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

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Message-ID: <20230921072655.GA14803@noisy.programming.kicks-ass.net>
Date:   Thu, 21 Sep 2023 09:26:55 +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 Tue, Sep 19, 2023 at 05:17:28PM -0700, Fangrui Song wrote:
> .discard.retpoline_safe sections do not have the SHF_ALLOC flag.  These
> sections referencing text sections' STT_SECTION symbols with PC-relative
> relocations like R_386_PC32 [0] is conceptually not suitable.  Newer
> LLD will report warnings for REL relocations even for relocatable links
> [1].
> 
>     ld.lld: warning: vmlinux.a(drivers/i2c/busses/i2c-i801.o):(.discard.retpoline_safe+0x120): has non-ABS relocation R_386_PC32 against symbol ''

What, why ?!? Please explain more.

> Switch to absolute relocations instead, which indicate link-time
> addresses.  In a relocatable link, these addresses are also output
> section offsets, used by checks in tools/objtool/check.c.  When linking
> vmlinux, these .discard.* sections will be discarded, therefore it is
> not a problem that R_X86_64_32 cannot represent a kernel address.
> 
> Alternatively, we could set the SHF_ALLOC flag for .discard.* sections,
> but I think non-SHF_ALLOC for sections to be discarded makes more sense.
> 
> Note: if we decide to never support REL architectures (e.g. arm, i386),

We have explicit support for REL (as opposed to RELA) architectures, so
I don't think we can do that.