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

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

On 2025-09-09 at 10:24:22 +0200, Maciej Wieczor-Retman wrote:
>On 2025-09-08 at 22:19:05 +0200, Andrey Konovalov wrote:
>>On Mon, Sep 8, 2025 at 3:09 PM Maciej Wieczor-Retman
>><maciej.wieczor-retman@intel.com> wrote:
>>>
>>> >>I recall there were some corner cases where this code path got called in outline
>>> >>mode, didn't have a mismatch but still died due to the die() below. But I'll
>>> >>recheck and either apply what you wrote above or get add a better explanation
>>> >>to the patch message.
>>> >
>>> >Okay, so the int3_selftest_ip() is causing a problem in outline mode.
>>> >
>>> >I tried disabling kasan with kasan_disable_current() but thinking of it now it
>>> >won't work because int3 handler will still be called and die() will happen.
>>>
>>> Sorry, I meant to write that kasan_disable_current() works together with
>>> if(!kasan_report()). Because without checking kasan_report()' return
>>> value, if kasan is disabled through kasan_disable_current() it will have no
>>> effect in both inline mode, and if int3 is called in outline mode - the
>>> kasan_inline_handler will lead to die().
>>
>>So do I understand correctly, that we have no way to distinguish
>>whether the int3 was inserted by the KASAN instrumentation or natively
>>called (like in int3_selftest_ip())?
>>
>>If so, I think that we need to fix/change the compiler first so that
>>we can distinguish these cases. And only then introduce
>>kasan_inline_handler(). (Without kasan_inline_handler(), the outline
>>instrumentation would then just work, right?)
>>
>>If we can distinguish them, then we should only call
>>kasan_inline_handler() for the KASAN-inserted int3's. This is what we
>>do on arm64 (via brk and KASAN_BRK_IMM). And then int3_selftest_ip()
>>should not be affected.
>
>Looking at it again I suppose LLVM does pass a number along metadata to the
>int3. I didn't notice because no other function checks anything in the x86 int3
>handler, compared to how it's done on arm64 with brk.
>
>So right, thanks, after fixing it up it shouldn't affect the int3_selftest_ip().

But as Peter Zijlstra noticed, x86 already uses the #UD instruction similarly to
BRK on arm64. So I think I'll use this one here, and then change INT3 to UD in
the LLVM patch.

-- 
Kind regards
Maciej Wieczór-Retman