[PATCH v3] hugetlbfs: Fix integer overflow check in hugetlbfs_file_mmap()

[Linke Li <lilinke99@foxmail.com>]

From: Linke Li <lilinke99@gmail.com>

```
	vma_len = (loff_t)(vma->vm_end - vma->vm_start);
	len = vma_len + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
	/* check for overflow */
	if (len < vma_len)
		return -EINVAL;
```

There is a signed integer overflow in the code, which is undefined
behavior according to the C stacnard. Although this works, it's
still a bit ugly and static checkers will complain.

Using macro "check_add_overflow" to do the overflow check can
effectively detect integer overflow and avoid any undefined behavior.

Signed-off-by: Linke Li <lilinke99@gmail.com>
---
v3: fix checkpatch warning and better description.
 fs/hugetlbfs/inode.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 7b17ccfa039d..326a8c0af5f6 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -154,10 +154,7 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
 	if (vma->vm_pgoff & (~huge_page_mask(h) >> PAGE_SHIFT))
 		return -EINVAL;
 
-	vma_len = (loff_t)(vma->vm_end - vma->vm_start);
-	len = vma_len + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
-	/* check for overflow */
-	if (len < vma_len)
+	if (check_add_overflow(vma_len, (loff_t)vma->vm_pgoff << PAGE_SHIFT, &len))
 		return -EINVAL;
 
 	inode_lock(inode);
-- 
2.25.1