Re: [RFC PATCH 00/11] Kernel FineIBT Support

Joao Moreira
2022-04-20T22:41:21Z

>>
>> If FineIBT needs it, I could reconsider. But I think there's a strong
>> case to be made that the linker should be doing that instead.
>
> That sounds reasonable to me (and reminds me of linker relaxation).
> Joao, can you please work with Fangrui (LLD) and HJ (GNU binutils) to
> determine how feasible this would be? I assume code outside the kernel
> might enjoy such an optimization, too. When that's the case, then it
> probably makes more sense to "upstream" such "optimizations" from the
> kernel-specific objtool into the toolchains.

Alright, these are the greenlights I was hoping for.

I went quickly into this with HJ and he mentioned that it should be
doable in the linker, and that he has a patch for it in gcc (for local
function, from what I could see):
https://gcc.gnu.org/pipermail/gcc-patches/2022-February/590832.html

If @Fangrui is fine with it, I would like to try implementing this
myself in lld (I'm still learning a lot about lld and having an actual
problem to solve is the kind of fuel I need). Should take me a while,
but I think this is not urgent, right? I can also go ahead and replicate
HJ's gcc patch into clang, so we can also handle the local functions
within the compiler (I think this makes a lot of sense).

Once we have these in, I'll revisit FineIBT and extend the features to
handle the FineIBT instrumentation. Hopefully we'll be released from
needing objtool (famous last words?!).

This sounds like a plan, but I'm ofc open to suggestions or different
ideas/plans.

Tks,
Joao