linux-kernel - Re: [PATCH v8 00/14] kasan: x86: arm64: KASAN tag-based mode for x86

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Message-ID: <e273571e-ab8f-46d6-a44e-c1d0d06d3cbf@gmail.com>
Date: Mon, 19 Jan 2026 17:33:35 +0100
From: Andrey Ryabinin <ryabinin.a.a@...il.com>
To: Maciej Wieczor-Retman <m.wieczorretman@...me>, corbet@....net,
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Subject: Re: [PATCH v8 00/14] kasan: x86: arm64: KASAN tag-based mode for x86



On 1/12/26 6:26 PM, Maciej Wieczor-Retman wrote:

> ======= Compilation
> Clang was used to compile the series (make LLVM=1) since gcc doesn't
> seem to have support for KASAN tag-based compiler instrumentation on
> x86.
> 


It appears that GCC nominally supports this, but in practice it does not work.
Here is a minimal reproducer: https://godbolt.org/z/s85e11T5r

As far as I understand, calling a function through a tagged pointer is not
supported by the hardware, so GCC attempts to clear the tag before the call.
This behavior seems to be inherited from the userspace implementation of HWASan (-fsanitize=hwaddress).

I have filed a GCC bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123696

For the kernel, we probably do not want this masking at all, as effectively 99.9–100%
of function pointer calls are expected to be untagged anyway.

Clang does not appear to do this, not even for userspace.