On 2017/6/29 5:43, hpa@xxxxxxxxx wrote:
> On June 27, 2017 9:35:10 PM PDT, zhong jiang <zhongjiang@xxxxxxxxxx> wrote:
>> Hi, Ingo
>>
>> Thank you for the comment.
>> 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.
>> as the above case, oparg is a negative is common.
>>
>> I think it can be avoided by following change.
>>
>> diff --git a/arch/x86/include/asm/futex.h
>> b/arch/x86/include/asm/futex.h
>> index b4c1f54..3205e86 100644
>> --- a/arch/x86/include/asm/futex.h
>> +++ b/arch/x86/include/asm/futex.h
>> @@ -50,7 +50,7 @@ static inline int futex_atomic_op_inuser(int
>> encoded_op, u32 __user *uaddr)
>> int oldval = 0, ret, tem;
>>
>> if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28))
>> - oparg = 1 << oparg;
>> + oparg = safe_shift(1, oparg);
>>
>> if (!access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))
>> return -EFAULT;
>> diff --git a/drivers/video/fbdev/core/fbmem.c
>> b/drivers/video/fbdev/core/fbmem.c
>> index 069fe79..b4edda3 100644
>> --- a/drivers/video/fbdev/core/fbmem.c
>> +++ b/drivers/video/fbdev/core/fbmem.c
>> @@ -190,11 +190,6 @@ char* fb_get_buffer_offset(struct fb_info *info,
>> struct fb_pixmap *buf, u32 size
>>
>> #ifdef CONFIG_LOGO
>>
>> -static inline unsigned safe_shift(unsigned d, int n)
>> -{
>> - return n < 0 ? d >> -n : d << n;
>> -}
>> -
>> static void fb_set_logocmap(struct fb_info *info,
>> const struct linux_logo *logo)
>> {
>> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
>> index d043ada..f3b8856 100644
>> --- a/include/linux/kernel.h
>> +++ b/include/linux/kernel.h
>> @@ -841,6 +841,10 @@ static inline void ftrace_dump(enum
>> ftrace_dump_mode oops_dump_mode) { }
>> */
>> #define clamp_val(val, lo, hi) clamp_t(typeof(val), val, lo, hi)
>>
>> +static inline unsigned safe_shift(unsigned d, int n)
>> +{
>> + return n < 0 ? d >> -n : d << n;
>> +}
>>
>> Thansk
>> zhongjiang
>>
>>> Thanks,
>>>
>>> Ingo
>>>
>>> .
>>>
> What makes it reasonable? It is totally ill-defined and doesn't do anything useful now?
Thanks you for comments.
Maybe I mismake the meaning. I test the negative cases in x86 , all case is zero. so I come to a conclusion.
zj.c:15:8: warning: left shift count is negative [-Wshift-count-negative]
j = 1 << -2048;
^
[root@localhost zhongjiang]# ./zj
j = 0
j.c:15:8: warning: left shift count is negative [-Wshift-count-negative]
j = 1 << -2047;
^
[root@localhost zhongjiang]# ./zj
j = 0
I insmod a module into kernel to test the testcasts, all of the result is zero.
I wonder whether I miss some point or not. Do you point out to me? please
Thanks
zhongjiang
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