[llvm-dev] Position independent code writes absolute pointer

[Tim Northover via llvm-dev]

Hi Gaier,

There's no way to do this automatically in LLVM at the moment. It
sounds kind of related to pointer compression techniques (also not
supported right now).

On Thu, 9 Jan 2020 at 08:14, Gaier, Bjoern via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Could it be possible to modify the code on the IR-Level to store PIC/offset address and not absolute address? I’m not familiar with the LLVM IR so I don’t know what is possible and how it effects the code at all.

It depends how much control you have over the code. You could
instrument code so that it converted all stores of pointers to be
relative to some fixed global (PC-relative doesn't work there because
it will be loaded at a different address, and "relative to the address
it's being stored to" would break memcpy). But that has some major
issues:

1. It's an ABI break, so you have to be able to recompile all code,
including any system libraries you make use of.
2. LLVM can only convert the pointers it knows about, so it would
still be broken by someone storing a pointer via an intptr_t cast and
probably other things I haven't thought of.
3. There probably isn't even a relocation for any statically
initialized pointers. You might be able to convert all of them to use
a dynamic module initializer instead though.
4. I'd expect debugging to go horribly wrong.

Cheers.

Tim.