[LLVMdev] MachO non-external X86_64_RELOC_UNSIGNED

Keno Fischer
Mon, 09 Jun 2014 03:59:06 -0000

Hello everybody,

I would like some insights on the semantics of the X86_64_RELOC_UNSIGNED
relocation type. When r_extern=1, the semantics seem pretty clear:

Let x be a pointer to r_offset of appropriate size given by r_size, then
*x += addr_of_symbol(r_symbolnum)

However, when r_extern=0 the correct behavior is not clear. By analogy with
the above, I would have expected

*x += addr_of_section(r_symbolnum)

but what LLVM implements is different. In RTDyld it implements

*x = (*x-addr_of_section(r_symbolnum)) + addr_of_section(r_symbolnum)

or equivalently

*x = *x

i.e. a noop. This works because llvm codegen also emits the absolute value
of the address. I am unsure what is intended and would appreciate some
clarification. A couple of points to consider:

1. I checked ld64 and as far as I can tell it doesn't consider non-external
X86_64_RELOC_UNSIGNED but does *x += addr_of_symbol(r_symbolnum)
regardless. That seems like a bug in ld64 to me because other relocations
in the same switch statement do check r_extern.

2. I implemented *x += addr_of_section(r_symbolnum) in LLVM and all tests
pass just fine

3. If the current implementation is correct r_symbolnum (and potentially
the entire relocation) basically meaningless, which could of course be
correct, but which is what originally caused me to look at this. If so I'd
appreciate an explanation as to why we need to have the relocation in the
first place.

That's all I could find on the subject. I hope somebody else knows more
than I.

Thanks,
Keno
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20140608/d966206f/attachment.html>