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

Oleg Nesterov
2011-04-12T19:09:28Z

__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;
}

/*