linux-kernel - Re: [PATCH 2/2] vdso: Reject absolute relocations during build

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Message-ID: <CAASaF6yGT0pDythQ9nTcn5=MHmLYD=gCNVc6dFXhWUO_iXJXqA@mail.gmail.com>
Date: Tue, 29 Apr 2025 17:17:06 +0200
From: Jan Stancek <jstancek@...hat.com>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Nam Cao <namcao@...utronix.de>, Anna-Maria Behnsen <anna-maria@...utronix.de>, 
	Thomas Gleixner <tglx@...utronix.de>, Andy Lutomirski <luto@...nel.org>, 
	Vincenzo Frascino <vincenzo.frascino@....com>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] vdso: Reject absolute relocations during build

On Tue, Apr 29, 2025 at 2:56 PM Thomas Weißschuh
<thomas.weissschuh@...utronix.de> wrote:
>
> All vDSO code needs to be completely position independent.
> Symbol references are marked as hidden so the compiler is forced to emit
> PC-relative relocations.
> If the compiler does however emit an absolute relocation this will be
> resolved by the linker and break at runtime.
>
> 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.
>
> Link: https://lore.kernel.org/lkml/aApGPAoctq_eoE2g@t14ultra/
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
> ---
>  lib/vdso/Makefile.include | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/lib/vdso/Makefile.include b/lib/vdso/Makefile.include
> index cedbf15f80874d4bb27c097244bc5b11272f261c..18fcc94e4d6bf7abf374c7a953349e7ad75f8a18 100644
> --- a/lib/vdso/Makefile.include
> +++ b/lib/vdso/Makefile.include
> @@ -12,7 +12,12 @@ 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 are present in the object files themselves.
>  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, $(real-prereqs)) | grep -q " R_\w*_ABS"; \
> +                      then (echo >&2 "$@: absolute relocations are not supported"; \
>                              rm -f $@; /bin/false); fi

Should this check only some sections? I'm getting lot of matches on
debuginfo related sections:

# readelf -rW arch/arm64/kernel/vdso/vgettimeofday.o | awk
'/section.*debug/ {count=10} count {print; count--}'
Relocation section '.rela.debug_info' at offset 0x4fc8 contains 524 entries:
    Offset             Info             Type               Symbol's
Value  Symbol's Name + Addend
0000000000000008  0000000a00000102 R_AARCH64_ABS32
0000000000000000 .debug_abbrev + 0
000000000000000d  0000000f00000102 R_AARCH64_ABS32
0000000000000000 .debug_str + 3b3
0000000000000012  0000001000000102 R_AARCH64_ABS32
0000000000000000 .debug_line_str + 0
0000000000000016  0000001000000102 R_AARCH64_ABS32
0000000000000000 .debug_line_str + 27
000000000000001a  0000000200000101 R_AARCH64_ABS64
0000000000000000 .text + 0
000000000000002a  0000000e00000102 R_AARCH64_ABS32
0000000000000000 .debug_line + 0
0000000000000031  0000000f00000102 R_AARCH64_ABS32
0000000000000000 .debug_str + 2d5
0000000000000038  0000000f00000102 R_AARCH64_ABS32
0000000000000000 .debug_str + 7f0
Relocation section '.rela.debug_loclists' at offset 0x80e8 contains 3 entries:
    Offset             Info             Type               Symbol's
Value  Symbol's Name + Addend
0000000000000504  0000000900000102 R_AARCH64_ABS32
0000000000000000 .debug_info + ab8
0000000000000b3a  0000000900000102 R_AARCH64_ABS32
0000000000000000 .debug_info + 1111
0000000000000bc5  0000000900000102 R_AARCH64_ABS32
0000000000000000 .debug_info + 143e

Relocation section '.rela.debug_aranges' at offset 0x8130 contains 2 entries:
    Offset             Info             Type               Symbol's
Value  Symbol's Name + Addend
0000000000000006  0000000900000102 R_AARCH64_ABS32
0000000000000000 .debug_info + 0
0000000000000010  0000000200000101 R_AARCH64_ABS64
0000000000000000 .text + 0

Relocation section '.rela.debug_line' at offset 0x8160 contains 27 entries:
    Offset             Info             Type               Symbol's
Value  Symbol's Name + Addend
0000000000000022  0000001000000102 R_AARCH64_ABS32
0000000000000000 .debug_line_str + 38
0000000000000026  0000001000000102 R_AARCH64_ABS32
0000000000000000 .debug_line_str + 49
000000000000002a  0000001000000102 R_AARCH64_ABS32
0000000000000000 .debug_line_str + 60
000000000000002e  0000001000000102 R_AARCH64_ABS32
0000000000000000 .debug_line_str + 7a
0000000000000032  0000001000000102 R_AARCH64_ABS32
0000000000000000 .debug_line_str + 98
0000000000000036  0000001000000102 R_AARCH64_ABS32
0000000000000000 .debug_line_str + a7
000000000000003a  0000001000000102 R_AARCH64_ABS32
0000000000000000 .debug_line_str + c2
000000000000003e  0000001000000102 R_AARCH64_ABS32
0000000000000000 .debug_line_str + d8

Regards,
Jan