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

[Florian Weimer <fweimer@redhat.com>]

* Kito Cheng:

> HI Florian:
>
> On Wed, Oct 30, 2024 at 5:22 PM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * Kito Cheng:
>>
>> > Do you remember if there are any other issues that need to be resolved
>> > other than the GP issue?
>>
>> In certain cases, IFUNC resolvers can be invoked when the object that
>> provides has not been relocated yet.  That's impossible to address
>> from within the IFUNC resolver.  (For the GP issue, the IFUNC resolver
>> could just assume that GP isn't defined, I think.)
>
> Hmmmm, I am a little surprised that since we are kinda learning the
> function multi-version (FMV) from AArch64[1] and x86[2], they both
> will reference global variables during the FMV IFUNC resolver, or is
> it just bad design from your point of view?

The default code model for AArch64 and x86-64 uses more or less direct
PC-relative addressing for local data references.  For this reason, that
data is in libgcc.a, with hidden visibility.  On x86-64, there is a
large code model that requires a register like GP, but every function is
required to set it up if it needs it (including PLT stubs, and %r11 is
reserved for that purposes).

The use of the GP register makes RISC-V a HIDDEN_VAR_NEEDS_DYNAMIC_RELOC
target in glibc's terms, and those can't use any data references in
IFUNC resolvers (unless you ban cyclic references, LD_PRELOAD etc.).

Thanks,
Florian