Re: [PATCH] mm: kmsan: fix instrumentation recursion on preempt_count

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Mar 11, 2024 at 07:23:30PM +0800, Changbin Du wrote:
> This disables msan check for preempt_count_{add,sub} to fix a
> instrumentation recursion issue on preempt_count:
> 
>   __msan_metadata_ptr_for_load_4() -> kmsan_virt_addr_valid() ->
> 	preempt_disable() -> __msan_metadata_ptr_for_load_4()
> 
> With this fix, I was able to run kmsan kernel with:
>   o CONFIG_DEBUG_KMEMLEAK=n
>   o CONFIG_KFENCE=n
>   o CONFIG_LOCKDEP=n
> 
> KMEMLEAK and KFENCE generate too many false positives in unwinding code.
> LOCKDEP still introduces instrumenting recursions issue. But these are
> other issues expected to be fixed.
> 
> Cc: Marco Elver <elver@google.com>
> Signed-off-by: Changbin Du <changbin.du@huawei.com>
> ---
>  kernel/sched/core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9116bcc90346..5b63bb98e60a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5848,7 +5848,7 @@ static inline void preempt_latency_start(int val)
>  	}
>  }
>  
> -void preempt_count_add(int val)
> +void __no_kmsan_checks preempt_count_add(int val)
>  {
>  #ifdef CONFIG_DEBUG_PREEMPT
>  	/*
> @@ -5880,7 +5880,7 @@ static inline void preempt_latency_stop(int val)
>  		trace_preempt_on(CALLER_ADDR0, get_lock_parent_ip());
>  }

What prevents a larger loop via one of the calles of preempt_count_{add,sub}()

For example, via preempt_latency_{start,stop}() ?

... or via some *other* instrumentation that might be placed in those?

I suspect we should be using noinstr or __always_inline in a bunch of places to
clean this up properly.

Mark.