Re: [RFC PATCH 1/2] x86/relocs: Improve diagnostic for rejected absolute references

[Ard Biesheuvel]

On Mon, 3 Feb 2025 at 10:40, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> > On Mon, 27 Jan 2025 at 17:57, Linus Torvalds
> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, 27 Jan 2025 at 03:43, Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
> > > >
> > > >   Absolute reference to symbol '.rodata+0x180' detected in .head.text (0xffffffff820cb4ba).
> > >
> > > Do we have any symbol name lookup logic anywhere?
> > >
> >
> > I can look into that. In this particular case, though, there is no
> > symbol to look up as it is a anonymous jump table generated by the
> > compiler. And the function name would be inaccurate too, as
> > snp_cpuid_postprocess() got inlined into its caller. But I guess with
> > the right DWARF data, at least the call site could be narrowed down a
> > bit better.
>
> So patch #2 is now upstream, but should I apply this diagnostic patch
> as-is, or will there be a -v2?
>

I'm looking into this. But give the points above, I'm reaching the
conclusion that producing a better diagnostic based solely on vmlinux
(which may be built without debug info) is intractable, and not even
the DWARF metadata will describe a compiler generated jump table using
a named ELF symbol.

So I am also looking into isolating the startup code like I did for
arm64 (and which has been adopted by RISC-V as well), but this is
rather hairy on x86 so it will take some time. But once that lands,
this diagnostic can be removed.

So I will leave it up to you to decide whether to merge this
improvement for now, or revert the diagnostic as you suggested before.
This code has already identified some issues that were subsequently
fixed, so it has already served its purpose.

As for the related changes: .head.text can be removed in its entirety
once this is all done, so no need to revert the linker script changes
at this point. (The startup code that only executes at boot on the BSP
can move into .init.text where it belongs)