[lldb-dev] LLDB does not support the default 8 byte build ID generated by LLD

[Pavel Labath via lldb-dev]

That sounds like a plan. I have started cleaning up the class a bit
(removing manual uuid string formatting in various places and such),
and then I'll send a patch which implements that.

Leonard, I'm not going to use your patch, as it's a bit un-llvm-y
(uses std::ostream and such). However, I wanted to check whether 20
bytes will be enough for your use cases (uuids in minidumps)?
On Thu, 21 Jun 2018 at 16:03, Greg Clayton <clayborg at gmail.com> wrote:
>
> I am fine if we go with any number of bytes. We should have the lldb_private::UUID class have an array of bytes that is in the class that is to to 20 bytes. We can increase it later if needed. I would rather not have a dynamically allocated buffer.
>
> That being said a few points:
> - Length can be set to zero to indicate invalid UUID. Better that than filling in all zeroes and having to check for that IMHO. I know there were some problems with the last patch around this.
> - Don't set length to a valid value and have UUID contain zeros unless that is a true UUID that was calculated. LLDB does a lot of things by matching UUID values so we can't have multiple modules claiming to have a UUID that is filled with zeroes, otherwise many matches will occur that we don't want
> - 32 bit GNU debug info CRCs from ELF notes could be filled in as 4 byte UUIDs
> - Comparing two UUIDs can start with the length field first the if they match proceed to compare the bytes (which is hopefully what is already happening)
>
>
> On Jun 20, 2018, at 11:01 AM, Leonard Mosescu via lldb-dev <lldb-dev at lists.llvm.org> wrote:
>
> Here's a snapshot of the old changes I had: https://reviews.llvm.org/D48381
> (hopefully it helps a bit but caveat emptor: this is a quick merge from an old patch, so it's for illustrative purposes only)
>
>
> On Wed, Jun 20, 2018 at 10:26 AM, Pavel Labath <labath at google.com> wrote:
>>
>> From the looks of it, the patch stalled on the part whether we can
>> consider all-zero UUIDs as valid or not. I've dug around the code a
>> bit now, and I've found this comment in ObjectFileMachO.cpp.
>>
>>            // "main bin spec" (main binary specification) data payload is
>>            // formatted:
>>            //    uint32_t version       [currently 1]
>>            //    uint32_t type          [0 == unspecified, 1 ==
>> kernel, 2 == user process]
>>            //    uint64_t address       [ UINT64_MAX if address not specified ]
>>            //    uuid_t   uuid          [ all zero's if uuid not specified ]
>>            //    uint32_t log2_pagesize [ process page size in log
>> base 2, e.g. 4k pages are 12.  0 for unspecified ]
>>
>>
>> So it looks like there are situations where we consider all-zero UUIDs
>> as invalid.
>>
>> I guess that means we either have to keep IsValid() definition as-is,
>> or make ObjectFileMachO check the all-zero case itself. (Some middle
>> ground may be where we have two SetFromStringRef functions, one which
>> treats all-zero case specially (sets m_num_uuid_bytes to 0), and one
>> which doesn't). Then clients can pick which semantics they want.
>>
>>
>> > 1. A variable-length UUID likely incurs an extra heap allocation.
>> Not really. If you're happy with the current <=20 limit, then you can
>> just use the existing data structure. Otherwise, you could use a
>> SmallVector<uint8_t, 20>.
>>
>> > 2. Formatting arbitrary length UUIDs as string is a bit inconvenient as you noted as well.
>> For the string representation, I would say we should just use the
>> existing layout of dashes (after 4, 6, 8, 10 and 16 bytes) and just
>> cut it short when we have less bytes. The implementation of that
>> should be about a dozen lines of code.
>>
>> The fact that these new UUIDs would not be real UUIDs could be solved
>> by renaming this class to something else, if anyone can think of a
>> good name for it (I can't). Then the "real" UUIDs will be just a
>> special case of the new object.
>>
>> pl
>
>
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