Re: [PATCH v3 3/3] vdso: Reject absolute relocations during build

[Nathan Chancellor <nathan@kernel.org>]

On Wed, Jun 11, 2025 at 11:22:14AM +0200, Thomas Weißschuh wrote:
> All vDSO code needs to be completely position independent.
> Symbol references are marked as hidden so the compiler emits
> PC-relative relocations. However there are cases where the compiler may
> still emit absolute relocations, as they are valid in regular PIC DSO code.
> These would be resolved by the linker and will break at runtime.
> This has been observed on arm64 under some circumstances, see
> commit 0c314cda9325 ("arm64: vdso: Work around invalid absolute relocations from GCC")
> 
> Introduce a build-time check for absolute relocations.
> The check is done on the object files as the relocations will not exist
> anymore in the final DSO. As there is no extension point for the
> compilation of each object file, perform the validation in vdso_check.
> 
> Debug information can contain legal absolute relocations and readelf can
> not print relocations from the .text section only. Make use of the fact
> that all global vDSO symbols follow the naming pattern "vdso_u_".
> 
> Link: https://lore.kernel.org/lkml/aApGPAoctq_eoE2g@t14ultra/
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120002
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

I ran this through a few different architectures with LLVM=1 and did not
see anything interesting.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  lib/vdso/Makefile.include | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/lib/vdso/Makefile.include b/lib/vdso/Makefile.include
> index cedbf15f80874d4bb27c097244bc5b11272f261c..04257d0f28c0ed324e31adbb68497181085752f8 100644
> --- a/lib/vdso/Makefile.include
> +++ b/lib/vdso/Makefile.include
> @@ -12,7 +12,13 @@ c-getrandom-$(CONFIG_VDSO_GETRANDOM) := $(addprefix $(GENERIC_VDSO_DIR), getrand
>  #
>  # As a workaround for some GNU ld ports which produce unneeded R_*_NONE
>  # dynamic relocations, ignore R_*_NONE.
> +#
> +# Also validate that no absolute relocations against global symbols are present
> +# in the object files.
>  quiet_cmd_vdso_check = VDSOCHK $@

I do not see VDSOCHK in the normal build log but I do see the check
being executed with V=1. That's obviously an outstanding issue but
figured it was worth mentioning.

>        cmd_vdso_check = if $(READELF) -rW $@ | grep -v _NONE | grep -q " R_\w*_"; \
>  		       then (echo >&2 "$@: dynamic relocations are not supported"; \
> +			     rm -f $@; /bin/false); fi && \
> +		       if $(READELF) -rW $(filter %.o, $(real-prereqs)) | grep -q " R_\w*_ABS.*vdso_u_"; \
> +		       then (echo >&2 "$@: absolute relocations are not supported"; \
>  			     rm -f $@; /bin/false); fi
> 
> -- 
> 2.49.0
>