Re: fake symbols to aid debugging

Daniel Jacobowitz
2003-07-29T03:11:08Z

On Tue, Jul 29, 2003 at 12:02:58PM +0930, Alan Modra wrote:
> PowerPC64 (and other architechures) use linker stubs for various things,
> most notably to make calls to functions in shared objects. Typical
> disassembly looks like:
>
> the stub
> 7ea138: 3d 82 00 01 addis r12,r2,1
> 7ea13c: f8 41 00 28 std r2,40(r1)
> 7ea140: e9 6c 11 88 ld r11,4488(r12)
> 7ea144: e8 4c 11 90 ld r2,4496(r12)
> 7ea148: 7d 69 03 a6 mtctr r11
> 7ea14c: e9 6c 11 98 ld r11,4504(r12)
> 7ea150: 4e 80 04 20 bctr
>
> call to stub
> 87d6f0: e8 7e 80 90 ld r3,-32624(r30)
> 87d6f4: 4b f6 ca 45 bl 7ea138 <._init+0xe9b8>
> 87d6f8: e8 41 00 28 ld r2,40(r1)
>
> Note the meaningless symbol given as target for the "bl". Now, if you
> know enough about the ABI, it's possible to figure out which function is
> being called, but that's a pain. In the case of PowerPC64 you need to
> a) Find the value of r2 by looking at the calling function's opd entry.
> (with multiple GOT/TOC r2 is not fixed).
> b) Perform the calculation done by the stub to find the plt entry
> address.
> c) Search the relocs to find the one for this particular plt entry.
> The symbol on the reloc gives the function name.
>
> I waste enough time doing this that I figure it's worth doing something
> about it. My first idea, already implemented, was to have the linker
> emit extra symbols to identify the stubs. This works well but bloats
> the symbol table and isn't on by default. A better idea would be to
> create the stub symbols on the fly. With that in mind, I propose to
> add two new bfd functions
>
> long bfd_get_fake_symtab_upper_bound (bfd *abfd);
> long bfd_canonicalize_fake_symtab (bfd *abfd, asymbol **buf);
>
> analogous to bfd_get_symtab_upper_bound and bfd_canonicalize_symtab.
>
> Comments?

I may regret suggesting this... outgrowth of a similar conversation
with Jim a month or so ago. Would it be feasible for the linker to
generate a DWARF compilation unit for these stubs, and fill it with
DW_AT_trampoline markers? One of the forms of DW_AT_trampoline is just
a string which gives the function name, which is all you'd want from
what you said above.

As link-time DWARF generation goes this is a pretty simple one; you'd
only need one form.

--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer