Re: [PATCH v2 14/14] x86: Make software tag-based kasan available

On 2025-02-22 at 16:06:34 +0100, Andrey Konovalov wrote:

On Fri, Feb 21, 2025 at 3:45 PM Maciej Wieczor-Retman
[off-list ref] wrote:

quoted

quoted

What's the purpose of this config option? I think we can just change
the value of the KASAN_SHADOW_SCALE_SHIFT define when KASAN_SW_TAGS is
enabled.

Well, I was aiming at later adding the "default 5 if KASAN_SW_TAGS_DENSE", and
this way it would look much cleaner than the:

if KASAN_SW_TAGS
        if KASAN_SW_TAGS_DENSE
                KASAN_SHADOW_SCALE_SHIFT = 5
        else
                KASAN_SHADOW_SCALE_SHIFT = 4
else
        KASAN_SHADOW_SCALE_SHIFT = 3

I think this is fine. It's still better than adding a non-configurable
config option.

quoted

But now that I think of it, it should be possible to overwrite the
KASAN_SHADOW_SCALE_SHIFT from non-arch code if dense mode is enabled.

This should also work. Especially since the dense mode will probably
work for arm64 as well.

But let's keep this series self-contained.

Yes, of course. Anyway I'll just do one preprocessor if else in the same place
that the old x86 KASAN_SHADOW_SCALE_SHIFT was.

quoted

That's a topic for the next series but I'd imagine all architectures would
normally use the 16 memory bytes / shadow byte and if they'd care for the dense
mode they'd go for 32 memory bytes / shadow byte. Or do you think that's a
faulty assumption?

Probably, but for sure I don't know, not that many architectures that
care about memory tagging yet :)

I'll keep this assumption for now then. If some arch will have a different idea
about granularity I suppose the relevant code can be moved to arch specific
directories.

-- 
Kind regards
Maciej Wieczór-Retman