryptGenRandom(CryptoProvider, sizeof(Ret),
                          reinterpret_cast<BYTE *>(&Ret)))
-    ReportLastErrorFatal("Could not generate a random number");
       return Ret;
   }
 
+  // If that fails, fall back to pseudo-random numbers.
+  return GetPseudoRandomNumber();
+}
+
 typedef NTSTATUS(WINAPI* RtlGetVersionPtr)(PRTL_OSVERSIONINFOW);
 #define STATUS_SUCCESS ((NTSTATUS)0x00000000L)
 
 llvm::VersionTuple llvm::GetWindowsOSVersion() {
   HMODULE hMod = ::GetModuleHandleW(L"ntdll.dll");
   if (hMod) {
     auto getVer = (RtlGetVersionPtr)::GetProcAddress(hMod, "RtlGetVersion");
     if (getVer) {