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

Thu Jun 21 09:49:56 PDT 2018

> On Jun 21, 2018, at 9:46 AM, Leonard Mosescu <mosescu at google.com> wrote:
> 
> 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)?
> 
> For minidumps we normally use either 16 or 20 byte UUIDs, so I don't see any immediate problems. Are you planning to make 20 a hard limit or have the 20 bytes "inlined" and dynamically allocate if larger?

We could use a llvm::SmallVector<uint8_t, 20> to have up to 20 bytes before going larger and allocating on the heap.

> 
> On Thu, Jun 21, 2018 at 8:18 AM, Pavel Labath <labath at google.com <mailto:labath at google.com>> wrote:
> 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 <mailto: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 <mailto: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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> >
> >
> 

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