On 2026-01-13 at 02:21:07 +0100, Andrey Konovalov wrote:

On Mon, Jan 12, 2026 at 6:27 PM Maciej Wieczor-Retman
[off-list ref] wrote:

quoted

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