[PATCH v3] RISC-V: Fix IFUNC resolver cannot access gp pointer

[Florian Weimer]

* Palmer Dabbelt:

> On Mon, 06 Jan 2025 18:32:54 PST (-0800), cyy@cyyself.name wrote:
>> Ping again. It has already been 1 month after v3 patch was submitted.
>>
>> I think this patch is critical for RISC-V GCC Function multi-versioning
>> to work correctly, as GCC 15, which has this feature, is going to be
>> released.
>
> IIUC the conclusion here was to avoid global symbol resolution in
> IFUNCs and instead use the pre-resolved function hwprobe VDSO function
> pointer passed in via a register.  There was a long discussion on this
> during the hwprobe/IFUNC support patches, and IIRC one during the FMV
> patches -- though I remember accepting those and now I'm not sure why,
> so maybe I'm just forgetting something here?

As far as I understand it, at least in some cases, the gp register needs
to be set up for the main program even for local static/hidden symbol
access.  Unfortunately, the ABI does not provide a completely reliable
way for the dynamic linker to discover the gp value the main program
requires, which leads to the code we have now.  I think we should have
something that is more declarative than running a piece of assembler
code at the right time (the existing code runs too late, after IFUNC
resolver), and which does not depend on dynamic symbol resolution.

Thanks,
Florian