Re: [syzbot] [kernel?] upstream test error: KMSAN: uninit-value in irqentry_exit_to_kernel_mode_preempt

[Thomas Gleixner]

On Mon, Jun 22 2026 at 11:37, Alexander Potapenko wrote:
>> I have a draft Clang patch implementing the following intrinsics at
>> https://github.com/llvm/llvm-project/pull/202603:
>> - llvm.kmsan.instrumentation.begin
>> - llvm.kmsan.instrumentation.end
>> - llvm.kmsan.instrumentation.update.context (to reload the context
>> pointer; I am not using it yet).
>
> For the record, Marco Elver has an alternative proposal: modify
> __kmsan_get_context() to take _RET_IP_ from the caller.
> Then, if noinstr function A calls an instrumented function B, B will
> pass the return address belonging to A to __kmsan_get_context(), and
> the latter can check that return address and wipe out the argument
> shadow if it belongs to the noinstr function.
>
> This solution is quite elegant; it requires only minor compiler
> changes, and won't require any kernel annotations.
> However, there are downsides:
> - We are slowing down each call, including those between instrumented functions.
> - For noinstr calls from modules, we'll need complex lookups involving
> RCU, with a potential for infinite recursion.

Let's not go there. I rather spend some effort to disentangle the
BUG/WARN maze.

Thanks,

        tglx