Re: PPC relocs in shared libraries

[Alan Modra <amodra at bigpond dot net dot au>]

On Thu, Sep 26, 2002 at 03:39:00PM -0400, Daniel Jacobowitz wrote:
> So we're looking at RELA sections.  RELA does not use the in-section
> data, does it?

RELA relocs shouldn't, but see eg. current glibc
sysdeps/ia64/dl-machine.h:elf_machine_rela_relative  :-(

>  But readelf goes on to do:
> 
>               compunit.cu_abbrev_offset += rp->r_addend;
> 
> Which then will apply the value twice, or to garbage, depending on what
> version of binutils you happen to be using.  So I fixed that in my
> local copy.

Needs committing. :)

> Then there are dozens of other places in readelf which do not apply
> relocations.  GDB doesn't do it either unless you have a patch I
> haven't finished cleaning up yet (first posted four or five months ago,
> and I got busy elsewhere).  So I get things like this:
> 
> -     DW_AT_stmt_list   : 0     
> -     DW_AT_high_pc     : 0x24 36       
> -     DW_AT_low_pc      : 0 0   
> +     DW_AT_stmt_list   : 142   
> +     DW_AT_high_pc     : 0x1a04 6660   
> +     DW_AT_low_pc      : 0x19e0 6624   
> 
> -  Extended opcode 2: set Address to 0x0
> +  Extended opcode 2: set Address to 0x182c
> 
> These two have the same relocations; one happens to write the relocated
> values into the section and the other doesn't.

A consequence of section contents being ignored for RELA relocs.

Note that there is one good reason to apply the reloc to the
section contents as well as emitting a reloc and that is glibc's
ELF_MACHINE_REL_RELATIVE optimization.  Prior to seeing that one
I was inclined to say the section contents should be left at zero.
Not that this is particularly relevant to debug sections.

> So: Do we require full relocation processing on debug sections?  If so,
> I'm going to need to finish up the patch to provide an easy way for a
> client like readelf to relocate a section reliably.  At present we
> don't implement this relocation except for the location of the abbrev
> table.
> 
> Also, should this code be supporting REL relocations?  Or do the
> DWARF-2 sections always have RELA for some reason?

Well, x86 is REL and current gcc seems to support -gdwarf-2 on x86.

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre