Re: [PATCH 1/2] riscv: vdso: Do not use LTO for the vDSO

[Nathan Chancellor <nathan@kernel.org>]

On Wed, Jul 01, 2026 at 11:21:22AM +0200, Thomas Weißschuh wrote:
> With LTO enabled the compiler assumes that the vDSO functions are not
> used and optimizes them away completely. Currently this happens to
> __vdso_clock_getres(), __vdso_clock_gettime(), __vdso_getrandom(),
> __vdso_gettimeofday() and __vdso_riscv_hwprobe().
> 
> Disable LTO for the vDSO, as these functions are hand-optimized anyways.
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202606301855.WvkSC4kD-lkp@intel.com/

While this change seems correct, is this really the fix for that report?
It seems like that error happens in clang but I would expect this sort
of issue to only appear once LTO has run through ld.lld?

> Fixes: 021d23428bdb ("RISC-V: build: Allow LTO to be selected")
> Cc: stable@vger.kernel.org
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> ---
>  arch/riscv/kernel/vdso/Makefile | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> index a842dc034571..43ee881f6c6f 100644
> --- a/arch/riscv/kernel/vdso/Makefile
> +++ b/arch/riscv/kernel/vdso/Makefile
> @@ -69,9 +69,9 @@ CPPFLAGS_$(vdso_lds) += -DHAS_VGETTIMEOFDAY
>  endif
>  
>  # Disable -pg to prevent insert call site
> -CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
> -CFLAGS_REMOVE_getrandom.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
> -CFLAGS_REMOVE_hwprobe.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
> +CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
> +CFLAGS_REMOVE_getrandom.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
> +CFLAGS_REMOVE_hwprobe.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
>  
>  # Force dependency
>  $(obj)/$(vdso_o): $(obj)/$(vdso_so)
> 
> -- 
> 2.55.0
> 

-- 
Cheers,
Nathan