[lldb-dev] Debugging ELF relocatable files using LLDB

[Tamas Berghammer via lldb-dev]

I uploaded a CL for review what fixes the crash you are experimenting at
https://reviews.llvm.org/D30251 (we are mapping the files into memory as
read only and then trying to write into it) but I think nobody tests LLDB
with relocable object files so you might run into a lot of bugs in the way.
Also I suggest to switch to a newer version of LLDB (preferably ToT) as 3.6
is fairly old and known to have a lot of bugs on Linux and on ARM.

Tamas

On Wed, Feb 22, 2017 at 4:56 AM Ramana via lldb-dev <lldb-dev at lists.llvm.org>
wrote:

> On Thu, Feb 16, 2017 at 10:26 PM, Greg Clayton <gclayton at apple.com> wrote:
> >
> > On Feb 16, 2017, at 3:51 AM, Ramana via lldb-dev <
> lldb-dev at lists.llvm.org>
> > wrote:
> >
> > It looks like LLDB doesn't like ELF relocatable files for debugging
> > and asserts with the following message when tried
> >
> >     <path to>/lldb/source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp:2228:
> > unsigned int ObjectFileELF::RelocateSection(.....):  Assertion `false
> > && "unexpected relocation type"' failed.
> >
> > Are we not supposed to debug ELF relocatable files on LLDB or am I
> > missing something?
> >
> > If we cannot debug the relocatable files, is it _simply_ because those
> > files lack program headers (program memory map) and relocations are
> > yet to be processed (for debug info) or there are other reasons?
> >
> > For our target, the assembler output itself is a self contained ELF
> > and hence will not have external references (both code and data). I am
> > wondering if I can debug these ELF files on LLDB with minimal changes
> > which does not require a full (or proper) linking step and would
> > appreciate any pointers on that.
> >
> > Thanks,
> > Ramana
> >
> >
> > Looks like you just need to add support for the 32 bit relocations:
> >
> >
> >     if (hdr->Is32Bit()) {
> >       switch (reloc_type(rel)) {
> >       case R_386_32:
> >       case R_386_PC32:
> >       default:
> >         assert(false && "unexpected relocation type");
> >       }
> >     } else {
> >       switch (reloc_type(rel)) {
> >       case R_X86_64_64: {
> >         symbol = symtab->FindSymbolByID(reloc_symbol(rel));
> >         if (symbol) {
> >           addr_t value = symbol->GetAddressRef().GetFileAddress();
> >           DataBufferSP &data_buffer_sp =
> debug_data.GetSharedDataBuffer();
> >           uint64_t *dst = reinterpret_cast<uint64_t *>(
> >               data_buffer_sp->GetBytes() + rel_section->GetFileOffset() +
> >               ELFRelocation::RelocOffset64(rel));
> >           *dst = value + ELFRelocation::RelocAddend64(rel);
> >         }
> >         break;
> >       }
> >       case R_X86_64_32:
> >       case R_X86_64_32S: {
> >         symbol = symtab->FindSymbolByID(reloc_symbol(rel));
> >         if (symbol) {
> >           addr_t value = symbol->GetAddressRef().GetFileAddress();
> >           value += ELFRelocation::RelocAddend32(rel);
> >           assert(
> >               (reloc_type(rel) == R_X86_64_32 && (value <= UINT32_MAX))
> ||
> >               (reloc_type(rel) == R_X86_64_32S &&
> >                ((int64_t)value <= INT32_MAX && (int64_t)value >=
> > INT32_MIN)));
> >           uint32_t truncated_addr = (value & 0xFFFFFFFF);
> >           DataBufferSP &data_buffer_sp =
> debug_data.GetSharedDataBuffer();
> >           uint32_t *dst = reinterpret_cast<uint32_t *>(
> >               data_buffer_sp->GetBytes() + rel_section->GetFileOffset() +
> >               ELFRelocation::RelocOffset32(rel));
> >           *dst = truncated_addr;
> >         }
> >         break;
> >       }
> >       case R_X86_64_PC32:
> >       default:
> >         assert(false && "unexpected relocation type");
> >       }
> >     }
> >
> >
> > I am guessing you will do something similar to the x86-64 stuff.
>
> I tried to mimic the x86_64 relocations handling for our target but
> getting segmentation fault while trying to write to the 'dst'
> location.
>
> In fact, the x86_64 also segfaults while trying to write to 'dst'
> location. I just tried to debug the following simple program for
> x86_64.
>
> main.c:
>     int main () {
>        return 0;
>     }
>
> $ clang main.c -o main_64b.o --target=x86_64 -c -g
> $  lldb main_64b.o
> (lldb) target create "main_64b.o"
> Current executable set to 'main_64b.o' (x86_64).
> (lldb) source list
> Segmentation fault (core dumped)
>
> Am I doing something wrong or support for debugging the x86_64 ELF
> relocatable files using LLDB is broken?
>
> BTW, I am using LLVM v3.6 and LLDB v3.6.
>
> Regards,
> Ramana
> _______________________________________________
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> lldb-dev at lists.llvm.org
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>
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