linux-kernel - Re: [PATCH v5 13/19] kasan: x86: Handle int3 for inline KASAN reports

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Message-ID: <63ocq6aadqn74e7g57a6p3cqkr3sf4hmejfp6hxmuju2b42iny@2du24bomvk4n>
Date: Wed, 10 Sep 2025 10:23:37 +0200
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Peter Zijlstra <peterz@...radead.org>
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Subject: Re: [PATCH v5 13/19] kasan: x86: Handle int3 for inline KASAN reports

On 2025-09-09 at 11:03:57 +0200, Peter Zijlstra wrote:
>On Tue, Sep 09, 2025 at 10:49:53AM +0200, Maciej Wieczor-Retman wrote:
>
>> >Specifically, look at arch/x86/kernel/traps.h:decode_bug(), UBSan uses
>> >UD1 /0, I would suggest KASAN to use UD1 /1.
>> 
>> Okay, that sounds great, I'll change it in this patchset and write the LLVM
>> patch later.
>
>Thanks! Also note how UBSAN encodes an immediate in the UD1 instruction.
>You can use that same to pass through your meta-data thing.
>
>MOD=1 gives you a single byte immediate, and MOD=2 gives you 4 bytes,
>eg:
>
>  0f b9 49 xx -- ud1 xx(%rcx), %rcx
>
>When poking at LLVM, try and convince the thing to not emit that
>'operand address size prefix' byte like UBSAN does, that's just a waste
>of bytes.

Thanks, that's good tip :)

-- 
Kind regards
Maciej Wieczór-Retman