[PATCH/RFC] kernel-defaults.mk: get rid of BuildID

Felix Fietkau nbd at nbd.name
Tue Apr 5 08:05:43 PDT 2022

On 05.04.22 03:14, Daniel Golle wrote:
> When building the Linux kernel, the linker generates a hash of all
> versions of tools involved in a build called BuildID in ELF header.
> This breaks reproducibility accross different buildhosts eventhough
> OpenWrt builds the toolchain from source -- the build-id hash ends up
> to be the only thing which differs in the resulting builds.
> The cause is most likely a result of the build hosts' architectures,
> OSs and standard C libraries being different.
> While in theory it is true that tools may produce a different output
> depending on archtecture, OS and libc of the buildhost, in practice
> this is (fortunately) hardly ever the case and hence it contradicts
> ld(1) which states:
>   'The "md5" and "sha1" styles produces an identifier that is always
>    the same in an identical output file, but will be unique among all
>    nonidentical output files.'
> (the kernel is using sha1 style build-id, rebuilding the kernel on a
> different buildhost results in everything being identical **except**
> for the build-id)
> Hence, to achieve reproducible builds we will either have to resort to
> identical containers/VMs for building or get rid of the BuildID hash
> alltogether (or use a different build-id style)
> At this point, this seems to be what Debian is doing as well in order
> to achieve reproducible kernel builds, see[1].
> [1]: https://wiki.debian.org/SameKernel#How_this_works
> Signed-off-by: Daniel Golle <daniel at makrotopia.org>
> diff --git a/include/kernel-defaults.mk b/include/kernel-defaults.mk
> index 1e82f7d739..9c8d5fbe97 100644
> --- a/include/kernel-defaults.mk
> +++ b/include/kernel-defaults.mk
> @@ -46,6 +46,7 @@ else
>   	if [ -d $(LINUX_DIR)/user_headers ]; then \
>   		rm -rf $(LINUX_DIR)/user_headers; \
>   	fi
> +	$(SED) -i $(LINUX_DIR)/Makefile  -e 's/--build-id=.*/--build-id=none/g'
I don't like running sed on the linux Makefile, as this interferes with 
creating patches for it. I think it would be better to simply override 
KBUILD_LDFLAGS_MODULE on the kernel/module build command line.

- Felix

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