Re: [PATCH] PR30437 aarch64: make RELA relocs idempotent

[Michael Matz <matz@suse.de>]

Hello,

On Thu, 25 May 2023, Richard Earnshaw (lists) wrote:

> Thanks for the detailed explanation.  That makes sense, overall.  However...
> 
> I didn't think we had separate relocation rules for REL/RELA format 
> relocs.  I know that we recommend RELA for aarch64, but how would an 
> object file with REL format relocs be handled if src_mask is always 
> zero?

Right now the aarch64 bfd backend only support RELA 
(elf_backend_may_use_rel_p is off).  elf64-mips is an example that 
supports both, and it has separate howto tables for rel and rela 
descriptors (e.g. mips_elf64_howto_table_rel and 
mips_elf64_howto_table_rela), with the appropriate 
src_mask/partial_inplace settings depending on rel/rela:

  HOWTO (R_MIPS_32,             /* type */
         ...
         true,                  /* partial_inplace */
         0xffffffff,            /* src_mask */
         0xffffffff,            /* dst_mask */

vs.

  HOWTO (R_MIPS_32,             /* type */
         ...
         false,                 /* partial_inplace */
         0,                     /* src_mask */
         0xffffffff,            /* dst_mask */

the rtype_to_howto backend routine appropriately switches between those 
depending on side-info passed in parameters.  (Whereas the name_lookup and 
type_lookup routines always use the rela tables and contain a FIXME 
comment to that effect :) ).  Various non-mips embedded targets try to 
also support this, but often only for read-in, not for generating both 
types of relocs, and so get away with only one howto table.  (I believe (!)
mips is the only one that actively tries to get REL+RELA support correct).


Ciao,
Michael.