lkml.org 
[lkml]   [2020]   [Feb]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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@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/ : { *(*) }
}

\
 
 \ /
  Last update: 2020-02-22 08:22    [W:0.245 / U:0.184 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site