Re: [PATCH v2 1/2] arm64: vdso: Work around invalid absolute relocations from GCC

[Catalin Marinas <catalin.marinas@arm.com>]

Hi Thomas,

On Wed, Apr 30, 2025 at 11:20:13AM +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 GCC emits absolute relocations for symbol-relative references with
> an offset >= 64KiB. After recent refactorings in the vDSO code this is the
> case in __arch_get_vdso_u_timens_data() with a page size of 64KiB.
> 
> Work around the issue by preventing the optimizer from seeing the offsets.
> 
> Reported-by: Jan Stancek <jstancek@redhat.com>
> Closes: https://lore.kernel.org/lkml/aApGPAoctq_eoE2g@t14ultra/
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120002
> Fixes: 83a2a6b8cfc5 ("vdso/gettimeofday: Prepare do_hres_timens() for introduction of struct vdso_clock")
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> ---
>  arch/arm64/include/asm/vdso/gettimeofday.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/vdso/gettimeofday.h b/arch/arm64/include/asm/vdso/gettimeofday.h
> index 92a2b59a9f3df4d20feb483e6d8ebd1d813b7932..3322c7047d84fecae316a2904f1adec0cb458f6f 100644
> --- a/arch/arm64/include/asm/vdso/gettimeofday.h
> +++ b/arch/arm64/include/asm/vdso/gettimeofday.h
> @@ -99,6 +99,19 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode,
>  	return res;
>  }
>  
> +#if IS_ENABLED(CONFIG_CC_IS_GCC) && IS_ENABLED(CONFIG_PAGE_SIZE_64KB)
> +static __always_inline const struct vdso_time_data *__arch_get_vdso_u_time_data(void)
> +{
> +	const struct vdso_time_data *ret = &vdso_u_time_data;
> +
> +	/* Work around invalid absolute relocations */
> +	OPTIMIZER_HIDE_VAR(ret);
> +
> +	return ret;
> +}
> +#define __arch_get_vdso_u_time_data __arch_get_vdso_u_time_data
> +#endif /* IS_ENABLED(CONFIG_CC_IS_GCC) && IS_ENABLED(CONFIG_PAGE_SIZE_64KB) */

Thanks for the fix. We may have to chase other such cases in the future
but I don't have any better suggestion.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>

Question for the other Thomas (tglx): what's your preference, do you
plan to take these through tip or shall I queue them via the arm64 tree?

Thanks.

-- 
Catalin