linux-kernel - Re: [PATCH v2 01/14] kasan: sw_tags: Use arithmetic shift for shadow computation

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Message-ID: <CA+fCnZfwcV40i_78qY1WTdJc5PgRfvu-_7LERfJyPt6Xf8Ln9Q@mail.gmail.com>
Date: Tue, 18 Mar 2025 16:31:40 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
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Subject: Re: [PATCH v2 01/14] kasan: sw_tags: Use arithmetic shift for shadow computation

On Thu, Mar 13, 2025 at 3:58 PM Maciej Wieczor-Retman
<maciej.wieczor-retman@...el.com> wrote:
>
> >So this was my brain converting things to assembly. Feel free to
> >reword/clarify the comments.
>
> Right, I focused too much on the signed aspect. Treating everything as
> overflowing sounds better, more unified.

Alright!

> >It could be that your checks are equivalent to mine. What I did was to
> >check that the address lies outside of both contiguous regions, which
> >makes the checks symmetrical and IMO easier to follow.
>
> I drew this out and yeah, it looks like it's the same, just grouping the logical
> expressions differently. What do you think about incorporating something like
> the following into your comment about the x86 part? :
>
>         Given the KASAN_SHADOW_OFFSET equal 0xffeffc0000000000
>         the following ranges are valid mem-to-shadow mappings:
>
>         0xFFFFFFFFFFFFFFFF
>                 INVALID
>         0xFFEFFBFFFFFFFFFF - kasan_mem_to_shadow(~0UL)
>                 VALID   - kasan shadow mem
>                 VALID   - non-canonical kernel virtual address
>         0xFFCFFC0000000000 - kasan_mem_to_shadow(0xFEUL << 56)
>                 INVALID
>         0x07EFFBFFFFFFFFFF - kasan_mem_to_shadow(~0UL >> 1)
>                 VALID   - non-canonical user virtual addresses
>                 VALID   - user addresses
>         0x07CFFC0000000000 - kasan_mem_to_shadow(0x7EUL << 56)
>                 INVALID
>         0x0000000000000000

Sounds good - I like this visual representation a lot! Thanks!