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

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Message-ID: <20200222072144.asqaxlv364s6ezbv@google.com>
Date:   Fri, 21 Feb 2020 23:21:44 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Michael Matz <matz@...e.de>
Subject: Re: [PATCH 2/2] x86/boot/compressed: Remove unnecessary sections
 from bzImage

On 2020-02-22, Nathan Chancellor wrote:
>On Sat, Feb 22, 2020 at 07:55:21AM +0100, Borislav Petkov wrote:
>> On Fri, Feb 21, 2020 at 10:08:45PM -0700, Nathan Chancellor wrote:
>> > On Thu, Jan 09, 2020 at 10:02:18AM -0500, Arvind Sankar wrote:
>> > > Discarding the sections that are unused in the compressed kernel saves
>> > > about 10 KiB on 32-bit and 6 KiB on 64-bit, mostly from .eh_frame.
>> > >
>> > > Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
>> > > ---
>> > >  arch/x86/boot/compressed/vmlinux.lds.S | 5 +++++
>> > >  1 file changed, 5 insertions(+)
>> > >
>> > > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
>> > > index 508cfa6828c5..12a20603d92e 100644
>> > > --- a/arch/x86/boot/compressed/vmlinux.lds.S
>> > > +++ b/arch/x86/boot/compressed/vmlinux.lds.S
>> > > @@ -73,4 +73,9 @@ SECTIONS
>> > >  #endif
>> > >  	. = ALIGN(PAGE_SIZE);	/* keep ZO size page aligned */
>> > >  	_end = .;
>> > > +
>> > > +	/* Discard all remaining sections */
>> > > +	/DISCARD/ : {
>> > > +		*(*)
>> > > +	}
>> > >  }
>> > > --
>> > > 2.24.1
>> > >
>> >
>> > This patch breaks linking with ld.lld:
>> >
>> > $ make -j$(nproc) -s CC=clang LD=ld.lld O=out.x86_64 distclean defconfig bzImage
>> > ld.lld: error: discarding .shstrtab section is not allowed
>>
>> Well, why is it not allowed? And why isn't the GNU linker complaining?
>
>No idea, unfortunately I am not a linker expert and the patch that
>changes this in lld does not really explain why it adds this
>restriction:
>
>https://github.com/llvm/llvm-project/commit/1e799942b37d04f30b73f6a9e792d551dadafeea
>
>CC'ing Fangrui as I don't know George's email and he is usually
>responsive to ld.lld issues/questions.
>
>Cheers,
>Nathan

In GNU ld, it seems that .shstrtab .symtab and .strtab are special
cased. Neither the input section description *(.shstrtab) nor *(*)
discards .shstrtab . I feel that this is a weird case (probably even a bug)
that lld should not implement.

In GNU ld, the following is not useful, while lld will discard the
synthesized .strtab as requested:

   /DISCARD/ : { *(.strtab) }

I think it is better making the intention (retaining .shstrtab)
explicit, by adding a .shstrtab beside /DISCARD/ :

   SECTIONS {
     ...
     .shstrtab : { *(.shstrtab) }
     /DISCARD/ : { *(*) }
   }