[lldb-dev] MCJIT Mach-O JIT debugging

Mon Jun 2 14:42:46 PDT 2014

Hmm, nevermind, it seems to be working just fine now. I'll clean it up and
submit a patch.

On Mon, Jun 2, 2014 at 4:54 PM, Keno Fischer <kfischer at college.harvard.edu>
wrote:

> We do for ELF (ObjectFileELF::RelocateSection), because LLVM doesn't do
> the debug info relocation for us in that case. It currently does for Mach-O
> so that shouldn't be an issue yet, the only question is whether lldb
> correctly loads the relocated section (which I think it should since the
> load address is being set correctly), or whether it loads the section
> directly from the object file.
>
>
> On Mon, Jun 2, 2014 at 4:50 PM, Greg Clayton <gclayton at apple.com> wrote:
>
>> We don't currently apply any relocations (that I know of) for debug info
>> in LLDB.
>>
>> > On Jun 2, 2014, at 12:35 PM, Keno Fischer <kfischer at college.harvard.edu>
>> wrote:
>> >
>> > I think I'm getting closer. The debug_info section is being relocated
>> correctly (I think):
>> >
>> > 0x00000000: Compile Unit: length = 0x00000045  version = 0x0003
>>  abbr_offset = 0x00000000  addr_size = 0x08  (next CU at 0x00000049)
>> >
>> > 0x0000000b: TAG_compile_unit [1] *
>> >              AT_producer( "julia" )
>> >              AT_language( DW_LANG_C89 )
>> >              AT_name( "string.jl" )
>> >              AT_stmt_list( 0x00000000 )
>> >              AT_comp_dir( "." )
>> >              AT_APPLE_optimized( 0x01 )
>> >              AT_low_pc( 0x0000000112f5f1c0 )
>> >              AT_high_pc( 0x000006fb )
>> >
>> > 0x0000002b:     TAG_subprogram [2]
>> >                  AT_low_pc( 0x0000000112f5f1c0 )
>> >                  AT_high_pc( 0x0000000112f5f8bb )
>> >                  AT_frame_base( rbp )
>> >                  AT_MIPS_linkage_name( "julia_parseint_nocheck;18749" )
>> >                  AT_name( "parseint_nocheck" )
>> >                  AT_external( 0x01 )
>> >                  AT_accessibility( DW_ACCESS_private )
>> >
>> > 0x00000048:     NULL
>> >
>> > but lldb is still showing it at the original location:
>> >
>> > 0x7ff3afca9280: SymbolVendor
>> > 0x7ff3afcafa20:   Type{0x0000002b} , name = "parseint_nocheck",
>> clang_type = 0x00007ff3ab548df0 void (void)
>> > 0x7ff3afca93e0:   CompileUnit{0x00000000}, language =
>> "Language(language = 0xafca93e0)", file = './string.jl'
>> > 0x7ff3afcafe20:     Function{0x0000002b}, mangled =
>> julia_parseint_nocheck;18749, type = 0x7ff3afcafa20
>> >
>> > even though the section seems to be loaded correctly:
>> >
>> > Sections for 'JIT(0x7fc4230f4e00)(0x00007fc4230f4e00)' (x86_64):
>> >   SectID     Type             Load Address
>> File Off.  File Size  Flags      Section Name
>> >   ---------- ---------------- ---------------------------------------
>>  ---------- ---------- ---------- ----------------------------
>> >   0x00000100 container        [0x0000000112efccf8-0x0000000112f5f8fb)*
>> 0x000003b0 0x00000950 0x00000000 JIT(0x7fc4230f4e00).__TEXT
>> >   0x00000001 code             [0x0000000112f5f1c0-0x0000000112f5f8fb)
>>  0x000003b0 0x0000073b 0x80000400 JIT(0x7fc4230f4e00).__TEXT.__text
>> >   0x00000009 eh-frame         [0x0000000112efccf8-0x0000000112efcd68)
>>  0x00000c90 0x00000070 0x6800000b JIT(0x7fc4230f4e00).__TEXT.__eh_frame
>> >   0x00000200 container        [0x0000000000000784-0x0000000112efce75)*
>> 0x00000aeb 0x00000160 0x00000000 JIT(0x7fc4230f4e00).__DWARF
>> >   0x00000002 dwarf-info       [0x0000000112efcd68-0x0000000112efcdb1)
>>  0x00000aeb 0x00000049 0x02000000 JIT(0x7fc4230f4e00).__DWARF.__debug_info
>> >   0x00000003 dwarf-abbrev     [0x00007fc4230f5934-0x00007fc4230f595f)
>>  0x00000b34 0x0000002b 0x02000000 JIT(0x7fc4230f4e00).__DWARF.__debug_abbrev
>> >   0x00000004 dwarf-line       [0x0000000112efcdc9-0x0000000112efce75)
>>  0x00000b5f 0x000000ac 0x02000000 JIT(0x7fc4230f4e00).__DWARF.__debug_line
>> >   0x00000005 dwarf-str        [0x00007fc4230f5a0b-0x00007fc4230f5a4b)
>>  0x00000c0b 0x00000040 0x02000000 JIT(0x7fc4230f4e00).__DWARF.__debug_str
>> >   0x00000006 dwarf-loc
>> 0x00000c4b 0x00000000 0x02000000 JIT(0x7fc4230f4e00).__DWARF.__debug_loc
>> >   0x00000007 dwarf-ranges
>>  0x00000c4b 0x00000000 0x02000000 JIT(0x7fc4230f4e00).__DWARF.__debug_ranges
>> >   0x00000300 container        [0x0000000112efce80-0x0000000112efcec0)*
>> 0x00000c50 0x00000040 0x00000000 JIT(0x7fc4230f4e00).__LD
>> >   0x00000008 regular          [0x0000000112efce80-0x0000000112efcec0)
>>  0x00000c50 0x00000040 0x02000000 JIT(0x7fc4230f4e00).__LD.__compact_unwind
>> >
>> > (the relocated address is
>> >
>> > julia> datapointer(filter(s->s.sectname == "__debug_info",sects)[1])
>> > Ptr{Uint8} @0x0000000112efcd68
>> >
>> > )
>> >
>> > so it seems like despite knowing the correct load address for the
>> __debug_info section, it's still somehow picking up on the old addresses.
>> I'll keep looking, but if something springs to mind, please let me know.
>> >
>> >
>> >
>> >
>> >
>> > On Mon, Jun 2, 2014 at 11:47 AM, Keno Fischer <
>> kfischer at college.harvard.edu> wrote:
>> > I didn't get to work on this more last week, but I'll look at
>> incorporating that suggestion.
>> >
>> > The other question of course is how to do this in LLDB. Right, now what
>> I'm doing is going through and adjusting the load address of every leaf in
>> the section tree. That basically works and gets me backtraces with the
>> correct function names and the ability to set breakpoints at functions in
>> JITed modules. What it doesn't get me yet is line numbers. I suspect that
>> is because the DWARF still refer to the old addresses. I thought
>> relocations should take care of that, but apparently they don't so I'll
>> have to look at whether to solve this in LLDB or in LLVM. Suggestions are
>> most welcome.
>> >
>> >
>> >
>> > On Wed, May 28, 2014 at 12:53 PM, Greg Clayton <gclayton at apple.com>
>> wrote:
>> >
>> > > On May 28, 2014, at 8:57 AM, Keno Fischer <
>> kfischer at college.harvard.edu> wrote:
>> > >
>> > > Hello,
>> > >
>> > > I'm finally getting back to getting JIT debugging work for MCJIT.
>> This has worked for ELF for a while in LLVM and support in lldb was added
>> in January (for ELF). I'm now trying to add support for Mach-O and would
>> appreciate some feedback (though I'm fighting my way through learning the
>> format, I'm still just a novice).
>> > >
>> > > My current patchset for llvm is here:
>> https://gist.github.com/loladiro/8d909ddd04e6d7e9a5d0 . I have a
>> corresponding patch for lldb and I basically got this working (modulo line
>> table information, though I'm sure I'm doing something stupid in lldb here).
>> > > The basic approach is to, when a section gets allocated rewrite the
>> sections `addr` and update every symbols `n_value` correspondingly. This is
>> very much in line with what is done for ELF, but I'm not sure if it's the
>> right approach, so I'd appreciate if somebody who has more experience with
>> Mach-O could look at the above patch and give some feedback. If this
>> approach looks sane in general, I'll finish up and post both the LLVM and
>> the LLDB patch for formal review.
>> >
>> > The one thing you might want to look into is the n_value only needs to
>> be updated "if ((N_TYPE & n_type) == N_SECT)" (the symbol is in a section
>> and therefore is has a address value). Other symbols have values that
>> usually don't need to be modified. You might also need to watch out for
>> absolute symbols (if ((N_TYPE & n_type) == N_ABS)) as there are a few that
>> sometimes don't claim to be a symbol that has a valid address, but they
>> actually do point to an address. The symbol named "mach_header" is one such
>> absolute symbol.
>> >
>> > If this is all new code, get it as close as you can and then we can
>> work the kinks out once it is in the codebase.
>> >
>> > Greg
>> >
>> >
>>
>>
>
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