[PATCH 13/25] mm: factor out VMA fault permission checking

[Dave Hansen <dave@sr71.net>]

From: Dave Hansen <dave.hansen@linux.intel.com>

This code matches a fault condition up with the VMA and ensures
that the VMA allows the fault to be handled instead of just
erroring out.

We will be extending this in a moment to comprehend protection
keys.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
---

 b/mm/gup.c |   15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff -puN mm/gup.c~pkeys-10-pte-fault mm/gup.c
--- a/mm/gup.c~pkeys-10-pte-fault	2015-09-28 11:39:46.790221164 -0700
+++ b/mm/gup.c	2015-09-28 11:39:46.794221345 -0700
@@ -554,6 +554,17 @@ next_page:
 }
 EXPORT_SYMBOL(__get_user_pages);
 
+bool vma_permits_fault(struct vm_area_struct *vma, unsigned int fault_flags)
+{
+        vm_flags_t vm_flags =
+		(fault_flags & FAULT_FLAG_WRITE) ? VM_WRITE : VM_READ;
+
+	if (!(vm_flags & vma->vm_flags))
+		return false;
+
+	return true;
+}
+
 /*
  * fixup_user_fault() - manually resolve a user page fault
  * @tsk:	the task_struct to use for page fault accounting, or
@@ -585,15 +596,13 @@ int fixup_user_fault(struct task_struct
 		     unsigned long address, unsigned int fault_flags)
 {
 	struct vm_area_struct *vma;
-	vm_flags_t vm_flags;
 	int ret;
 
 	vma = find_extend_vma(mm, address);
 	if (!vma || address < vma->vm_start)
 		return -EFAULT;
 
-	vm_flags = (fault_flags & FAULT_FLAG_WRITE) ? VM_WRITE : VM_READ;
-	if (!(vm_flags & vma->vm_flags))
+	if (!vma_permits_fault(vma, fault_flags))
 		return -EFAULT;
 
 	ret = handle_mm_fault(mm, vma, address, fault_flags);
_