Correctly set reserved bits for UUID version 4.

[ruiu]

Summary

Correctly set reserved bits for UUID version 4.

[emaste]

LGTM

[• espindola]

would it be possible to test this?

lld/ELF/SyntheticSections.cpp

case BuildIdKind::Md5:
     });
     break;
   case BuildIdKind::Sha1:
     computeHash(Buf, [](uint8_t *Dest, ArrayRef<uint8_t> Arr) {
       memcpy(Dest, SHA1::hash(Arr).data(), 20);
     });
     break;
   case BuildIdKind::Uuid:
-    if (auto EC = getRandomBytes(HashBuf, HashSize))
+    if (auto EC = getUuidV4(HashBuf))
       error("entropy source failure: " + EC.message());
     break;
   case BuildIdKind::Hexstring:
     memcpy(HashBuf, Config->BuildIdVector.data(), Config->BuildIdVector.size());
     break;
   default:
     llvm_unreachable("unknown BuildIdKind");
   }

llvm/include/llvm/Support/RandomNumberGenerator.h

private:
   RandomNumberGenerator(const RandomNumberGenerator &other) = delete;
   RandomNumberGenerator &operator=(const RandomNumberGenerator &other) = delete;
 
   friend class Module;
 };
 
 // Get random vector of specified size
 std::error_code getRandomBytes(void *Buffer, size_t Size);
+
+// Get UUID v4.
+std::error_code getUuidV4(void *Buffer);
 }
 
 #endif

llvm/lib/Support/RandomNumberGenerator.cpp

if (Fd != -1) {
     if (close(Fd) == -1)
       Ret = std::error_code(errno, std::system_category());
 
     return Ret;
   }
   return std::error_code(errno, std::system_category());
 #endif
 }
+
+// UUID v4 is a 122-bit random number in the form of
+// RRRRRRRR-RRRR-4RRR-xRRR-RRRRRRRRRRRR, where R is a random byte and
+// x's most siginificant two bits are 0b10.
+std::error_code llvm::getUuidV4(void *Buffer) {
+  if (auto EC = getRandomBytes(Buffer, 16))
+    return EC;
+  char *Buf = (char *)Buffer;
+  Buf[7] = 0x40 | (Buf[7] & 0xf);
+  Buf[9] = 0x80 | (Buf[9] & 0x3f);
+  return std::error_code();
+}