Re: [PATCH v6 17/18] x86/kasan: Logical bit shift for kasan_mem_to_shadow

[Marco Elver]

On Wed, 29 Oct 2025 at 21:11, Maciej Wieczor-Retman
<m.wieczorretman@xxxxx> wrote:
>
> From: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>
>
> While generally tag-based KASAN adopts an arithemitc bit shift to
> convert a memory address to a shadow memory address, it doesn't work for
> all cases on x86. Testing different shadow memory offsets proved that
> either 4 or 5 level paging didn't work correctly or inline mode ran into
> issues. Thus the best working scheme is the logical bit shift and
> non-canonical shadow offset that x86 uses for generic KASAN, of course
> adjusted for the increased granularity from 8 to 16 bytes.
>
> Add an arch specific implementation of kasan_mem_to_shadow() that uses
> the logical bit shift.
>
> The non-canonical hook tries to calculate whether an address came from
> kasan_mem_to_shadow(). First it checks whether this address fits into
> the legal set of values possible to output from the mem to shadow
> function.
>
> Tie both generic and tag-based x86 KASAN modes to the address range
> check associated with generic KASAN.
>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>
> ---
> Changelog v4:
> - Add this patch to the series.
>
>  arch/x86/include/asm/kasan.h | 7 +++++++
>  mm/kasan/report.c            | 5 +++--
>  2 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/kasan.h b/arch/x86/include/asm/kasan.h
> index 375651d9b114..2372397bc3e5 100644
> --- a/arch/x86/include/asm/kasan.h
> +++ b/arch/x86/include/asm/kasan.h
> @@ -49,6 +49,13 @@
>  #include <linux/bits.h>
>
>  #ifdef CONFIG_KASAN_SW_TAGS
> +static inline void *__kasan_mem_to_shadow(const void *addr)
> +{
> +       return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
> +               + KASAN_SHADOW_OFFSET;
> +}

You're effectively undoing "kasan: sw_tags: Use arithmetic shift for
shadow computation" for x86 - why?
This function needs a comment explaining this.

Also, the commit message just says "it doesn't work for all cases" - why?