On 1/21/21 12:16 PM, Yu, Yu-cheng 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.
Are you sure?
Usually, the compiler is better at making code efficient than humans. I
find that coding it in the most human-readable way is best unless I
*know* the compiler is unable to generate god code.
