[PATCH 4/7] mm: Introduce verify_page_range()

[Peter Zijlstra <peterz@infradead.org>]

Introduce and EXPORT a read-only counterpart to apply_to_page_range().

It only exposes the PTE value, not a pointer to the pagetables itself
and is thus quite a bit safer to export. A number of
apply_to_page_range() users can be converted to this primitive.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/mm.h |    4 ++++
 mm/memory.c        |   24 ++++++++++++++++++++++++
 2 files changed, 28 insertions(+)

--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2876,6 +2876,10 @@ extern int apply_to_page_range(struct mm
 extern int apply_to_existing_page_range(struct mm_struct *mm,
 				   unsigned long address, unsigned long size,
 				   pte_fn_t fn, void *data);
+extern int verify_page_range(struct mm_struct *mm,
+			     unsigned long addr, unsigned long size,
+			     int (*fn)(pte_t pte, unsigned long addr, void *data),
+			     void *data);
 
 extern void init_mem_debugging_and_hardening(void);
 #ifdef CONFIG_PAGE_POISONING
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2559,6 +2559,30 @@ int apply_to_existing_page_range(struct
 	return __apply_to_page_range(mm, addr, size, fn, data, false);
 }
 
+struct vpr_data {
+	int (*fn)(pte_t pte, unsigned long addr, void *data);
+	void *data;
+};
+
+static int vpr_fn(pte_t *pte, unsigned long addr, void *data)
+{
+	struct vpr_data *vpr = data;
+	return vpr->fn(*pte, addr, vpr->data);
+}
+
+int verify_page_range(struct mm_struct *mm,
+		      unsigned long addr, unsigned long size,
+		      int (*fn)(pte_t pte, unsigned long addr, void *data),
+		      void *data)
+{
+	struct vpr_data vpr = {
+		.fn = fn,
+		.data = data,
+	};
+	return apply_to_page_range(mm, addr, size, vpr_fn, &vpr);
+}
+EXPORT_SYMBOL_GPL(verify_page_range);
+
 /*
  * handle_pte_fault chooses page fault handler according to an entry which was
  * read non-atomically.  Before making any commitment, on those architectures