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

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Message-ID: <7ddda233-99f7-468b-842d-8469f0a86e77@ghiti.fr>
Date: Thu, 12 Jun 2025 10:31:20 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Vincenzo Frascino <vincenzo.frascino@....com>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 llvm@...ts.linux.dev
Subject: Re: [PATCH v3 3/3] vdso: Reject absolute relocations during build

Hi Thomas,

On 6/11/25 11:22, 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@...utronix.de>
> ---
>   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 $@
>         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_"; \


This only works for arm64 relocations right? I can't find any *ABS* 
relocations in riscv elf abi 
(https://github.com/riscv-non-isa/riscv-elf-psabi-doc/releases/tag/v1.0).

Thanks,

Alex


> +		       then (echo >&2 "$@: absolute relocations are not supported"; \
>   			     rm -f $@; /bin/false); fi
>