Re: [PATCH v8 04/14] x86/kasan: Add arch specific kasan functions

[Andrey Konovalov <andreyknvl@gmail.com>]

On Mon, Jan 12, 2026 at 6:27 PM Maciej Wieczor-Retman
<m.wieczorretman@pm.me> wrote:
>
> From: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
>
> KASAN's software tag-based mode needs multiple macros/functions to
> handle tag and pointer interactions - to set, retrieve and reset tags
> from the top bits of a pointer.
>
> Mimic functions currently used by arm64 but change the tag's position to
> bits [60:57] in the pointer.
>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
> ---
> Changelog v7:
> - Add KASAN_TAG_BYTE_MASK to avoid circular includes and avoid removing
>   KASAN_TAG_MASK from mmzone.h.
> - Remove Andrey's Acked-by tag.
>
> Changelog v6:
> - Remove empty line after ifdef CONFIG_KASAN_SW_TAGS
> - Add ifdef 64 bit to avoid problems in vdso32.
> - Add Andrey's Acked-by tag.
>
> Changelog v4:
> - Rewrite __tag_set() without pointless casts and make it more readable.
>
> Changelog v3:
> - Reorder functions so that __tag_*() etc are above the
>   arch_kasan_*() ones.
> - Remove CONFIG_KASAN condition from __tag_set()
>
>  arch/x86/include/asm/kasan.h | 42 ++++++++++++++++++++++++++++++++++--
>  include/linux/kasan-tags.h   |  2 ++
>  include/linux/mmzone.h       |  2 +-
>  3 files changed, 43 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/kasan.h b/arch/x86/include/asm/kasan.h
> index d7e33c7f096b..eab12527ed7f 100644
> --- a/arch/x86/include/asm/kasan.h
> +++ b/arch/x86/include/asm/kasan.h
> @@ -3,6 +3,8 @@
>  #define _ASM_X86_KASAN_H
>
>  #include <linux/const.h>
> +#include <linux/kasan-tags.h>
> +#include <linux/types.h>
>  #define KASAN_SHADOW_OFFSET _AC(CONFIG_KASAN_SHADOW_OFFSET, UL)
>  #define KASAN_SHADOW_SCALE_SHIFT 3
>
> @@ -24,8 +26,43 @@
>                                                   KASAN_SHADOW_SCALE_SHIFT)))
>
>  #ifndef __ASSEMBLER__
> +#include <linux/bitops.h>
> +#include <linux/bitfield.h>
> +#include <linux/bits.h>
> +
> +#ifdef CONFIG_KASAN_SW_TAGS
> +#define __tag_shifted(tag)             FIELD_PREP(GENMASK_ULL(60, 57), tag)
> +#define __tag_reset(addr)              (sign_extend64((u64)(addr), 56))
> +#define __tag_get(addr)                        ((u8)FIELD_GET(GENMASK_ULL(60, 57), (u64)addr))
> +#else
> +#define __tag_shifted(tag)             0UL
> +#define __tag_reset(addr)              (addr)
> +#define __tag_get(addr)                        0
> +#endif /* CONFIG_KASAN_SW_TAGS */
> +
> +#ifdef CONFIG_64BIT
> +static inline void *__tag_set(const void *__addr, u8 tag)
> +{
> +       u64 addr = (u64)__addr;
> +
> +       addr &= ~__tag_shifted(KASAN_TAG_BYTE_MASK);
> +       addr |= __tag_shifted(tag & KASAN_TAG_BYTE_MASK);
> +
> +       return (void *)addr;
> +}
> +#else
> +static inline void *__tag_set(void *__addr, u8 tag)
> +{
> +       return __addr;
> +}
> +#endif
> +
> +#define arch_kasan_set_tag(addr, tag)  __tag_set(addr, tag)
> +#define arch_kasan_reset_tag(addr)     __tag_reset(addr)
> +#define arch_kasan_get_tag(addr)       __tag_get(addr)
>
>  #ifdef CONFIG_KASAN
> +
>  void __init kasan_early_init(void);
>  void __init kasan_init(void);
>  void __init kasan_populate_shadow_for_vaddr(void *va, size_t size, int nid);
> @@ -34,8 +71,9 @@ static inline void kasan_early_init(void) { }
>  static inline void kasan_init(void) { }
>  static inline void kasan_populate_shadow_for_vaddr(void *va, size_t size,
>                                                    int nid) { }
> -#endif
>
> -#endif
> +#endif /* CONFIG_KASAN */
> +
> +#endif /* __ASSEMBLER__ */
>
>  #endif
> diff --git a/include/linux/kasan-tags.h b/include/linux/kasan-tags.h
> index ad5c11950233..e4f26bec3673 100644
> --- a/include/linux/kasan-tags.h
> +++ b/include/linux/kasan-tags.h
> @@ -10,6 +10,8 @@
>  #define KASAN_TAG_WIDTH                0
>  #endif
>
> +#define KASAN_TAG_BYTE_MASK    ((1UL << KASAN_TAG_WIDTH) - 1)

How about KASAN_TAG_BITS_MASK?

When KASAN_TAG_WIDTH == 4, the mask does not cover a whole byte.


> +
>  #ifndef KASAN_TAG_KERNEL
>  #define KASAN_TAG_KERNEL       0xFF /* native kernel pointers tag */
>  #endif
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 75ef7c9f9307..3839052121d4 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1177,7 +1177,7 @@ static inline bool zone_is_empty(const struct zone *zone)
>  #define NODES_MASK             ((1UL << NODES_WIDTH) - 1)
>  #define SECTIONS_MASK          ((1UL << SECTIONS_WIDTH) - 1)
>  #define LAST_CPUPID_MASK       ((1UL << LAST_CPUPID_SHIFT) - 1)
> -#define KASAN_TAG_MASK         ((1UL << KASAN_TAG_WIDTH) - 1)
> +#define KASAN_TAG_MASK         KASAN_TAG_BYTE_MASK
>  #define ZONEID_MASK            ((1UL << ZONEID_SHIFT) - 1)
>
>  static inline enum zone_type memdesc_zonenum(memdesc_flags_t flags)
> --
> 2.52.0
>
>