[PATCH 1/3] x86: Initialize CPU info via IFUNC relocation [BZ 26203]

[Florian Weimer]

* H. J. Lu via Libc-alpha:

> X86 CPU features in ld.so are initialized by init_cpu_features, which is
> invoked by DL_PLATFORM_INIT from _dl_sysdep_start.  But when ld.so is
> loaded by static executable, DL_PLATFORM_INIT is never called.

I don't think that's accurate.  It's called from elf/dl-support.c in the
static case.  But I agree that it's too late in this case.

> Also x86 cache info in libc.o and libc.a is initialized by a
> constructor which may be called too late.  Instead, we should
> initialize x86 CPU features and cache info by initializing dummy
> function pointers via IFUNC relocation.

Why would ld put this special IRELATIVE relocation first?  I'm sorry,
but this looks very brittle.

I think there's an existing mechanism for this corner case,
ARCH_SETUP_IREL.  Could you use that?

Thanks,
Florian
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