Re: [PATCH v3 05/20] kbuild: only emit vmlinux relocations when required

[Kees Cook]

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) <ljs@xxxxxxxxxx>
> ---
>  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 <kees@xxxxxxxxxx>

-- 
Kees Cook