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Subject: Re: [PATCH v2 2/2] x86/process: Silence KASAN warnings in get_wchan()
To: Ingo Molnar <mingo@kernel.org>
References: <1444739750-29241-1-git-send-email-aryabinin@virtuozzo.com>
 <1444739750-29241-3-git-send-email-aryabinin@virtuozzo.com>
 <20151013134859.GB8843@gmail.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>
From: Andrey Ryabinin <aryabinin@virtuozzo.com>
Message-ID: <561D0DAE.6010903@virtuozzo.com>
Date: Tue, 13 Oct 2015 16:57:02 +0300
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On 10/13/2015 04:48 PM, Ingo Molnar wrote:
> 
> * 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?

We doing get_whcan() *only* on other tasks:

520:	if (!p || p == current || p->state == TASK_RUNNING)
521:		return 0;


Current wouldn't be a problem for KASAN.

 


> Thanks,
> 
> 	Ingo
>