[PATCH] D40623: Correctly set reserved bits for UUID version 4.

[Rui Ueyama via llvm-commits]

No, I just find that UUID v4 is not a 128-bit random number but a 122-bit
random number. I don't think the previous code caused a symptom or
anything, but that's technically incorrect.

On Wed, Nov 29, 2017 at 3:01 PM, Zachary Turner <zturner at google.com> wrote:

> How did you find this?  Was it causing a symptom of something breaking?
>
> On Wed, Nov 29, 2017 at 2:54 PM Rui Ueyama via Phabricator <
> reviews at reviews.llvm.org> wrote:
>
>> ruiu created this revision.
>> Herald added subscribers: hiraditya, emaste.
>>
>> Correctly set reserved bits for UUID version 4.
>>
>>
>> https://reviews.llvm.org/D40623
>>
>> Files:
>>   lld/ELF/SyntheticSections.cpp
>>   llvm/include/llvm/Support/RandomNumberGenerator.h
>>   llvm/lib/Support/RandomNumberGenerator.cpp
>>
>>
>> Index: llvm/lib/Support/RandomNumberGenerator.cpp
>> ===================================================================
>> --- llvm/lib/Support/RandomNumberGenerator.cpp
>> +++ llvm/lib/Support/RandomNumberGenerator.cpp
>> @@ -89,3 +89,15 @@
>>    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();
>> +}
>> Index: llvm/include/llvm/Support/RandomNumberGenerator.h
>> ===================================================================
>> --- llvm/include/llvm/Support/RandomNumberGenerator.h
>> +++ llvm/include/llvm/Support/RandomNumberGenerator.h
>> @@ -65,6 +65,9 @@
>>
>>  // 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
>> Index: lld/ELF/SyntheticSections.cpp
>> ===================================================================
>> --- lld/ELF/SyntheticSections.cpp
>> +++ lld/ELF/SyntheticSections.cpp
>> @@ -368,7 +368,7 @@
>>      });
>>      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:
>>
>>
>>
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