linux-kernel - Re: [PATCH 2/2] x86/boot/compressed: Remove unnecessary sections from bzImage

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Message-ID: <20200224221703.eqql5hrx4ccngwa5@google.com>
Date:   Mon, 24 Feb 2020 14:17:03 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Michael Matz <matz@...e.de>, Borislav Petkov <bp@...en8.de>,
        Nathan Chancellor <natechancellor@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 2/2] x86/boot/compressed: Remove unnecessary sections
 from bzImage

On 2020-02-24, Arvind Sankar wrote:
>On Mon, Feb 24, 2020 at 01:28:28PM -0800, Fangrui Song wrote:
>> Hi Michael, please see my other reply on this thread: https://lkml.org/lkml/2020/2/24/47
>>
>> Synthesized sections can be matched as well. For example, SECTIONS { .pltfoo : { *(.plt) }} can rename the output section .plt to .pltfoo
>> It seems that in GNU ld, the synthesized section is associated with the
>> original object file, so it can be written as:
>>
>>    SECTIONS { .pltfoo : { a.o(.plt) }}
>>
>> In lld, you need a wildcard to match the synthesized section *(.plt)
>>
>> .rela.dyn is another example.
>>
>
>With the BFD toolchain, file matching doesn't actually seem to work at
>least for .rela.dyn. I've tried playing around with it in the past and
>if you try to use file-matching to capture relocations from a particular
>input file, it just doesn't work sensibly.

I think most things are working in GNU ld...

/* a.x */
SECTIONS {
   .rela.pltfoo : { a.o(.rela.plt) }  /* *(.rela.plt) with lld */
   .rela.dynfoo : { a.o(.rela.data) } /* *(.rela.dyn) with lld */
}

% cat <<e > a.s
  .globl foo
  foo:
    call bar
  .data
  .quad quz
e
% as a.s -o a.o
% ld.bfd -T a.x a.o -shared -o a.so