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

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Message-ID: <CAFP8O3JpkWTXnxYxa45uANNMQ34g2VUPnfj7-oWn3oq9egUb-A@mail.gmail.com>
Date:   Thu, 21 Sep 2023 10:36:27 -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 10:19 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Sep 21, 2023 at 09:26:43AM -0700, Fangrui Song wrote:
>
> > I do not see why absolute things need 12 byte entries.
> > We can freely use `.long .text.foo` even in ELFCLASS64 object files.
> > There is no risk of overflow (the ultimate link .text.foo may have an
> > address of 0xffffffff........) since the section will be discarded.
>
> And you're sure no toolchain is going to be clever and tell me that the
> absolute relocation will want to be 8 bytes and does not fit in the 4
> bytes allotted?
> Because clearly that is something some clever assembler is going to
> complain about any day now.

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.

Of course, I cannot predict new toolchains for new architectures from
doing insensible thing, but
DWARF32 support and other metadata section uses are pretty strong
arguments for them to add a 32-bit absolute relocation type.

Note: some .discard.* sections before commit 1c0c1faf5692 ("objtool:
Use relative pointers for annotations") used
absolute relocations.

-- 
宋方睿