[RFT][PATCH 4/7] s390/pci_dma: Avoid overflow at boundary_size

[Nicolin Chen <nicoleotsuka@gmail.com>]

The boundary_size might be as large as ULONG_MAX, which means
that a device has no specific boundary limit. So either "+ 1"
or passing it to ALIGN() would potentially overflow.

According to kernel defines:
    #define ALIGN_MASK(x, mask) (((x) + (mask)) & ~(mask))
    #define ALIGN(x, a)	ALIGN_MASK(x, (typeof(x))(a) - 1)

We can simplify the logic here:
  ALIGN(boundary + 1, 1 << shift) >> shift
= ALIGN_MASK(b + 1, (1 << s) - 1) >> s
= {[b + 1 + (1 << s) - 1] & ~[(1 << s) - 1]} >> s
= [b + 1 + (1 << s) - 1] >> s
= [b + (1 << s)] >> s
= (b >> s) + 1

So fixing a potential overflow with the safer shortcut.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
---
 arch/s390/pci/pci_dma.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/s390/pci/pci_dma.c b/arch/s390/pci/pci_dma.c
index 64b1399a73f0..ecb067acc6d5 100644
--- a/arch/s390/pci/pci_dma.c
+++ b/arch/s390/pci/pci_dma.c
@@ -263,8 +263,8 @@ static unsigned long __dma_alloc_iommu(struct device *dev,
 	struct zpci_dev *zdev = to_zpci(to_pci_dev(dev));
 	unsigned long boundary_size;
 
-	boundary_size = ALIGN(dma_get_seg_boundary(dev) + 1,
-			      PAGE_SIZE) >> PAGE_SHIFT;
+	/* Overflow-free shortcut for: ALIGN(b + 1, 1 << s) >> s */
+	boundary_size = (dma_get_seg_boundary(dev) >> PAGE_SHIFT) + 1;
 	return iommu_area_alloc(zdev->iommu_bitmap, zdev->iommu_pages,
 				start, size, zdev->start_dma >> PAGE_SHIFT,
 				boundary_size, 0);
-- 
2.17.1