linux-kernel - [RFT][PATCH 7/7] parisc: Avoid overflow at boundary

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Message-Id: <20200820232106.23969-1-nicoleotsuka@gmail.com>
Date:   Thu, 20 Aug 2020 16:21:06 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     James.Bottomley@...senPartnership.com, deller@....de
Cc:     hch@....de, linux-parisc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [RFT][PATCH 7/7] parisc: Avoid overflow at boundary_size

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@...il.com>
Cc: Christoph Hellwig <hch@....de>
---
 drivers/parisc/ccio-dma.c  | 4 ++--
 drivers/parisc/sba_iommu.c | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/parisc/ccio-dma.c b/drivers/parisc/ccio-dma.c
index a5507f75b524..c667d6aba764 100644
--- a/drivers/parisc/ccio-dma.c
+++ b/drivers/parisc/ccio-dma.c
@@ -356,8 +356,8 @@ ccio_alloc_range(struct ioc *ioc, struct device *dev, size_t size)
 	** ggg sacrifices another 710 to the computer gods.
 	*/
 
-	boundary_size = ALIGN((unsigned long long)dma_get_seg_boundary(dev) + 1,
-			      1ULL << IOVP_SHIFT) >> IOVP_SHIFT;
+	/* Overflow-free shortcut for: ALIGN(b + 1, 1 << s) >> s */
+	boundary_size = (dma_get_seg_boundary(dev) >> IOVP_SHIFT) + 1;
 
 	if (pages_needed <= 8) {
 		/*
diff --git a/drivers/parisc/sba_iommu.c b/drivers/parisc/sba_iommu.c
index d4314fba0269..96bc2c617cbd 100644
--- a/drivers/parisc/sba_iommu.c
+++ b/drivers/parisc/sba_iommu.c
@@ -342,8 +342,8 @@ sba_search_bitmap(struct ioc *ioc, struct device *dev,
 	unsigned long shift;
 	int ret;
 
-	boundary_size = ALIGN((unsigned long long)dma_get_seg_boundary(dev) + 1,
-			      1ULL << IOVP_SHIFT) >> IOVP_SHIFT;
+	/* Overflow-free shortcut for: ALIGN(b + 1, 1 << s) >> s */
+	boundary_size = (dma_get_seg_boundary(dev) >> IOVP_SHIFT) + 1;
 
 #if defined(ZX1_SUPPORT)
 	BUG_ON(ioc->ibase & ~IOVP_MASK);
-- 
2.17.1