[LLVMdev] MachO non-external X86_64_RELOC_UNSIGNED

[Nick Kledzik]

On Jun 8, 2014, at 8:59 PM, Keno Fischer <kfischer at college.harvard.edu> wrote:

> 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 
In ld64 relocations are parsed into “Fixups”.  A Fixup is a location to fix up and a value/expression of what to set it to.  All sections are parsed up into “atoms”.  A location is an atom and an offset (within the atom).  The expression for a fixup is a target atom and optional addend (e.g. &foo + 10).  

For X86_64_RELOC_UNSIGNED when r_extern=1, the location is the atom containing the r_address (offset in the section), and the expression is the atom corresponding to r_symbolnum plus the added that is the current content of the location.  In the JIT case where you are trying to prepare a object file for execution, that boils down to adding the final address of the r_symbolnum atom to the current content (addend) in the fixup location.

For X86_64_RELOC_UNSIGNED when r_extern=0, the fixup location is the atom containing the r_address (offset in the section), and the expression is whatever atom+offset the current contents of location points to in that object file.  In the JIT case, the boils down to adjusting the location by the amount the target atom slid from its address in the object file to its final address for execution.  For instance, if the location contains 0x00000218 which points into section __DATA,__data (0x200 thru 0x280) and the __data section winds up at address 0x100001000 at runtime, then the location needs to have 0x100000E00 added to it (0x100001000 - 0x200).

-Nick


> 
> 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
> 
> 
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