Re: [PATCH v2] RISC-V: Fix IFUNC resolver cannot access gp pointer

[Kito Cheng <kito.cheng@gmail.com>]

On Thu, Oct 31, 2024 at 2:29 AM Yangyu Chen <cyy@cyyself.name> wrote:
>
>
>
> > On Oct 30, 2024, at 19:09, Florian Weimer <fweimer@redhat.com> wrote:
> >
> > * Kito Cheng:
> >
> >> Hi Florian:
> >>
> >> Thanks for your explanation! one more question around the
> >> HIDDEN_VAR_NEEDS_DYNAMIC_RELOC,
> >> RISC-V don't set that to 1 but seems to work well so far...?
> >> Is it because the GP register is limited to executable so that we are
> >> safe without that?
> >
> > Yes, exactly.  The dynamic linker doesn't use the GP register for its
> > own purposes, and it doesn't impact code generation for the dynamic
> > linker itself.
> >
> > However, the way GP setup is currently handled is highly problematic
> > because the ABI does not give us a proper way to set it up correctly.
> > The use of a dynamic symbol is too fragile and seems to have been added
> > as an afterthought.  This should be a dynamic tag in the main
> > executable.  If we had that, we could set GP in all relevant calls to
> > application code (of course this assumes that applications do not use GP
> > as a global register variable for a different purpose, but hopefully
> > that's not the case).
> >
> > Thanks,
> > Florian
>
> Thanks for clarifying that!
>
> I proposed an idea to bypass the GP initialization by restricting
> the use of the norelax attribute for all callees of IFUNC. In this
> case, the linker will retain auipc + load/store instructions to
> access global variables based on pc-relative address mode instead
> of relaxing them to a single load/store at a +- 2KB offset from the
> GP.
>
> I'm not sure if this solution is the right way to go. Are there any
> potential edge cases that I might have missed?

That seems the right way to me :)


>
> Thanks,
> Yangyu Chen
>