Re: [PATCH v2 2/2] x86/process: Silence KASAN warnings in get_wchan()

[Dmitry Vyukov]

On Tue, Oct 13, 2015 at 3:48 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> 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@xxxxxxxxxx>
>> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
>> ---
>>  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?


When code is compiled with AddressSanitizer, most variables on stack
have redzones around them, on entry function "poisons" these redzones
(any accesses to them will be flagged), on exit function "unpoisons"
these redzones.
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