linux-kernel - Re: [PATCH 4/7] x86/cpu: Elide KCSAN for cpu

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Message-ID: <YpTDRFGcEGIO0h06@elver.google.com>
Date:   Mon, 30 May 2022 15:14:44 +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,
        mark.rutland@....com, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 4/7] x86/cpu: Elide KCSAN for cpu_has() and friends

On Thu, May 26, 2022 at 12:52PM +0200, Peter Zijlstra wrote:
> As x86 uses the <asm-generic/bitops/instrumented-*.h> headers, the
> regular forms of all bitops are instrumented with explicit calls to
> KASAN and KCSAN checks. As these are explicit calls, these are not
> suppressed by the noinstr function attribute.
> 
> This can result in calls to those check functions in noinstr code, which
> objtool warns about:
> 
> vmlinux.o: warning: objtool: enter_from_user_mode+0x24: call to __kcsan_check_access() leaves .noinstr.text section
> vmlinux.o: warning: objtool: syscall_enter_from_user_mode+0x28: call to __kcsan_check_access() leaves .noinstr.text section
> vmlinux.o: warning: objtool: syscall_enter_from_user_mode_prepare+0x24: call to __kcsan_check_access() leaves .noinstr.text section
> vmlinux.o: warning: objtool: irqentry_enter_from_user_mode+0x24: call to __kcsan_check_access() leaves .noinstr.text section
> 
> Prevent this by using the arch_*() bitops, which are the underlying
> bitops without explciit instrumentation.
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: https://lkml.kernel.org/r/20220502111216.290518605@infradead.org

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

> ---
>  arch/x86/include/asm/cpufeature.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/arch/x86/include/asm/cpufeature.h
> +++ b/arch/x86/include/asm/cpufeature.h
> @@ -51,7 +51,7 @@ extern const char * const x86_power_flag
>  extern const char * const x86_bug_flags[NBUGINTS*32];
>  
>  #define test_cpu_cap(c, bit)						\
> -	 test_bit(bit, (unsigned long *)((c)->x86_capability))
> +	 arch_test_bit(bit, (unsigned long *)((c)->x86_capability))
>  
>  /*
>   * There are 32 bits/features in each mask word.  The high bits
> 
>