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

[Yangyu Chen <cyy@cyyself.name>]

> 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?

Thanks,
Yangyu Chen