On Fri, Dec 02, 2022 at 04:35:38PM -0800, Rick Edgecombe wrote:

 static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
 {
+	pteval_t _page_chg_mask_no_dirty = _PAGE_CHG_MASK & ~_PAGE_DIRTY;
 	pteval_t val = pte_val(pte), oldval = val;
+	pte_t pte_result;
 
 	/*
 	 * Chop off the NX bit (if present), and add the NX portion of
 	 * the newprot (if present):
 	 */
-	val &= _PAGE_CHG_MASK;
-	val |= check_pgprot(newprot) & ~_PAGE_CHG_MASK;
+	val &= _page_chg_mask_no_dirty;
+	val |= check_pgprot(newprot) & ~_page_chg_mask_no_dirty;
 	val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);
-	return __pte(val);
+
+	pte_result = __pte(val);
+
+	/*
+	 * Dirty bit is not preserved above so it can be done

Just for my own understanding: are you saying here that
flip_protnone_guard() might end up setting _PAGE_DIRTY in val...

+	 * in a special way for the shadow stack case, where it
+	 * needs to set _PAGE_COW. pte_mkcow() will do this in
+	 * the case of shadow stack.
+	 */
+	if (pte_dirty(pte_result))
+		pte_result = pte_mkcow(pte_result);

... and in that case we need to turn it into a _PAGE_COW setting?

Thx.

-- 
Regards/Gruss,
    Boris.

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