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

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Message-ID: <20200222070218.GA27571@ubuntu-m2-xlarge-x86>
Date:   Sat, 22 Feb 2020 00:02:18 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        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>, Fangrui Song <maskray@...gle.com>
Subject: Re: [PATCH 2/2] x86/boot/compressed: Remove unnecessary sections
 from bzImage

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