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Re: Multiple relocation in ELF

From: Alan Modra <amodra at bigpond dot net dot au>
To: Michael Eager <eager at eagercon dot com>
Cc: Santosh <santosh at acmet dot com>, binutils at sourceware dot org
Date: Tue, 25 Jul 2006 09:21:50 +0930
Subject: Re: Multiple relocation in ELF
References: <000001c6af0f$4a9a8a50$eb00a8c0@santosh> <20060724140259.GE6872@bubble.grove.modra.org> <44C510D3.5020702@eagercon.com>

On Mon, Jul 24, 2006 at 11:26:27AM -0700, Michael Eager wrote:
> Alan Modra wrote:
> >ELF can support multiple symbols in a relocation expression.  The trick
> >is to apply multiple suitably defined relocations to the one location.
> 
> 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.

Suppose you want to calculate 32-bit expressions of the form

  (sym1 + addend) * sym2

then use two relocs R_XYZ_ADDR32 and R_XYZ_MULT32 at the same location,
with the MULT32 reloc appearing after the ADDR32 one.
R_XYZ_ADDR32 operates as per usual ELF relocs, calculating sym + addend
and storing to the section contents.  R_XYZ_MULT32 is defined to read
the section contents and multiply that value by sym, then store the
result back to the section.

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre

References:
- Multiple relocation in ELF
  - From: Santosh
- Re: Multiple relocation in ELF
  - From: Alan Modra
- Re: Multiple relocation in ELF
  - From: Michael Eager

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