Re: Multiple relocation in ELF

[Richard Earnshaw <rearnsha at arm dot com>]

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.

R.