[LLVMdev] [lldb-dev] MCJIT Mach-O JIT debugging
Keno Fischer
kfischer at college.harvard.edu
Mon Jun 2 13:54:23 PDT 2014
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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