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

[Fangrui Song]

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@xxxxxxxxxxxx>
> > > > ---
> > > >  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/ : { *(*) }
  }