Hi, Thomas
Thank you for clarification.
On 2017/6/29 6:13, Thomas Gleixner wrote:
> On Wed, 28 Jun 2017, zhong jiang wrote:
>> On 2017/6/22 0:40, Ingo Molnar wrote:
>>> * zhong jiang <zhongjiang@xxxxxxxxxx> wrote:
>>>
>>>> when shift expoment is negative, left shift alway zero. therefore, we
>>>> modify the logic to avoid the warining.
>>>>
>>>> Signed-off-by: zhong jiang <zhongjiang@xxxxxxxxxx>
>>>> ---
>>>> arch/x86/include/asm/futex.h | 8 ++++++--
>>>> 1 file changed, 6 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/x86/include/asm/futex.h b/arch/x86/include/asm/futex.h
>>>> index b4c1f54..2425fca 100644
>>>> --- a/arch/x86/include/asm/futex.h
>>>> +++ b/arch/x86/include/asm/futex.h
>>>> @@ -49,8 +49,12 @@ static inline int futex_atomic_op_inuser(int encoded_op, u32 __user *uaddr)
>>>> int cmparg = (encoded_op << 20) >> 20;
>>>> int oldval = 0, ret, tem;
>>>>
>>>> - if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28))
>>>> - oparg = 1 << oparg;
>>>> + if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) {
>>>> + if (oparg >= 0)
>>>> + oparg = 1 << oparg;
>>>> + else
>>>> + oparg = 0;
>>>> + }
>>> Could we avoid all these complications by using an unsigned type?
>> I think it is not feasible. a negative shift exponent is likely
>> existence and reasonable.
> What is reasonable about a negative shift value?
>
>> as the above case, oparg is a negative is common.
> That's simply wrong. If oparg is negative and the SHIFT bit is set then the
> result is undefined today and there is no way that this can be used at
> all.
>
> On x86:
>
> 1 << -1 = 0x80000000
> 1 << -2048 = 0x00000001
> 1 << -2047 = 0x00000002
but I test the cases in x86_64 all is zero. I wonder whether it is related to gcc or not
zj.c:15:8: warning: left shift count is negative [-Wshift-count-negative]
j = 1 << -2048;
^
[root@localhost zhongjiang]# ./zj
j = 0
Thanks
zhongjiang
> Anything using a shift value < 0 or > 31 will get crap as a
> result. Rightfully so because it's just undefined.
>
> Yes I know that the insanity of user space is unlimited, but anything
> attempting this is so broken that we cannot break it further by making that
> shift arg unsigned and actually limit it to 0-31
> Thanks,
>
> tglx
>
>
>
> .
>
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