linux-kernel - Re: [PATCH] x86/speculation, objtool: Use absolute relocations for annotations

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Date:   Thu, 21 Sep 2023 21:31:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Fangrui Song <maskray@...gle.com>
Cc:     x86@...nel.org, Josh Poimboeuf <jpoimboe@...nel.org>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH] x86/speculation, objtool: Use absolute relocations for
 annotations

On Thu, Sep 21, 2023 at 09:22:53PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 21, 2023 at 10:36:27AM -0700, Fangrui Song wrote:
> 
> > Well, only if the clever assembler doesn't support 32-bit absolute
> > relocation for a 64-bit architecture.
> > I don't know such an architecture. In addition, as long as the
> > architecture intends to support DWARF32, it has to support 32-bit
> > absolute relocations for a 64-bit architecture.
> 
> Ooh... my bad. For some reason I thought that absolute meant native word
> size. But you already mentioned R_X86_64_32 (and I failed to check) and
> that is indeed an absolute (S+A) relocation of 32bit (dword) size.
> 
> And apparently we also have R_X64_64_16 and R_X86_64_8, which would even
> allow something like:

Hurm, just checked PPC/PPC64 and ARM64 and they only do 16bit (and up)
absolute relocations, not the single byte form.

So if I want to keep this portable, I suppose I shouldn't go smaller.