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.
>
> # 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. :)
Reviewed-by: Kees Cook <[email protected]>
--
Kees Cook
