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

[Nathan Chancellor <natechancellor@gmail.com>]

On Fri, Feb 21, 2020 at 11:21:44PM -0800, Fangrui Song wrote:
> 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@alum.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/ : { *(*) }
>   }

Thanks for the clarity. With your suggestion (diff below), I see the
following error:

arch/x86/boot/compressed/vmlinux: no symbols
ld.lld: error: undefined symbol: ZO_input_data
>>> referenced by arch/x86/boot/header.o:(.header+0x59)

ld.lld: error: undefined symbol: ZO_z_input_len
>>> referenced by arch/x86/boot/header.o:(.header+0x5D)
make[3]: *** [../arch/x86/boot/Makefile:108: arch/x86/boot/setup.elf]

It seems like the section still isn't being added?

Cheers,
Nathan

diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
index 12a20603d92e..a6c3c7440c27 100644
--- a/arch/x86/boot/compressed/vmlinux.lds.S
+++ b/arch/x86/boot/compressed/vmlinux.lds.S
@@ -74,6 +74,10 @@ SECTIONS
 	. = ALIGN(PAGE_SIZE);	/* keep ZO size page aligned */
 	_end = .;
 
+	.shstrtab : {
+		*(.shstrtab)
+	}
+
 	/* Discard all remaining sections */
 	/DISCARD/ : {
 		*(*)