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Date: Tue, 13 Oct 2015 15:48:59 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Andrey Konovalov <andreyknvl@google.com>,
        Kostya Serebryany <kcc@google.com>,
        Alexander Potapenko <glider@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>, Borislav Petkov <bp@alien8.de>,
        Denys Vlasenko <dvlasenk@redhat.com>, Andi Kleen <ak@linux.intel.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        Wolfram Gloger <wmglo@dent.med.uni-muenchen.de>
Subject: Re: [PATCH v2 2/2] x86/process: Silence KASAN warnings in get_wchan()
Message-ID: <20151013134859.GB8843@gmail.com>
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* Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:

> get_wchan() is racy by design, it may access volatile stack
> of running task, thus it may access redzone in a stack frame
> and cause KASAN to warn about this.
> 
> Use READ_ONCE_NOCHECK() to silence these warnings.
> 
> Reported-by: Sasha Levin <sasha.levin@oracle.com>
> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> ---
>  arch/x86/kernel/process.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 39e585a..e28db18 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -550,14 +550,14 @@ unsigned long get_wchan(struct task_struct *p)
>  	if (sp < bottom || sp > top)
>  		return 0;
>  
> -	fp = READ_ONCE(*(unsigned long *)sp);
> +	fp = READ_ONCE_NOCHECK(*(unsigned long *)sp);
>  	do {
>  		if (fp < bottom || fp > top)
>  			return 0;
> -		ip = READ_ONCE(*(unsigned long *)(fp + sizeof(unsigned long)));
> +		ip = READ_ONCE_NOCHECK(*(unsigned long *)(fp + sizeof(unsigned long)));
>  		if (!in_sched_functions(ip))
>  			return ip;
> -		fp = READ_ONCE(*(unsigned long *)fp);
> +		fp = READ_ONCE_NOCHECK(*(unsigned long *)fp);
>  	} while (count++ < 16 && p->state != TASK_RUNNING);
>  	return 0;
>  }

Hm, exactly how is the 'red zone' defined? Is this about the current task mostly, 
or when doing get_wchan() on other tasks?

Thanks,

	Ingo