Re: Multiple relocation in ELF

[Michael Eager <eager@eagercon.com>]

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.

-- 
Michael Eager	 eager@eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077