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

[Michael Matz <matz@suse.de>]

Hello,

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

> I'd be surprised if even that is totally correct.  ELF permits 
> relocations of a single 'place' to be chained provided that they are of 
> the same type (all REL or all RELA); not that I've ever seen that done 
> in practice.  When this is done the addend for the second and subsequent 
> relocations use the result of the previous relocation - they must be 
> processed in order.  Only the last relocation in a sequence writes a 
> value back to the location being relocated.
> 
> So really, we shouldn't need two tables, we just need to know where to 
> get the addend value from at the start of a chain.

Sort of, except ... it's not just for a chain of relocs (to the same 
place) steming from a single input file, but also about the similar 
situation when the same place is relocated multiple times due to 'ld -r' 
or pre-existing (non-zero) section content in an input file to a final 
link.  In this situation it's not easily determined what's the start of a 
reloc chain.

I still agree with you that abstractly multiple howto tables shouldn't be 
needed, because RELA relocs _always_ should ignore section content on 
read-in, and hence the reloc routines could simply special-case the 
RELA/REL distinction by just not looking at src_mask or partial_inplace 
for RELA relocs.  It's just that BFD isn't structured in this way.  
The internal representation of relocs always has an addend, no matter if 
it comes from read-in section contents or a RELA reloc, and the 
information of where it came from is lost (and section content isn't 
zeroed on read-in for REL relocs, when transferring the addend to the bfd 
reloc structure).  So, to get this all somewhat working correctly right 
now multiple howto entries are necessary (or special-casing target 
reloc-swap-in and swap-out routines).

I'm totally sure, once one tries to _really_ start mixing REL and RELA 
relocs in a link, at the latest when ld -r and/or other toolchains are 
involved, that a heap of bugs will be uncovered :-)

(And of course, it's always possible that REL relocations cannot always be 
used to achieve idempotence even in presence of 'ld -r', in which case 
RELA must be used.  The aarch64 psABI thankfully even spells this out.)


Ciao,
Michael.