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.