linux-kernel - Re: [PATCH v5 04/19] x86: Add arch specific kasan functions

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Message-ID: <CA+fCnZd7tM5i5jOriaYyR1GgjgREv0PhyxpFuEC5506FkndzAg@mail.gmail.com>
Date: Sat, 6 Sep 2025 19:17:43 +0200
From: Andrey Konovalov <andreyknvl@...il.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
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Subject: Re: [PATCH v5 04/19] x86: Add arch specific kasan functions

On Mon, Aug 25, 2025 at 10:27 PM Maciej Wieczor-Retman
<maciej.wieczor-retman@...el.com> wrote:
>
> 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@...el.com>
> ---
> 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 | 36 ++++++++++++++++++++++++++++++++++--
>  1 file changed, 34 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/kasan.h b/arch/x86/include/asm/kasan.h
> index d7e33c7f096b..1963eb2fcff3 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 +26,37 @@
>                                                   KASAN_SHADOW_SCALE_SHIFT)))
>
>  #ifndef __ASSEMBLER__
> +#include <linux/bitops.h>
> +#include <linux/bitfield.h>
> +#include <linux/bits.h>
> +
> +#ifdef CONFIG_KASAN_SW_TAGS
> +

Nit: can remove this empty line.

> +#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 */
> +
> +static inline void *__tag_set(const void *__addr, u8 tag)
> +{
> +       u64 addr = (u64)__addr;
> +
> +       addr &= ~__tag_shifted(KASAN_TAG_MASK);
> +       addr |= __tag_shifted(tag);
> +
> +       return (void *)addr;
> +}
> +
> +#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 +65,9 @@ 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
> --
> 2.50.1
>

Acked-by: Andrey Konovalov <andreyknvl@...il.com>