[Buildroot] [PATCH] toolchain/toolchain-wrapper: explicitly set Build ID to none if BR2_REPRODUCIBLE

[Yann E. MORIN <yann.morin.1998@free.fr>]

Atharva, All,

On 2019-08-16 22:33 +0530, Atharva Lele spake thusly:
> Build ID is added to binaries at link time. Building in different output
> directories causes some packages to have different Build IDs, thus resulting in
> non-reproducibility.
> 
> Adding "-Wl,--build-id=none" fixes this issue by disabling setting of Build ID.
> 
> Diffoscope output for Build ID issue: https://gitlab.com/snippets/1886180/raw
> 
> After this patch, build is reproducible - i.e. diffoscope does not produce any
> output.
> 
> Signed-off-by: Atharva Lele <itsatharva@gmail.com>
> ---
>  toolchain/toolchain-wrapper.c  | 3 +++
>  toolchain/toolchain-wrapper.mk | 4 ++++
>  2 files changed, 7 insertions(+)
> 
> diff --git a/toolchain/toolchain-wrapper.c b/toolchain/toolchain-wrapper.c
> index 7a4b9c4007..f7f2a9ec97 100644
> --- a/toolchain/toolchain-wrapper.c
> +++ b/toolchain/toolchain-wrapper.c
> @@ -98,6 +98,9 @@ static char *predef_args[] = {
>  #if defined(BR_MIPS_TARGET_BIG_ENDIAN) || defined(BR_ARC_TARGET_BIG_ENDIAN)
>  	"-EB",
>  #endif
> +#ifdef BR2_REPRODUCIBLE
> +    "-Wl,--build-id=none",

Actually, I would have preferred that we do pass a reproducible value
instead of none, probably something based on SOURCE_DATE_EPOCH for
eaxmple:

    #ifdef BR2_BUILD_ID
        "-Wl,--build-id=" BR2_BUILD_ID,
    #endif

And then, below....

> +#endif
>  #ifdef BR_ADDITIONAL_CFLAGS
>  	BR_ADDITIONAL_CFLAGS
>  #endif
> diff --git a/toolchain/toolchain-wrapper.mk b/toolchain/toolchain-wrapper.mk
> index 970bde76a0..21fc08f3ee 100644
> --- a/toolchain/toolchain-wrapper.mk
> +++ b/toolchain/toolchain-wrapper.mk
> @@ -59,6 +59,10 @@ else ifeq ($(BR2_RELRO_FULL),y)
>  TOOLCHAIN_WRAPPER_ARGS += -DBR2_RELRO_FULL
>  endif
>  
> +ifeq ($(BR2_REPRODUCIBLE),y)
> +TOOLCHAIN_WRAPPER_ARGS += -DBR2_REPRODUCIBLE

... here, you'd set something like:

    ifeq ($(BR2_REPRODUCIBLE),y)
    TOOLCHAIN_WRAPPER_ARGS += -DBR2_BUILD_ID="$(BR2_BUILD_ID)"
    endif

and then in the main Makefile, in the BR2_REPRODUCIBLE condition (lines
518 and following), you'd need something like:

    ifeq ($(BR2_REPRODUCIBLE),y)
    [...]
    BR2_BUILD_ID = $(shell echo $(SOURCE_DATE_EPOCH) |sha256sum |cut -d ' ' -f 1)
    endif

Note: as per the docs, build-id can be any hex-string.

But before re-sending, please wait for feedback from others.

Regards,
Yann E. MORIN.

> +endif
> +
>  define TOOLCHAIN_WRAPPER_BUILD
>  	$(HOSTCC) $(HOST_CFLAGS) $(TOOLCHAIN_WRAPPER_ARGS) \
>  		-s -Wl,--hash-style=$(TOOLCHAIN_WRAPPER_HASH_STYLE) \
> -- 
> 2.22.0
> 
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