[MIPS] Drop .dynsym symbol ordering requirement and nullify DT_GNU_XHASH

[Rich Felker]

On Wed, Jan 29, 2020 at 06:59:01PM +0000, Maciej W. Rozycki wrote:
> On Sat, 28 Dec 2019, Fangrui Song wrote:
> 
> > As I understand it, the benefits of the symbol ordering requirement come from
> > the following:
> 
>  There are no benefits; the requirement comes from the GOT being relocated 
> implicitly (i.e. without the use of dynamic relocations, as it is usually 
> done, and based solely on dynamic entries providing ranges and then the 
> correlation between GOT entries and the corresponding dynsym entries), as 
> defined by the ELF MIPS psABI, long ago in mid 1990s when ELF was new and 
> people were still experimenting with it in various ways.
> 
>  You could lift the requirement, but it would be a significant ABI change, 
> requiring the addition of suitable dynamic relocations for GOT entries and 
> then updating toolchains and runtimes across the world.
> 
>  This would typically only be done with an otherwise major incompatible 
> change to the ABI, and it has actually been, for the nanoMIPS ISA.

I don't see how it requires any incompatible change. R_MIPS_REL32 is
valid for non-PLT GOT slots and R_MIPS_JUMP_SLOT is valid for PLT
ones.

Rich