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

[Fangrui Song]

On 2020-02-24, Arvind Sankar wrote:
> On Mon, Feb 24, 2020 at 02:17:03PM -0800, Fangrui Song wrote:
> > On 2020-02-24, Arvind Sankar wrote:
> > >On Mon, Feb 24, 2020 at 01:28:28PM -0800, Fangrui Song wrote:
> > >> Hi Michael, please see my other reply on this thread: https://lkml.org/lkml/2020/2/24/47
> > >>
> > >> Synthesized sections can be matched as well. For example, SECTIONS { .pltfoo : { *(.plt) }} can rename the output section .plt to .pltfoo
> > >> It seems that in GNU ld, the synthesized section is associated with the
> > >> original object file, so it can be written as:
> > >>
> > >>    SECTIONS { .pltfoo : { a.o(.plt) }}
> > >>
> > >> In lld, you need a wildcard to match the synthesized section *(.plt)
> > >>
> > >> .rela.dyn is another example.
> > >>
> > >
> > >With the BFD toolchain, file matching doesn't actually seem to work at
> > >least for .rela.dyn. I've tried playing around with it in the past and
> > >if you try to use file-matching to capture relocations from a particular
> > >input file, it just doesn't work sensibly.
> >
> > I think most things are working in GNU ld...
> >
> > /* a.x */
> > SECTIONS {
> >    .rela.pltfoo : { a.o(.rela.plt) }  /* *(.rela.plt) with lld */
> >    .rela.dynfoo : { a.o(.rela.data) } /* *(.rela.dyn) with lld */
> > }
>
> The file matching doesn't do anything sensible. If you split your .data
> section out into b.s, and update the linker script so it filters for
> b.o(.rela.data), .rela.dynfoo doesn't get created, instead the default
> .rela.dyn will contains the .data section relocation. If you keep the
> filter as a.o(.rela.data), you get .rela.dynfoo, even though a.o doesn't
> actually contain any .rela.data section any more.

I raised the examples to support my viewpoint "synthesized sections can
be matched, as well as input sections."

If there is really a need (rare, not recommended) to rename output
sections only consisting of synthesized sections (e.g. .plt .rela.dyn),
for linker portability, it is better using a wildcard for the input
filename pattern.

As another example, SECTIONS { /DISCARD/ : { *(.rela.*) } } discards synthesized .rela.*

> > % cat <<e > a.s
> >   .globl foo
> >   foo:
> >     call bar
> >   .data
> >   .quad quz
> > e
> > % as a.s -o a.o
> > % ld.bfd -T a.x a.o -shared -o a.so