On 1/21/21 2:16 PM, David Laight wrote:
> From: Yu, Yu-cheng
>>
>> On 1/21/2021 10:44 AM, Borislav Petkov wrote:
>>> On Tue, Dec 29, 2020 at 01:30:35PM -0800, Yu-cheng Yu wrote:
>> [...]
>>>> @@ -343,6 +349,16 @@ static inline pte_t pte_mkold(pte_t pte)
>>>>
>>>> static inline pte_t pte_wrprotect(pte_t pte)
>>>> {
>>>> + /*
>>>> + * Blindly clearing _PAGE_RW might accidentally create
>>>> + * a shadow stack PTE (RW=0, Dirty=1). Move the hardware
>>>> + * dirty value to the software bit.
>>>> + */
>>>> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
>>>> + pte.pte |= (pte.pte & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW;
>>>
>>> Why the unreadable shifting when you can simply do:
>>>
>>> if (pte.pte & _PAGE_DIRTY)
>>> pte.pte |= _PAGE_COW;
>>>
>
>>> ?
>>
>> It clears _PAGE_DIRTY and sets _PAGE_COW. That is,
>>
>> if (pte.pte & _PAGE_DIRTY) {
>> pte.pte &= ~_PAGE_DIRTY;
>> pte.pte |= _PAGE_COW;
>> }
>>
>> So, shifting makes resulting code more efficient.
>
> Does the compiler manage to do one shift?
>
> How can it clear anything?
It could shift it off either end since there are both
<< and >>.
> There is only an |= against the target.
>
> Something horrid with ^= might set and clear.
--
~Randy
"He closes his eyes and drops the goggles. You can't get hurt
by looking at a bitmap. Or can you?"
(Neal Stephenson: Snow Crash)
