llvm dev - Aug 2018 - How does LLDB locate the DWARF information stored inside MACH-O .o files produced by LLVM and LLD?

When I make a simple test program and call abort() and use LLDB to debug
it, it has address-to-line mapping. If I then delete the .o file and run
LLDB again, it loses the address-to-line mapping. So there must be DWARF
info, or otherwise address-to-line info in the .o file.

I'm parsing the MACH-O executable, which has a LC_SYMTAB  entry, and one of
the entries is the file path of the .o file. So I can find the file that
way. Then I parse the MACH-O .o file, which also has the LC_SYMTAB, as well
as an LC_SEGMENT_64 and LC_DATA_IN_CODE, and then finally a LC_DYSYMTAB.
But I don't see where the address-to-line information is, or any DWARF
information.

Where is LLDB getting this address-to-line info?
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You'll need to go a bit further than the load commands - I don't know
much
about MachO files, so I'm just reading:
https://lowlevelbits.org/parsing-mach-o-files/ and
https://samhuri.net/posts/2010/01/basics-of-the-mach-o-file-format - so
inside the section (described by the LC_SEGMENT_64) are probably a bunch of
sections, including debug info sections. Tools like llvm-objdump will show
you those (llvm-objdump -h), and also tools like llvm-dwarfdump can print
the contents of the debug info sections (those with names starting with
"__debug") in human-readable form.

On Thu, Aug 23, 2018 at 12:19 AM Andrew Kelley via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> When I make a simple test program and call abort() and use LLDB to debug
> it, it has address-to-line mapping. If I then delete the .o file and run
> LLDB again, it loses the address-to-line mapping. So there must be DWARF
> info, or otherwise address-to-line info in the .o file.
>
> I'm parsing the MACH-O executable, which has a LC_SYMTAB  entry, and
one
> of the entries is the file path of the .o file. So I can find the file that
> way. Then I parse the MACH-O .o file, which also has the LC_SYMTAB, as well
> as an LC_SEGMENT_64 and LC_DATA_IN_CODE, and then finally a LC_DYSYMTAB.
> But I don't see where the address-to-line information is, or any DWARF
> information.
>
> Where is LLDB getting this address-to-line info?
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Thanks David.

I figured it out. Here's the scoop:

The executable has a LC_SYMTAB. One of those nlist_64 entries has the
string set to the .o file path, and the n_type set to N_OSO.
So you can go from symbol to .o file.
Then you go read the .o file, and the first load command is LC_SEGMENT_64.
That has some number of sections following it, among which are the DWARF
sections.

I had tried this, but I forgot to increment a pointer in a loop, so I
thought all the sections were the same as the first one. D'oh!

Regards,
-Andrew

On Thu, Aug 23, 2018 at 3:15 PM David Blaikie <dblaikie at gmail.com>
wrote:
> You'll need to go a bit further than the load commands - I don't
know much
> about MachO files, so I'm just reading:
> https://lowlevelbits.org/parsing-mach-o-files/ and
> https://samhuri.net/posts/2010/01/basics-of-the-mach-o-file-format - so
> inside the section (described by the LC_SEGMENT_64) are probably a bunch of
> sections, including debug info sections. Tools like llvm-objdump will show
> you those (llvm-objdump -h), and also tools like llvm-dwarfdump can print
> the contents of the debug info sections (those with names starting with
> "__debug") in human-readable form.
>
> On Thu, Aug 23, 2018 at 12:19 AM Andrew Kelley via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> When I make a simple test program and call abort() and use LLDB to
debug
>> it, it has address-to-line mapping. If I then delete the .o file and
run
>> LLDB again, it loses the address-to-line mapping. So there must be
DWARF
>> info, or otherwise address-to-line info in the .o file.
>>
>> I'm parsing the MACH-O executable, which has a LC_SYMTAB  entry,
and one
>> of the entries is the file path of the .o file. So I can find the file
that
>> way. Then I parse the MACH-O .o file, which also has the LC_SYMTAB, as
well
>> as an LC_SEGMENT_64 and LC_DATA_IN_CODE, and then finally a
LC_DYSYMTAB.
>> But I don't see where the address-to-line information is, or any
DWARF
>> information.
>>
>> Where is LLDB getting this address-to-line info?
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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