From: Samuel Holland <samuel.holland@sifive.com>
KASAN's tag-based mode defines multiple special tag values. They're
reserved for:
- Native kernel value. On arm64 it's 0xFF and it causes an early return
in the tag checking function.
- Invalid value. 0xFE marks an area as freed / unallocated. It's also
the value that is used to initialize regions of shadow memory.
- Min and max values. 0xFD is the highest value that can be randomly
generated for a new tag. 0 is the minimal value with the exception of
arm64's hardware mode where it is equal to 0xF0.
Metadata macro is also defined:
- Tag width equal to 8.
Tag-based mode on x86 is going to use 4 bit wide tags so all the above
values need to be changed accordingly.
Make tag width and native kernel tag arch specific for x86 and arm64.
Base the invalid tag value and the max value on the native kernel tag
since they follow the same pattern on both mentioned architectures.
Also generalize KASAN_SHADOW_INIT and 0xff used in various
page_kasan_tag* helpers.
Give KASAN_TAG_MIN the default value of zero, and move the special value
for hw_tags arm64 to its arch specific kasan-tags.h.
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Co-developed-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Acked-by: Will Deacon <will@kernel.org> (for the arm part)
---
Changelog v7:
- Reorder defines of arm64 tag width to prevent redefinition warnings.
- Remove KASAN_TAG_MASK so it's only defined in mmzone.h (Andrey
Konovalov)
- Merge the 'support tag widths less than 8 bits' with this patch since
they do similar things and overwrite each other. (Alexander)
Changelog v6:
- Add hardware tags KASAN_TAG_WIDTH value to the arm64 arch file.
- Keep KASAN_TAG_MASK in the mmzone.h.
- Remove ifndef from KASAN_SHADOW_INIT.
Changelog v5:
- Move KASAN_TAG_MIN to the arm64 kasan-tags.h for the hardware KASAN
mode case.
Changelog v4:
- Move KASAN_TAG_MASK to kasan-tags.h.
Changelog v2:
- Remove risc-v from the patch.
MAINTAINERS | 2 +-
arch/arm64/include/asm/kasan-tags.h | 14 ++++++++++++++
arch/arm64/include/asm/kasan.h | 2 --
arch/arm64/include/asm/uaccess.h | 1 +
arch/x86/include/asm/kasan-tags.h | 9 +++++++++
include/linux/kasan-tags.h | 19 ++++++++++++++-----
include/linux/kasan.h | 3 +--
include/linux/mm.h | 6 +++---
include/linux/page-flags-layout.h | 9 +--------
9 files changed, 44 insertions(+), 21 deletions(-)
create mode 100644 arch/arm64/include/asm/kasan-tags.h
create mode 100644 arch/x86/include/asm/kasan-tags.h
diff --git a/MAINTAINERS b/MAINTAINERS
index 0d044a58cbfe..84fdf497a97c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -13581,7 +13581,7 @@ L: kasan-dev@googlegroups.com
S: Maintained
B: https://bugzilla.kernel.org/buglist.cgi?component=Sanitizers&product=Memory%20Management
F: Documentation/dev-tools/kasan.rst
-F: arch/*/include/asm/*kasan.h
+F: arch/*/include/asm/*kasan*.h
F: arch/*/mm/kasan_init*
F: include/linux/kasan*.h
F: lib/Kconfig.kasan
diff --git a/arch/arm64/include/asm/kasan-tags.h b/arch/arm64/include/asm/kasan-tags.h
new file mode 100644
index 000000000000..259952677443
--- /dev/null
+++ b/arch/arm64/include/asm/kasan-tags.h
@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_KASAN_TAGS_H
+#define __ASM_KASAN_TAGS_H
+
+#define KASAN_TAG_KERNEL 0xFF /* native kernel pointers tag */
+
+#ifdef CONFIG_KASAN_HW_TAGS
+#define KASAN_TAG_MIN 0xF0 /* minimum value for random tags */
+#define KASAN_TAG_WIDTH 4
+#else
+#define KASAN_TAG_WIDTH 8
+#endif
+
+#endif /* ASM_KASAN_TAGS_H */
diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h
index b167e9d3da91..fd4a8557d736 100644
--- a/arch/arm64/include/asm/kasan.h
+++ b/arch/arm64/include/asm/kasan.h
@@ -6,8 +6,6 @@
#include <linux/linkage.h>
#include <asm/memory.h>
-#include <asm/mte-kasan.h>
-#include <asm/pgtable-types.h>
#define arch_kasan_set_tag(addr, tag) __tag_set(addr, tag)
#define arch_kasan_reset_tag(addr) __tag_reset(addr)
diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index 6490930deef8..ccd41a39e3a1 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -22,6 +22,7 @@
#include <asm/cpufeature.h>
#include <asm/mmu.h>
#include <asm/mte.h>
+#include <asm/mte-kasan.h>
#include <asm/ptrace.h>
#include <asm/memory.h>
#include <asm/extable.h>
diff --git a/arch/x86/include/asm/kasan-tags.h b/arch/x86/include/asm/kasan-tags.h
new file mode 100644
index 000000000000..68ba385bc75c
--- /dev/null
+++ b/arch/x86/include/asm/kasan-tags.h
@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_KASAN_TAGS_H
+#define __ASM_KASAN_TAGS_H
+
+#define KASAN_TAG_KERNEL 0xF /* native kernel pointers tag */
+
+#define KASAN_TAG_WIDTH 4
+
+#endif /* ASM_KASAN_TAGS_H */
diff --git a/include/linux/kasan-tags.h b/include/linux/kasan-tags.h
index 4f85f562512c..ad5c11950233 100644
--- a/include/linux/kasan-tags.h
+++ b/include/linux/kasan-tags.h
@@ -2,13 +2,22 @@
#ifndef _LINUX_KASAN_TAGS_H
#define _LINUX_KASAN_TAGS_H
+#if defined(CONFIG_KASAN_SW_TAGS) || defined(CONFIG_KASAN_HW_TAGS)
+#include <asm/kasan-tags.h>
+#endif
+
+#ifndef KASAN_TAG_WIDTH
+#define KASAN_TAG_WIDTH 0
+#endif
+
+#ifndef KASAN_TAG_KERNEL
#define KASAN_TAG_KERNEL 0xFF /* native kernel pointers tag */
-#define KASAN_TAG_INVALID 0xFE /* inaccessible memory tag */
-#define KASAN_TAG_MAX 0xFD /* maximum value for random tags */
+#endif
+
+#define KASAN_TAG_INVALID (KASAN_TAG_KERNEL - 1) /* inaccessible memory tag */
+#define KASAN_TAG_MAX (KASAN_TAG_KERNEL - 2) /* maximum value for random tags */
-#ifdef CONFIG_KASAN_HW_TAGS
-#define KASAN_TAG_MIN 0xF0 /* minimum value for random tags */
-#else
+#ifndef KASAN_TAG_MIN
#define KASAN_TAG_MIN 0x00 /* minimum value for random tags */
#endif
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 0f65e88cc3f6..1c7acdb5f297 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -40,8 +40,7 @@ typedef unsigned int __bitwise kasan_vmalloc_flags_t;
/* Software KASAN implementations use shadow memory. */
#ifdef CONFIG_KASAN_SW_TAGS
-/* This matches KASAN_TAG_INVALID. */
-#define KASAN_SHADOW_INIT 0xFE
+#define KASAN_SHADOW_INIT KASAN_TAG_INVALID
#else
#define KASAN_SHADOW_INIT 0
#endif
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6f959d8ca4b4..8ba91f38a794 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1949,7 +1949,7 @@ static inline u8 page_kasan_tag(const struct page *page)
if (kasan_enabled()) {
tag = (page->flags.f >> KASAN_TAG_PGSHIFT) & KASAN_TAG_MASK;
- tag ^= 0xff;
+ tag ^= KASAN_TAG_KERNEL;
}
return tag;
@@ -1962,7 +1962,7 @@ static inline void page_kasan_tag_set(struct page *page, u8 tag)
if (!kasan_enabled())
return;
- tag ^= 0xff;
+ tag ^= KASAN_TAG_KERNEL;
old_flags = READ_ONCE(page->flags.f);
do {
flags = old_flags;
@@ -1981,7 +1981,7 @@ static inline void page_kasan_tag_reset(struct page *page)
static inline u8 page_kasan_tag(const struct page *page)
{
- return 0xff;
+ return KASAN_TAG_KERNEL;
}
static inline void page_kasan_tag_set(struct page *page, u8 tag) { }
diff --git a/include/linux/page-flags-layout.h b/include/linux/page-flags-layout.h
index 760006b1c480..b2cc4cb870e0 100644
--- a/include/linux/page-flags-layout.h
+++ b/include/linux/page-flags-layout.h
@@ -3,6 +3,7 @@
#define PAGE_FLAGS_LAYOUT_H
#include <linux/numa.h>
+#include <linux/kasan-tags.h>
#include <generated/bounds.h>
/*
@@ -72,14 +73,6 @@
#define NODE_NOT_IN_PAGE_FLAGS 1
#endif
-#if defined(CONFIG_KASAN_SW_TAGS)
-#define KASAN_TAG_WIDTH 8
-#elif defined(CONFIG_KASAN_HW_TAGS)
-#define KASAN_TAG_WIDTH 4
-#else
-#define KASAN_TAG_WIDTH 0
-#endif
-
#ifdef CONFIG_NUMA_BALANCING
#define LAST__PID_SHIFT 8
#define LAST__PID_MASK ((1 << LAST__PID_SHIFT)-1)
--
2.52.0On 1/12/26 6:27 PM, Maciej Wieczor-Retman wrote: > From: Samuel Holland <samuel.holland@sifive.com> > > KASAN's tag-based mode defines multiple special tag values. They're > reserved for: > - Native kernel value. On arm64 it's 0xFF and it causes an early return > in the tag checking function. > - Invalid value. 0xFE marks an area as freed / unallocated. It's also > the value that is used to initialize regions of shadow memory. > - Min and max values. 0xFD is the highest value that can be randomly > generated for a new tag. 0 is the minimal value with the exception of > arm64's hardware mode where it is equal to 0xF0. > > Metadata macro is also defined: > - Tag width equal to 8. > > Tag-based mode on x86 is going to use 4 bit wide tags so all the above > values need to be changed accordingly. > > Make tag width and native kernel tag arch specific for x86 and arm64. > > Base the invalid tag value and the max value on the native kernel tag > since they follow the same pattern on both mentioned architectures. > > Also generalize KASAN_SHADOW_INIT and 0xff used in various > page_kasan_tag* helpers. > > Give KASAN_TAG_MIN the default value of zero, and move the special value > for hw_tags arm64 to its arch specific kasan-tags.h. > > Signed-off-by: Samuel Holland <samuel.holland@sifive.com> > Co-developed-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com> > Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com> > Acked-by: Will Deacon <will@kernel.org> (for the arm part) > --- Reviewed-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
On Mon, Jan 12, 2026 at 6:27 PM Maciej Wieczor-Retman
<m.wieczorretman@pm.me> wrote:
>
> From: Samuel Holland <samuel.holland@sifive.com>
>
> KASAN's tag-based mode defines multiple special tag values. They're
> reserved for:
> - Native kernel value. On arm64 it's 0xFF and it causes an early return
> in the tag checking function.
> - Invalid value. 0xFE marks an area as freed / unallocated. It's also
> the value that is used to initialize regions of shadow memory.
> - Min and max values. 0xFD is the highest value that can be randomly
> generated for a new tag. 0 is the minimal value with the exception of
> arm64's hardware mode where it is equal to 0xF0.
>
> Metadata macro is also defined:
> - Tag width equal to 8.
>
> Tag-based mode on x86 is going to use 4 bit wide tags so all the above
> values need to be changed accordingly.
>
> Make tag width and native kernel tag arch specific for x86 and arm64.
>
> Base the invalid tag value and the max value on the native kernel tag
> since they follow the same pattern on both mentioned architectures.
>
> Also generalize KASAN_SHADOW_INIT and 0xff used in various
> page_kasan_tag* helpers.
>
> Give KASAN_TAG_MIN the default value of zero, and move the special value
> for hw_tags arm64 to its arch specific kasan-tags.h.
>
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
> Co-developed-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
> Acked-by: Will Deacon <will@kernel.org> (for the arm part)
> ---
> Changelog v7:
> - Reorder defines of arm64 tag width to prevent redefinition warnings.
> - Remove KASAN_TAG_MASK so it's only defined in mmzone.h (Andrey
> Konovalov)
> - Merge the 'support tag widths less than 8 bits' with this patch since
> they do similar things and overwrite each other. (Alexander)
>
> Changelog v6:
> - Add hardware tags KASAN_TAG_WIDTH value to the arm64 arch file.
> - Keep KASAN_TAG_MASK in the mmzone.h.
> - Remove ifndef from KASAN_SHADOW_INIT.
>
> Changelog v5:
> - Move KASAN_TAG_MIN to the arm64 kasan-tags.h for the hardware KASAN
> mode case.
>
> Changelog v4:
> - Move KASAN_TAG_MASK to kasan-tags.h.
>
> Changelog v2:
> - Remove risc-v from the patch.
>
> MAINTAINERS | 2 +-
> arch/arm64/include/asm/kasan-tags.h | 14 ++++++++++++++
> arch/arm64/include/asm/kasan.h | 2 --
> arch/arm64/include/asm/uaccess.h | 1 +
> arch/x86/include/asm/kasan-tags.h | 9 +++++++++
> include/linux/kasan-tags.h | 19 ++++++++++++++-----
> include/linux/kasan.h | 3 +--
> include/linux/mm.h | 6 +++---
> include/linux/page-flags-layout.h | 9 +--------
> 9 files changed, 44 insertions(+), 21 deletions(-)
> create mode 100644 arch/arm64/include/asm/kasan-tags.h
> create mode 100644 arch/x86/include/asm/kasan-tags.h
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 0d044a58cbfe..84fdf497a97c 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -13581,7 +13581,7 @@ L: kasan-dev@googlegroups.com
> S: Maintained
> B: https://bugzilla.kernel.org/buglist.cgi?component=Sanitizers&product=Memory%20Management
> F: Documentation/dev-tools/kasan.rst
> -F: arch/*/include/asm/*kasan.h
> +F: arch/*/include/asm/*kasan*.h
> F: arch/*/mm/kasan_init*
> F: include/linux/kasan*.h
> F: lib/Kconfig.kasan
> diff --git a/arch/arm64/include/asm/kasan-tags.h b/arch/arm64/include/asm/kasan-tags.h
> new file mode 100644
> index 000000000000..259952677443
> --- /dev/null
> +++ b/arch/arm64/include/asm/kasan-tags.h
> @@ -0,0 +1,14 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ASM_KASAN_TAGS_H
> +#define __ASM_KASAN_TAGS_H
> +
> +#define KASAN_TAG_KERNEL 0xFF /* native kernel pointers tag */
> +
> +#ifdef CONFIG_KASAN_HW_TAGS
> +#define KASAN_TAG_MIN 0xF0 /* minimum value for random tags */
> +#define KASAN_TAG_WIDTH 4
> +#else
> +#define KASAN_TAG_WIDTH 8
> +#endif
> +
> +#endif /* ASM_KASAN_TAGS_H */
> diff --git a/arch/arm64/include/asm/kasan.h b/arch/arm64/include/asm/kasan.h
> index b167e9d3da91..fd4a8557d736 100644
> --- a/arch/arm64/include/asm/kasan.h
> +++ b/arch/arm64/include/asm/kasan.h
> @@ -6,8 +6,6 @@
>
> #include <linux/linkage.h>
> #include <asm/memory.h>
> -#include <asm/mte-kasan.h>
> -#include <asm/pgtable-types.h>
>
> #define arch_kasan_set_tag(addr, tag) __tag_set(addr, tag)
> #define arch_kasan_reset_tag(addr) __tag_reset(addr)
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index 6490930deef8..ccd41a39e3a1 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -22,6 +22,7 @@
> #include <asm/cpufeature.h>
> #include <asm/mmu.h>
> #include <asm/mte.h>
> +#include <asm/mte-kasan.h>
> #include <asm/ptrace.h>
> #include <asm/memory.h>
> #include <asm/extable.h>
> diff --git a/arch/x86/include/asm/kasan-tags.h b/arch/x86/include/asm/kasan-tags.h
> new file mode 100644
> index 000000000000..68ba385bc75c
> --- /dev/null
> +++ b/arch/x86/include/asm/kasan-tags.h
> @@ -0,0 +1,9 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ASM_KASAN_TAGS_H
> +#define __ASM_KASAN_TAGS_H
> +
> +#define KASAN_TAG_KERNEL 0xF /* native kernel pointers tag */
One thing that stood out to me here was that for x86, KASAN_TAG_KERNEL
is defined as a 4-bit value (0xF). Which makes sense, as
KASAN_TAG_WIDTH == 4.
But for arm64, KASAN_TAG_KERNEL and others are defined as 8-bit values
(0xFF, etc.), even though for HW_TAGS, KASAN_TAG_WIDTH is also == 4
and only the lower 4 bits of these values define the tags.
This happens to work out: for HW_TAGS, __tag_set resets the top byte
but then uses the given value as is, so the higher 4 bits gets set to
0xF and the lower set to the tag. And for saving/restoring the tag in
page->flags, everything also works, as we only store the meaningful
lower 4 bits in flags, and restore the higher 0xF when doing ^ 0xFF.
But this is not related to this series: I think the way x86 defines
KASAN_TAG_KERNEL to be 0xF makes sense; we might just need to clean up
the arm64 implementation at some point.
> +
> +#define KASAN_TAG_WIDTH 4
> +
> +#endif /* ASM_KASAN_TAGS_H */
> diff --git a/include/linux/kasan-tags.h b/include/linux/kasan-tags.h
> index 4f85f562512c..ad5c11950233 100644
> --- a/include/linux/kasan-tags.h
> +++ b/include/linux/kasan-tags.h
> @@ -2,13 +2,22 @@
> #ifndef _LINUX_KASAN_TAGS_H
> #define _LINUX_KASAN_TAGS_H
>
> +#if defined(CONFIG_KASAN_SW_TAGS) || defined(CONFIG_KASAN_HW_TAGS)
> +#include <asm/kasan-tags.h>
> +#endif
> +
> +#ifndef KASAN_TAG_WIDTH
> +#define KASAN_TAG_WIDTH 0
> +#endif
> +
> +#ifndef KASAN_TAG_KERNEL
> #define KASAN_TAG_KERNEL 0xFF /* native kernel pointers tag */
> -#define KASAN_TAG_INVALID 0xFE /* inaccessible memory tag */
> -#define KASAN_TAG_MAX 0xFD /* maximum value for random tags */
> +#endif
> +
> +#define KASAN_TAG_INVALID (KASAN_TAG_KERNEL - 1) /* inaccessible memory tag */
> +#define KASAN_TAG_MAX (KASAN_TAG_KERNEL - 2) /* maximum value for random tags */
>
> -#ifdef CONFIG_KASAN_HW_TAGS
> -#define KASAN_TAG_MIN 0xF0 /* minimum value for random tags */
> -#else
> +#ifndef KASAN_TAG_MIN
> #define KASAN_TAG_MIN 0x00 /* minimum value for random tags */
> #endif
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 0f65e88cc3f6..1c7acdb5f297 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -40,8 +40,7 @@ typedef unsigned int __bitwise kasan_vmalloc_flags_t;
> /* Software KASAN implementations use shadow memory. */
>
> #ifdef CONFIG_KASAN_SW_TAGS
> -/* This matches KASAN_TAG_INVALID. */
> -#define KASAN_SHADOW_INIT 0xFE
> +#define KASAN_SHADOW_INIT KASAN_TAG_INVALID
> #else
> #define KASAN_SHADOW_INIT 0
> #endif
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6f959d8ca4b4..8ba91f38a794 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1949,7 +1949,7 @@ static inline u8 page_kasan_tag(const struct page *page)
>
> if (kasan_enabled()) {
> tag = (page->flags.f >> KASAN_TAG_PGSHIFT) & KASAN_TAG_MASK;
> - tag ^= 0xff;
> + tag ^= KASAN_TAG_KERNEL;
> }
>
> return tag;
> @@ -1962,7 +1962,7 @@ static inline void page_kasan_tag_set(struct page *page, u8 tag)
> if (!kasan_enabled())
> return;
>
> - tag ^= 0xff;
> + tag ^= KASAN_TAG_KERNEL;
> old_flags = READ_ONCE(page->flags.f);
> do {
> flags = old_flags;
> @@ -1981,7 +1981,7 @@ static inline void page_kasan_tag_reset(struct page *page)
>
> static inline u8 page_kasan_tag(const struct page *page)
> {
> - return 0xff;
> + return KASAN_TAG_KERNEL;
> }
>
> static inline void page_kasan_tag_set(struct page *page, u8 tag) { }
> diff --git a/include/linux/page-flags-layout.h b/include/linux/page-flags-layout.h
> index 760006b1c480..b2cc4cb870e0 100644
> --- a/include/linux/page-flags-layout.h
> +++ b/include/linux/page-flags-layout.h
> @@ -3,6 +3,7 @@
> #define PAGE_FLAGS_LAYOUT_H
>
> #include <linux/numa.h>
> +#include <linux/kasan-tags.h>
> #include <generated/bounds.h>
>
> /*
> @@ -72,14 +73,6 @@
> #define NODE_NOT_IN_PAGE_FLAGS 1
> #endif
>
> -#if defined(CONFIG_KASAN_SW_TAGS)
> -#define KASAN_TAG_WIDTH 8
> -#elif defined(CONFIG_KASAN_HW_TAGS)
> -#define KASAN_TAG_WIDTH 4
> -#else
> -#define KASAN_TAG_WIDTH 0
> -#endif
> -
> #ifdef CONFIG_NUMA_BALANCING
> #define LAST__PID_SHIFT 8
> #define LAST__PID_MASK ((1 << LAST__PID_SHIFT)-1)
> --
> 2.52.0
>
>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
On 2026-01-13 at 02:21:07 +0100, Andrey Konovalov wrote: >On Mon, Jan 12, 2026 at 6:27 PM Maciej Wieczor-Retman ><m.wieczorretman@pm.me> wrote: >> >> From: Samuel Holland <samuel.holland@sifive.com> >> diff --git a/arch/x86/include/asm/kasan-tags.h b/arch/x86/include/asm/kasan-tags.h >> new file mode 100644 >> index 000000000000..68ba385bc75c >> --- /dev/null >> +++ b/arch/x86/include/asm/kasan-tags.h >> @@ -0,0 +1,9 @@ >> +/* SPDX-License-Identifier: GPL-2.0 */ >> +#ifndef __ASM_KASAN_TAGS_H >> +#define __ASM_KASAN_TAGS_H >> + >> +#define KASAN_TAG_KERNEL 0xF /* native kernel pointers tag */ > >One thing that stood out to me here was that for x86, KASAN_TAG_KERNEL >is defined as a 4-bit value (0xF). Which makes sense, as >KASAN_TAG_WIDTH == 4. > >But for arm64, KASAN_TAG_KERNEL and others are defined as 8-bit values >(0xFF, etc.), even though for HW_TAGS, KASAN_TAG_WIDTH is also == 4 >and only the lower 4 bits of these values define the tags. > >This happens to work out: for HW_TAGS, __tag_set resets the top byte >but then uses the given value as is, so the higher 4 bits gets set to >0xF and the lower set to the tag. And for saving/restoring the tag in >page->flags, everything also works, as we only store the meaningful >lower 4 bits in flags, and restore the higher 0xF when doing ^ 0xFF. > >But this is not related to this series: I think the way x86 defines >KASAN_TAG_KERNEL to be 0xF makes sense; we might just need to clean up >the arm64 implementation at some point. > I suppose while there is only one such mode that stands out from the other two there is little hint as to what should be generalized. As you said so far this scheme we have works - altough it is somewhat convoluted. One thing I was thinking of was cleaning up all the #ifdefs for different modes into a more ordered structure. I think there are ~24 ifdefs in mm/kasan.h and include/linux/kasan.h and many of them could potentially be merged. ... > >Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com> Thanks :) -- Kind regards Maciej Wieczór-Retman
© 2016 - 2026 Red Hat, Inc.