linux-kernel - Re: [PATCH 3/3] jump_label,noinstr: Avoid instrumentation for JUMP

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Date:   Mon, 2 May 2022 13:23:49 +0200
From:   Marco Elver <elver@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, jpoimboe@...hat.com, linux-kernel@...r.kernel.org,
        jbaron@...mai.com, rostedt@...dmis.org, ardb@...nel.org,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 3/3] jump_label,noinstr: Avoid instrumentation for
 JUMP_LABEL=n builds

On Mon, 2 May 2022 at 13:12, Peter Zijlstra <peterz@...radead.org> wrote:
>
> When building x86_64 with JUMP_LABEL=n it's possible for
> instrumentation to sneak into noinstr:
>
> vmlinux.o: warning: objtool: exit_to_user_mode+0x14: call to static_key_count.constprop.0() leaves .noinstr.text section
> vmlinux.o: warning: objtool: syscall_exit_to_user_mode+0x2d: call to static_key_count.constprop.0() leaves .noinstr.text section
> vmlinux.o: warning: objtool: irqentry_exit_to_user_mode+0x1b: call to static_key_count.constprop.0() leaves .noinstr.text section
>
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  include/linux/jump_label.h |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> --- a/include/linux/jump_label.h
> +++ b/include/linux/jump_label.h
> @@ -256,9 +256,9 @@ extern void static_key_disable_cpuslocke
>  #include <linux/atomic.h>
>  #include <linux/bug.h>
>
> -static inline int static_key_count(struct static_key *key)
> +static __always_inline int static_key_count(struct static_key *key)
>  {
> -       return atomic_read(&key->enabled);
> +       return READ_ONCE_NOCHECK(&key->enabled.count);
>  }

Did arch_atomic_read() not work? Or should this also be unchecked in
instrumented functions?