mach-o objfmt, amd64 relocation issues

[Peter Johnson]

On Fri, 5 Jan 2007, Henryk Richter wrote:
> Am 05.01.2007 um 18:32 schrieb Peter Johnson:
>> Great work so far!  Looking at reloc.h, it appears that the equivalent of 
>> R_X86_64_32 is X86_64_RELOC_SIGNED, which the header file claims is "for 
>> signed 32-bit displacement".
>
> That was my first thought as well. Unfortunately, the current linker insists 
> on X86_64_RELOC_SIGNED in conjunction with "r_pcrel=1", which is in turn 
> "movq _foo(%rip), %rdx".
> The (seemingly) absolute form "movq (_label),%rdx" is not covered by those 
> relocations, unless appropriate extra actions are taken. My first idea was 
> explicitly using "movq rax,[qword label]",
> but that form works only with the "rax" register. *sigh* x86.

Yes, Windows on x86-64 does much the same thing, basically requiring 
RIP-relative for 32-bit relocs, and only allowing absolute relocs for 
64-bit values (although if I recall, this is enforced more by the linker 
than the object format itself).  You can simply treat everything else as 
an error.  What this means is peppering your asm code with (%rip) or WRT 
rip, but that's the price we have to pay. :)

When it comes to the Yasm code, If you've not noticed yet, the yasm_value 
structure has an ip_rel member that indicates a RIP-relative relocation.

Peter