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

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Message-ID: <CAFP8O3+_8H+fetuoqwwcfqJLNtYmpsWZhY2arD7HMuPmcN+X_g@mail.gmail.com>
Date:   Thu, 21 Sep 2023 00:58:13 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 12:26 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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.

This can be read as a pedantic warning from the linker.

A location relocated by an R_386_PC32 relocation in
.discard.retpoline_safe records an offset from the current location
(non-allocable) to an text symbol.
This offset is conceptually not suitable: in the ELF object file
format's model, the non-SHF_ALLOC section is not part of the memory
image, so
we cannot say that the offset from the non-memory thing to a text
symbol is a fixed value.

> > 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.
>


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