Read-only data in ELF libraries may be remapped writable at runtime (upcoming NDSS'17 paper)

[Richard Henderson]

On 12/27/2016 09:48 PM, Cary Coutant wrote:
> Nevertheless, I'd like to suggest that we're fixing the wrong problem.
> Copy relocations have many problems, and I think we should focus more
> on simply eliminating them entirely. The paper talks about that, but
> only to the extent of using -fpic, which is overkill. It talks about
> how Windows and macOS deal with the problem, but it overlooks some
> other Unix platforms like HP-UX that do not use copy relocations at
> all.
>
> On HP-UX, the compiler can be told to assume that any undefined data
> symbol may be external to the load module, and generates an indirect
> reference through the linkage table (aka, GOT). The linker is able to
> rewrite the indirect reference as a direct reference if it determines
> that the reference is within the same load module. The rewritten
> sequence involves an extra register copy, but eliminates a much more
> expensive memory load. The compiler can also make the opposite
> assumption, and uses "#pragma external" to flag symbols that are known
> to be external and should use the indirect sequence. All system header
> files on HP-UX are decorated with these pragmas for all exported
> variables.
>
> A similar technique could be used on most other platforms that today
> require copy relocations.

Indeed, this is exactly what I did for alpha-elf, back in 1996.

It's a bit of a shame that we didn't do the same thing for one of the several 
reincarnations of ppc64(le)-elf, nor the initial aarch64.  Both have ISAs and 
ABIs that would not have suffered at all for having an indirect reference that 
relaxes to a direct reference iff the symbol turns out to be local.


r~