[PATCH 1/1] __mlock_vma_pages_range: stack_guard_page() case returns the wrong value

[Oleg Nesterov <oleg@redhat.com>]

__mlock_vma_pages_range() simply changes addr/nr_pages when
stack_guard_page(vma, start). But this means that __get_user_pages()
returns a number which doesn't match the [start, end) interval and
the caller can be confused.

If we skip the first page, we should return 1 if gup fails, or add
1 to the number it returns.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 mm/mlock.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

--- sigprocmask/mm/mlock.c~do_mlock_pages_stack_guard_page	2011-04-06 21:33:50.000000000 +0200
+++ sigprocmask/mm/mlock.c	2011-04-12 20:50:30.000000000 +0200
@@ -159,9 +159,8 @@ static long __mlock_vma_pages_range(stru
 				    int *nonblocking)
 {
 	struct mm_struct *mm = vma->vm_mm;
-	unsigned long addr = start;
 	int nr_pages = (end - start) / PAGE_SIZE;
-	int gup_flags;
+	int gup_flags, skip_page, ret;
 
 	VM_BUG_ON(start & ~PAGE_MASK);
 	VM_BUG_ON(end   & ~PAGE_MASK);
@@ -189,13 +188,22 @@ static long __mlock_vma_pages_range(stru
 		gup_flags |= FOLL_MLOCK;
 
 	/* We don't try to access the guard page of a stack vma */
+	skip_page = 0;
 	if (stack_guard_page(vma, start)) {
-		addr += PAGE_SIZE;
+		skip_page = 1;
+		start += PAGE_SIZE;
 		nr_pages--;
 	}
 
-	return __get_user_pages(current, mm, addr, nr_pages, gup_flags,
+	ret = __get_user_pages(current, mm, start, nr_pages, gup_flags,
 				NULL, NULL, nonblocking);
+
+	if (ret >= 0)
+		ret += skip_page;
+	else if (skip_page)
+		ret = 1;
+
+	return ret;
 }
 
 /*