[LLVMdev] MachO non-external X86_64_RELOC_UNSIGNED

Thu Jun 12 21:07:25 PDT 2014

Adjusting ->offset doesn't work because it's an unsigned 32bit integer, but
the data may easily be moved to an address before the header of the object
file. I'm honestly out of ideas at this point (other than manually
iterating through every single relocation and adjusting it). I do have to
say, these semantics of X86_RELOC_UNSIGNED make it pretty JIT-unfriendly.
Unless somebody comes up with a clever idea (or we agree on a way to pass
the required info to the debugger out of band), I guess we'll have to live
with always relocating debug section :(.

On Thu, Jun 12, 2014 at 6:10 PM, Keno Fischer <kfischer at college.harvard.edu>
wrote:

> Realized my original reply accidentally didn't go to the mailing list (see
> below). I'm also still pondering the question of
> whether for jitting purposes setting ->addr or ->offset would be better.
>
>
>
> On Mon, Jun 9, 2014 at 9:46 PM, Keno Fischer <kfischer at college.harvard.edu
> > wrote:
>
>>
>>
>>
>> On Mon, Jun 9, 2014 at 9:30 PM, Nick Kledzik <kledzik at apple.com> wrote:
>>
>>>
>>> On Jun 9, 2014, at 6:01 PM, Keno Fischer <kfischer at college.harvard.edu>
>>> wrote:
>>>
>>> Also, may I ask what the semantics for X86_64_RELOC_SIGNED are with an
>>> r_extern=0 relocation?
>>>
>>> That is only used for 32-bit fixups such as in RIP-relative
>>> instructions.  The r_extern=0 case might occur if the instruction
>>> references something in a section that has no symbols.  The JIT would need
>>> to do an analogous update of adding to the fixup location the (32-bit
>>> signed) difference between the final runtime address minus the object file
>>> address of the start of the section containing the thing being referenced
>>> by the RIP relative instruction.
>>>
>>>
>> Ok, a non-external X86_64_RELOC_SIGNED doesn't make sense then since the
>> address would always be positive so the unsigned variant could just be
>> used.
>>
>>
>>>
>>> On Mon, Jun 9, 2014 at 8:50 PM, Keno Fischer <
>>> kfischer at college.harvard.edu> wrote:
>>>
>>>> Thank you for the explanation. Does that mean r_symbolnum is basically
>>>> redundant in that case?
>>>>
>>> It usually is not needed.  The r_symbolnum (which is the section index
>>> when r_extern=0) is needed when the target of the relocation is the start
>>> of end of a section.  For instance if section __foo ends at address 0x300
>>> and section __bar starts at sections 0x300 and the fixup location content
>>> points to 0x300, you don’t know which section it is pointing to without
>>> that r_symbolnum.  The sections may be split apart in the final execution
>>> layout, so which section it is referencing is important in that edge case.
>>>
>>>
>>>
>> Ah, hadn't considered that edge case, thanks!
>>
>>>  Also, let me ask you how to handle the following use case which is
>>>> somewhat related. Currently in MCJIT for MachO we are relocating all the
>>>> debug sections. Eventually (as ELF does), it would be good to avoid this.
>>>> However, this means that the debugger would have to handle relocations (as
>>>> lldb currently does for ELF). With this scheme it seems impossible to me to
>>>> adjust the vaddr of one section without adjusting the relocations that
>>>> point at it. Is my interpretation of that correct? I guess the best we can
>>>> do then is to to the relocations inline in the original copy of the object
>>>> file.
>>>>
>>> In darwin tools, we leave the debug info in the .o file.  lldb can find
>>> it there if it needs it.  To aid that, the linker generates “debug notes”
>>> in the final linked image which contain the paths of the original .o files.
>>>  These are STABS N_OSO symbol table entries.   Can you just ignore (not
>>> copy to execution space) the DWARF debug sections in MCJIT for darwin?
>>>
>>>
>> The way this works in ELF is that the vaddr in the object header is
>> adjusted to the vaddr of the relocated section. I mirrored this approach in
>> my pending patch to add MachO support (i.e. adjusting
>> (section_(64))->addr). This means that if we don't relocate the debug
>> section (i.e. don't copy it) then we'll have lost the information where the
>> section used to be. I am now wondering if there is a better approach. Maybe
>> by modifying (section_(64))->offset instead?
>>
>>
>>> -Nick
>>>
>>>
>>>
>>>> Also, I'm not sure who at Apple does documentation, but would it be
>>>> possible to include the gist of your response in the reference
>>>> documentation? It's basically impossible to discern the semantics just from
>>>> what's written there.
>>>>
>>>>
>>>> On Mon, Jun 9, 2014 at 7:19 PM, Nick Kledzik <kledzik at apple.com> wrote:
>>>>
>>>>>
>>>>> 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
>>>>> >
>>>>> >
>>>>> > _______________________________________________
>>>>> > LLVM Developers mailing list
>>>>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>>
>>>>>
>>>>
>>>
>>>
>>
>
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