Multiple relocation in ELF

[Michael Eager]

Richard Earnshaw wrote:
> On Mon, 2006-07-24 at 19:26, Michael Eager wrote:
>> Alan Modra wrote:
>>> On Mon, Jul 24, 2006 at 04:22:41PM +0530, Santosh wrote:
>>>> ADDIU r2, RELOCATABLE_SYMBOL + RELOCATABLE_SYMBOL 
>>>>
>>>> In ELF, we can emit relocation information for only one symbol having
>>>> its symbol index, offset and relocation value to apply. But when I try
>>>> to give more than one relocatable symbol, it cannot be represented by
>>>> ELF format. 
>>> ELF can support multiple symbols in a relocation expression.  The trick
>>> is to apply multiple suitably defined relocations to the one location.
>> Alan --
>>
>> Can you give an example of how this might be done?  I don't think that
>> it is possible to apply two relocations to a single location in ELF.
>>
>> I can think of one way that this might work, but it's problematic:
>>
>> If the relocations are in REL format, the addend is saved in the
>> object file.  (If RELA is used, the addend is in the reloc entry.)
>> In either case, it seems that the result value from the first
>> relocation will computed and stored in the output, then overwritten
>> by the computation of the second relocation.  The only way that
>> these two relocations could be composed is if REL format is used,
>> and the result of the relocation overwrote the value in the *input*
>> object file, so that the result value from the first relocation
>> is used as the addend of the second.
>>
>> If this is the scheme, I don't think that this behavior is defined
>> by the ELF spec.
> 
> The ELF spec is pretty specific on this issue.  It's known as a composed
> sequence
> 
> - Multiple relocations on the same place must all be of the same type
> (all REL or all RELA).
> - Composed relocations are processed in order in the table
> - The initial addend for the first relocation in the sequence is
> extracted as normal
> - Subsequent relocations use the result of the previous relocation
> calculation as the initial addend value.
> 
> The result is only written back at the end of the composed sequence.
> 
> Note, however, that since all relocations are machine specific the rules
> can be varied somewhat.  In particular a relocation code can be
> specified to terminate a sequence or to start a new one.

Thanks.  You're right -- it is specified in the ELF 4.1 draft.


-- 
Michael Eager	 eager@eagercon.com
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