linux-kernel - Re: [PATCH v2 1/1] context_tracking: Use arch_atomic_read() in __ct

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Date:   Wed, 22 Jun 2022 18:58:26 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     rcu <rcu@...r.kernel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Rushikesh S Kadam <rushikesh.s.kadam@...el.com>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Neeraj upadhyay <neeraj.iitr10@...il.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>, vineeth@...byteword.org,
        Marco Elver <elver@...gle.com>
Subject: Re: [PATCH v2 1/1] context_tracking: Use arch_atomic_read() in
 __ct_state for KASAN

Apologies, I accidentally picked this specific commit up in my recent
git format-patch which ended up sending it out. Totally unintended.

On Wed, Jun 22, 2022 at 6:51 PM Joel Fernandes (Google)
<joel@...lfernandes.org> wrote:
>
> From: "Paul E. McKenney" <paulmck@...nel.org>
>
> Context tracking's __ct_state() function can be invoked from noinstr state
> where RCU is not watching.  This means that its use of atomic_read()
> causes KASAN to invoke the non-noinstr __kasan_check_read() function
> from the noinstr function __ct_state().  This is problematic because
> someone tracing the __kasan_check_read() function could get a nasty
> surprise because of RCU not watching.
>
> This commit therefore replaces the __ct_state() function's use of
> atomic_read() with arch_atomic_read(), which KASAN does not attempt to
> add instrumention to.
>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Cc: Marco Elver <elver@...gle.com>
> Reviewed-by: Marco Elver <elver@...gle.com>
> ---
>  include/linux/context_tracking_state.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
> index 0aecc07fb4f5..81c51e5f0314 100644
> --- a/include/linux/context_tracking_state.h
> +++ b/include/linux/context_tracking_state.h
> @@ -49,7 +49,7 @@ DECLARE_PER_CPU(struct context_tracking, context_tracking);
>
>  static __always_inline int __ct_state(void)
>  {
> -       return atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_STATE_MASK;
> +       return arch_atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_STATE_MASK;
>  }
>  #endif
>
> --
> 2.37.0.rc0.104.g0611611a94-goog
>