netdev - [PATCH net-next] net: ipa: avoid 64-bit modulus

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Message-Id: <20210323010505.2149882-1-elder@linaro.org>
Date:   Mon, 22 Mar 2021 20:05:05 -0500
From:   Alex Elder <elder@...aro.org>
To:     davem@...emloft.net, kuba@...nel.org
Cc:     rdunlap@...radead.org, bjorn.andersson@...aro.org,
        evgreen@...omium.org, cpratapa@...eaurora.org,
        subashab@...eaurora.org, elder@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH net-next] net: ipa: avoid 64-bit modulus

It is possible for a 32 bit x86 build to use a 64 bit DMA address.

There are two remaining spots where the IPA driver does a modulo
operation to check alignment of a DMA address, and under certain
conditions this can lead to a build error on i386 (at least).

The alignment checks we're doing are for power-of-2 values, and this
means the lower 32 bits of the DMA address can be used.  This ensures
both operands to the modulo operator are 32 bits wide.

Reported-by: Randy Dunlap <rdunlap@...radead.org>
Signed-off-by: Alex Elder <elder@...aro.org>
---
 drivers/net/ipa/gsi.c       | 11 +++++++----
 drivers/net/ipa/ipa_table.c |  9 ++++++---
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ipa/gsi.c b/drivers/net/ipa/gsi.c
index 7f3e338ca7a72..b6355827bf900 100644
--- a/drivers/net/ipa/gsi.c
+++ b/drivers/net/ipa/gsi.c
@@ -1436,15 +1436,18 @@ static void gsi_evt_ring_rx_update(struct gsi_evt_ring *evt_ring, u32 index)
 /* Initialize a ring, including allocating DMA memory for its entries */
 static int gsi_ring_alloc(struct gsi *gsi, struct gsi_ring *ring, u32 count)
 {
-	size_t size = count * GSI_RING_ELEMENT_SIZE;
+	u32 size = count * GSI_RING_ELEMENT_SIZE;
 	struct device *dev = gsi->dev;
 	dma_addr_t addr;
 
-	/* Hardware requires a 2^n ring size, with alignment equal to size */
+	/* Hardware requires a 2^n ring size, with alignment equal to size.
+	 * The size is a power of 2, so we can check alignment using just
+	 * the bottom 32 bits for a DMA address of any size.
+	 */
 	ring->virt = dma_alloc_coherent(dev, size, &addr, GFP_KERNEL);
-	if (ring->virt && addr % size) {
+	if (ring->virt && lower_32_bits(addr) % size) {
 		dma_free_coherent(dev, size, ring->virt, addr);
-		dev_err(dev, "unable to alloc 0x%zx-aligned ring buffer\n",
+		dev_err(dev, "unable to alloc 0x%x-aligned ring buffer\n",
 			size);
 		return -EINVAL;	/* Not a good error value, but distinct */
 	} else if (!ring->virt) {
diff --git a/drivers/net/ipa/ipa_table.c b/drivers/net/ipa/ipa_table.c
index 988f2c2886b95..4236a50ff03ae 100644
--- a/drivers/net/ipa/ipa_table.c
+++ b/drivers/net/ipa/ipa_table.c
@@ -658,10 +658,13 @@ int ipa_table_init(struct ipa *ipa)
 		return -ENOMEM;
 
 	/* We put the "zero rule" at the base of our table area.  The IPA
-	 * hardware requires rules to be aligned on a 128-byte boundary.
-	 * Make sure the allocation satisfies this constraint.
+	 * hardware requires route and filter table rules to be aligned
+	 * on a 128-byte boundary.  As long as the alignment constraint
+	 * is a power of 2, we can check alignment using just the bottom
+	 * 32 bits for a DMA address of any size.
 	 */
-	if (addr % IPA_TABLE_ALIGN) {
+	BUILD_BUG_ON(!is_power_of_2(IPA_TABLE_ALIGN));
+	if (lower_32_bits(addr) % IPA_TABLE_ALIGN) {
 		dev_err(dev, "table address %pad not %u-byte aligned\n",
 			&addr, IPA_TABLE_ALIGN);
 		dma_free_coherent(dev, size, virt, addr);
-- 
2.27.0