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

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Message-ID: <CAFP8O3Kmep3zXNk3=s5qpMQ3-TnfEYxeCkLi3e-DTqQc=Fs4TQ@mail.gmail.com>
Date:   Thu, 21 Sep 2023 13:07:09 -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 12:22 PM Peter Zijlstra <peterz@...radead.org> 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:
>
> #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>

Thanks!  I assume that the patch can be picked up without me doing anything :)

Yes, x86 also supports 1-byte and 2-byte absolute relocation types,
which are missing from some other architectures.
If we want to optimize for RELA, we can use R_*_NONE by utilizing
`.reloc ., BFD_RELOC_NONE, .text.foo` (as mentioned in the commit
message).
(binutils 2.26 https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=740bdc67c057ee8012327420848eb134e1db4211;
I added the support to most LLVM targets in 2021).

However, this is just 4 byte per entry difference compared with the
large sizeof(Elf{32,64}_Rel), so not worth the trouble:)

-- 
宋方睿