On Thu, Sep 17, 2026 at 10:41:50AM -0700, Kees Cook wrote:
> On Thu, Sep 17, 2026 at 05:06:15PM +0100, Lorenzo Stoakes (ARM) wrote:
> > A kernel build consists of more than one linking pass on vmlinux.o and
> > vmlinux, at minimum two, and with CONFIG_KALLSYMS and BTF enabled on x86-64
> > for example there are 5 such stages.
> >
> > For architectures that build their own relocation tables (x86, riscv, mips,
> > s390), vmlinux is linked with the --emit-relocs parameter specified.
> >
> > However, this is only required on the final vmlinux link.
> >
> > Symbol tables of trial links preceding it don't need it because they
> > already check that System.map matches kallsyms symbols on each build.
> >
> > GNU ld is slow at emitting relocation tables, so this results in a
> > reduction in build time.
> >
> > Whole build, 128-thread Threadripper 9980X, best of N runs:
> >
> > before after delta
> > -------------------------------
> > x86 defconfig, touch mm/vma.c, gcc 9.5s 9.1s -0.42s (-4%)
> > x86 defconfig, clean, gcc 30.1s 29.5s -0.59s (-2%)
> > x86 allmodconfig, touch mm/vma.c, gcc 37.2s 35.8s -1.4s (-4%)
> >
> > Note that this has little impact on LLVM ld which performs this operation
> > more efficiently.
> >
> > Assisted-by: LLM
> > Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]>
> > ---
> > Makefile | 2 +-
> > scripts/link-vmlinux.sh | 5 +++++
> > 2 files changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/Makefile b/Makefile
> > index 0f1b80100b47..dd4c10afbb7f 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -1263,7 +1263,7 @@ LDFLAGS_vmlinux +=
> > --orphan-handling=$(CONFIG_LD_ORPHAN_WARN_LEVEL)
> > endif
> >
> > ifneq ($(CONFIG_ARCH_VMLINUX_NEEDS_RELOCS),)
> > -LDFLAGS_vmlinux += --emit-relocs --discard-none
> > +LDFLAGS_vmlinux += --discard-none
> > endif
>
> Nit: I think a comment here is worth adding, like in link-vmlinux.sh.
> They're separate files, and it's kind of weird that ...NEEDS_RELOCS
> doesn't actually add --emit-relocs here unless you go find how
> --emit-relocs is used later.
Ack will add!
>
> >
> > # Align the architecture of userspace programs with the kernel
> > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> > index 970ca10f8fa9..ed5ff97c0667 100755
> > --- a/scripts/link-vmlinux.sh
> > +++ b/scripts/link-vmlinux.sh
> > @@ -96,6 +96,11 @@ vmlinux_link()
> > ldflags="${ldflags} ${wl}--strip-debug"
> > fi
> >
> > + # Only the final link actually requires the relocations.
> > + if [ "${output}" = "${VMLINUX}" ] && is_enabled
> > CONFIG_ARCH_VMLINUX_NEEDS_RELOCS; then
> > + ldflags="${ldflags} ${wl}--emit-relocs"
> > + fi
> > +
> > if [ -n "${generate_map}" ]; then
> > ldflags="${ldflags} ${wl}-Map=vmlinux.map"
> > fi
>
> But yeah, otherwise, another "don't generate something that doesn't get
> used" improvement. :)
Yes :) it's funny how simple a lot of these changes are
- Don't do work you don't need to
- Cache stuff so you don't have to look it up again (a variant on the first
one)
- Do other work at the same time
>
> Reviewed-by: Kees Cook <[email protected]>
Thanks!
>
> --
> Kees Cook
--
Cheers, Lorenzo
