On 1/21/21 12:16 PM, Yu, Yu-cheng wrote:

quoted

quoted

@@ -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.