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

[Greg Clayton via lldb-dev]

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 <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 <mailto: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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