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

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Message-ID: <20230921192253.GA39480@noisy.programming.kicks-ass.net>
Date:   Thu, 21 Sep 2023 21:22:53 +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 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:

#define OBJTOOL_ANNOTATE(type)				\
	"999:\n\t"					\
	".pushsection .discard.objtool_annotate\n\t"	\
	".byte 999b\n\t"				\
	".byte " __stringify(type) "\n\t"		\
	".popsection\n\t"

And since we only read the relocation and don't care for the actual
result that would actually work just fine.

Anyway, thanks for bearing with me.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>