linux-kernel - Re: [ammarfaizi2-block:paulmck/linux-rcu/pmladek.2022.06.15a 133/140] vmlinux.o: warning: objtool: __ct_user_exit+0x41: call to __kasan_check

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Date:   Thu, 16 Jun 2022 19:32:14 +0200
From:   Marco Elver <elver@...gle.com>
To:     paulmck@...nel.org
Cc:     kernel test robot <lkp@...el.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        kbuild-all@...ts.01.org,
        "GNU/Weeb Mailing List" <gwml@...r.gnuweeb.org>,
        linux-kernel@...r.kernel.org, dvyukov@...gle.com
Subject: Re: [ammarfaizi2-block:paulmck/linux-rcu/pmladek.2022.06.15a 133/140]
 vmlinux.o: warning: objtool: __ct_user_exit+0x41: call to __kasan_check_read()
 leaves .noinstr.text section

On Thu, 16 Jun 2022 at 18:44, Paul E. McKenney <paulmck@...nel.org> wrote:
[...]
> > > > > > >> vmlinux.o: warning: objtool: __ct_user_exit+0x41: call to __kasan_check_read() leaves .noinstr.text section
> > > > > > >> vmlinux.o: warning: objtool: __ct_user_enter+0x7f: call to __kasan_check_read() leaves .noinstr.text section
[...]
> > > It's due to the atomic_read()s within the noinstr function. Within
> > > noinstr you can use arch_atomic_read() to avoid the instrumentation.
> >
> > Thank you, will fix!
>
> And please see below for an alleged fix.

Looks good, if this made the above warnings go away.

Btw, it's not just KASAN, but arch_atomic_read() also disables KCSAN
checking. A similar warning would have been generated in a KCSAN
config.

Reviewed-by: Marco Elver <elver@...gle.com>

>                                                         Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 81e24ca26ee9933bcacf67a61e3f6ae41a025442
> Author: Paul E. McKenney <paulmck@...nel.org>
> Date:   Thu Jun 16 09:30:37 2022 -0700
>
>     context_tracking: Use arch_atomic_read() in __ct_state for KASAN
>
>     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>
>
> diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
> index 0aecc07fb4f50..81c51e5f03143 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
>