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

[Peter Zijlstra]

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.