Re: [PATCH v2 2/2] kcsan: Never set up watchpoints on NULL pointers

[Dmitry Vyukov]

On Thu, Oct 22, 2020 at 1:45 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> Avoid setting up watchpoints on NULL pointers, as otherwise we would
> crash inside the KCSAN runtime (when checking for value changes) instead
> of the instrumented code.
>
> Because that may be confusing, skip any address less than PAGE_SIZE.
>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

> ---
>  kernel/kcsan/encoding.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
> index f03562aaf2eb..64b3c0f2a685 100644
> --- a/kernel/kcsan/encoding.h
> +++ b/kernel/kcsan/encoding.h
> @@ -48,7 +48,11 @@
>
>  static inline bool check_encodable(unsigned long addr, size_t size)
>  {
> -       return size <= MAX_ENCODABLE_SIZE;
> +       /*
> +        * While we can encode addrs<PAGE_SIZE, avoid crashing with a NULL
> +        * pointer deref inside KCSAN.
> +        */
> +       return addr >= PAGE_SIZE && size <= MAX_ENCODABLE_SIZE;
>  }
>
>  static inline long
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>