Re: [PATCH v2 13/14] x86: runtime_const used for KASAN_SHADOW_END

[Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>]

On 2025-02-21 at 16:10:54 +0100, Maciej Wieczor-Retman wrote:
>On 2025-02-20 at 00:31:08 +0100, Andrey Konovalov wrote:
>>On Tue, Feb 18, 2025 at 9:20 AM Maciej Wieczor-Retman
>><maciej.wieczor-retman@intel.com> wrote:
>>>
>>> On x86, generic KASAN is setup in a way that needs a single
>>> KASAN_SHADOW_OFFSET value for both 4 and 5 level paging. It's required
>>> to facilitate boot time switching and it's a compiler ABI so it can't be
>>> changed during runtime.
>>>
>>> Software tag-based mode doesn't tie shadow start and end to any linear
>>> addresses as part of the compiler ABI so it can be changed during
>>> runtime.
>>
>>KASAN_SHADOW_OFFSET is passed to the compiler via
>>hwasan-mapping-offset, see scripts/Makefile.kasan (for the INLINE
>>mode). So while we can change its value, it has to be known at compile
>>time. So I don't think using a runtime constant would work.
>
>I don't know about arm64, but this doesn't seem to work right now on x86. I
>think I recall that hwasan-mapping-offset isn't implemented on the x86 LLVM or
>something like that? I'm sure I saw some note about it a while ago on the
>internet but I couldn't find it today.
>
>Anyway if KASAN_SHADOW_OFFSET is not set at compile time it defaults to nothing
>and just doesn't get passed into kasan-params a few lines below. I assume that
>result seems a little too makeshift for runtime const to make sense here?
>
>>
>>Which means that KASAN_SHADOW_OFFSET has to have such a value that
>>works for both 4 and 5 level page tables. This possibly means we might
>>need something different than the first patch in this series.
>
>I'll think again about doing one offset for both paging levels so that it's as
>optimal as possible.
>

Also I was wondering if you know what "hwasan-mapping-offset-dynamic" option is?
I noticed it in the llvm docs but can't find a good description what it does,
even from looking at the code in HWAddressSanitizer.cpp. If
hwasan-mapping-offset is not implemeneted for x86 I doubt this is but maybe it
could help in a cleaner makefile for x86 at least? Especially once these options
will be working in x86 llvm.

-- 
Kind regards
Maciej Wieczór-Retman