sfc: fix unaligned access in loopback selftests

PATCH Superseded [email protected] 3 participants 7 messages Jun 22, 2023

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Networking Network drivers Sfc network driver

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[email protected]

author

2023-06-22 10:19 UTC #0 lore raw

cover sfc: fix unaligned access in loopback selftests

From: Edward Cree <[email protected]>

Arnd reported that the sfc drivers each define a packed loopback_payload
structure with an ethernet header followed by an IP header, whereas the kernel definition of iphdr specifies that this is 4-byte aligned, causing a W=1 warning.
Fix this in each case by adding two bytes of leading padding to the
struct, taking care that these are not sent on the wire.
Tested on EF10; build-tested on Siena and Falcon.

Edward Cree (3):
sfc: use padding to fix alignment in loopback test
sfc: siena: use padding to fix alignment in loopback test
sfc: falcon: use padding to fix alignment in loopback test

 drivers/net/ethernet/sfc/falcon/selftest.c | 45 +++++++++++++---------
 drivers/net/ethernet/sfc/selftest.c        | 45 +++++++++++++---------
 drivers/net/ethernet/sfc/siena/selftest.c  | 45 +++++++++++++---------
 3 files changed, 81 insertions(+), 54 deletions(-)

[email protected]

author

2023-06-22 10:19 UTC #1 lore raw

PATCH 1/3 sfc: use padding to fix alignment in loopback test

From: Edward Cree <[email protected]>

Add two bytes of padding to the start of struct efx_loopback_payload,
which are not sent on the wire. This ensures the 'ip' member is
4-byte aligned, preventing the following W=1 warning:
net/ethernet/sfc/selftest.c:46:15: error: field ip within 'struct efx_loopback_payload' is less aligned than 'struct iphdr' and is usually due to 'struct efx_loopback_payload' being packed, which can lead to unaligned accesses [-Werror,-Wunaligned-access]

        struct iphdr ip;

Reported-by Arnd Bergmann <[email protected]>

Signed-off-by Edward Cree <[email protected]>

---
 drivers/net/ethernet/sfc/selftest.c | 45 +++++++++++++++++------------
 1 file changed, 27 insertions(+), 18 deletions(-)

drivers/net/ethernet/sfc/selftest.c +27 ~~-18~~

index 3c5227afd497..74b42efe7267 100644
@@ -42,12 +42,16 @@
  * Falcon only performs RSS on TCP/UDP packets.
  */
 struct efx_loopback_payload {
+	char pad[2]; /* Ensures ip is 4-byte aligned */
 	struct ethhdr header;
 	struct iphdr ip;
 	struct udphdr udp;
 	__be16 iteration;
 	char msg[64];
 } __packed;
+#define EFX_LOOPBACK_PAYLOAD_LEN	(sizeof(struct efx_loopback_payload) - \
+					 offsetof(struct efx_loopback_payload, \
+					 	  header))
 
 /* Loopback test source MAC address */
 static const u8 payload_source[ETH_ALEN] __aligned(2) = {

@@ -282,7 +286,7 @@ void efx_loopback_rx_packet(struct efx_nic *efx,
 			    const char *buf_ptr, int pkt_len)
 {
 	struct efx_loopback_state *state = efx->loopback_selftest;
-	struct efx_loopback_payload *received;
+	struct efx_loopback_payload received;
 	struct efx_loopback_payload *payload;
 
 	BUG_ON(!buf_ptr);

@@ -293,13 +297,14 @@ void efx_loopback_rx_packet(struct efx_nic *efx,
 
 	payload = &state->payload;
 
-	received = (struct efx_loopback_payload *) buf_ptr;
-	received->ip.saddr = payload->ip.saddr;
+	memcpy(&received.header, buf_ptr,
+	       min_t(int, pkt_len, EFX_LOOPBACK_PAYLOAD_LEN));
+	received.ip.saddr = payload->ip.saddr;
 	if (state->offload_csum)
-		received->ip.check = payload->ip.check;
+		received.ip.check = payload->ip.check;
 
 	/* Check that header exists */
-	if (pkt_len < sizeof(received->header)) {
+	if (pkt_len < sizeof(received.header)) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw runt RX packet (length %d) in %s loopback "
 			  "test\n", pkt_len, LOOPBACK_MODE(efx));

@@ -307,7 +312,7 @@ void efx_loopback_rx_packet(struct efx_nic *efx,
 	}
 
 	/* Check that the ethernet header exists */
-	if (memcmp(&received->header, &payload->header, ETH_HLEN) != 0) {
+	if (memcmp(&received.header, &payload->header, ETH_HLEN) != 0) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw non-loopback RX packet in %s loopback test\n",
 			  LOOPBACK_MODE(efx));

@@ -315,16 +320,16 @@ void efx_loopback_rx_packet(struct efx_nic *efx,
 	}
 
 	/* Check packet length */
-	if (pkt_len != sizeof(*payload)) {
+	if (pkt_len != EFX_LOOPBACK_PAYLOAD_LEN) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw incorrect RX packet length %d (wanted %d) in "
-			  "%s loopback test\n", pkt_len, (int)sizeof(*payload),
-			  LOOPBACK_MODE(efx));
+			  "%s loopback test\n", pkt_len,
+			  (int)EFX_LOOPBACK_PAYLOAD_LEN, LOOPBACK_MODE(efx));
 		goto err;
 	}
 
 	/* Check that IP header matches */
-	if (memcmp(&received->ip, &payload->ip, sizeof(payload->ip)) != 0) {
+	if (memcmp(&received.ip, &payload->ip, sizeof(payload->ip)) != 0) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw corrupted IP header in %s loopback test\n",
 			  LOOPBACK_MODE(efx));

@@ -332,7 +337,7 @@ void efx_loopback_rx_packet(struct efx_nic *efx,
 	}
 
 	/* Check that msg and padding matches */
-	if (memcmp(&received->msg, &payload->msg, sizeof(received->msg)) != 0) {
+	if (memcmp(&received.msg, &payload->msg, sizeof(received.msg)) != 0) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw corrupted RX packet in %s loopback test\n",
 			  LOOPBACK_MODE(efx));

@@ -340,10 +345,10 @@ void efx_loopback_rx_packet(struct efx_nic *efx,
 	}
 
 	/* Check that iteration matches */
-	if (received->iteration != payload->iteration) {
+	if (received.iteration != payload->iteration) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw RX packet from iteration %d (wanted %d) in "
-			  "%s loopback test\n", ntohs(received->iteration),
+			  "%s loopback test\n", ntohs(received.iteration),
 			  ntohs(payload->iteration), LOOPBACK_MODE(efx));
 		goto err;
 	}

@@ -363,7 +368,8 @@ void efx_loopback_rx_packet(struct efx_nic *efx,
 			       buf_ptr, pkt_len, 0);
 		netif_err(efx, drv, efx->net_dev, "expected packet:\n");
 		print_hex_dump(KERN_ERR, "", DUMP_PREFIX_OFFSET, 0x10, 1,
-			       &state->payload, sizeof(state->payload), 0);
+			       &state->payload.header, EFX_LOOPBACK_PAYLOAD_LEN,
+			       0);
 	}
 #endif
 	atomic_inc(&state->rx_bad);

@@ -385,14 +391,15 @@ static void efx_iterate_state(struct efx_nic *efx)
 	payload->ip.daddr = htonl(INADDR_LOOPBACK);
 	payload->ip.ihl = 5;
 	payload->ip.check = (__force __sum16) htons(0xdead);
-	payload->ip.tot_len = htons(sizeof(*payload) - sizeof(struct ethhdr));
+	payload->ip.tot_len = htons(sizeof(*payload) -
+				    offsetof(struct efx_loopback_payload, ip));
 	payload->ip.version = IPVERSION;
 	payload->ip.protocol = IPPROTO_UDP;
 
 	/* Initialise udp header */
 	payload->udp.source = 0;
-	payload->udp.len = htons(sizeof(*payload) - sizeof(struct ethhdr) -
-				 sizeof(struct iphdr));
+	payload->udp.len = htons(sizeof(*payload) -
+				 offsetof(struct efx_loopback_payload, udp));
 	payload->udp.check = 0;	/* checksum ignored */
 
 	/* Fill out payload */

@@ -418,7 +425,7 @@ static int efx_begin_loopback(struct efx_tx_queue *tx_queue)
 	for (i = 0; i < state->packet_count; i++) {
 		/* Allocate an skb, holding an extra reference for
 		 * transmit completion counting */
-		skb = alloc_skb(sizeof(state->payload), GFP_KERNEL);
+		skb = alloc_skb(EFX_LOOPBACK_PAYLOAD_LEN, GFP_KERNEL);
 		if (!skb)
 			return -ENOMEM;
 		state->skbs[i] = skb;

@@ -429,6 +436,8 @@ static int efx_begin_loopback(struct efx_tx_queue *tx_queue)
 		payload = skb_put(skb, sizeof(state->payload));
 		memcpy(payload, &state->payload, sizeof(state->payload));
 		payload->ip.saddr = htonl(INADDR_LOOPBACK | (i << 2));
+		/* Strip off the leading padding */
+		skb_pull(skb, offsetof(struct efx_loopback_payload, header));
 
 		/* Ensure everything we've written is visible to the
 		 * interrupt handler. */

Arnd Bergmann

2023-06-22 11:39 UTC #2 lore raw

Re: [PATCH net-next 1/3] sfc: use padding to fix alignment in loopback test

Quoted 9 lines · On Thu, Jun 22, 2023, at 12:18, [email protected] wrote:

 drivers/net/ethernet/sfc/selftest.c | 45 +++++++++++++++++------------
 1 file changed, 27 insertions(+), 18 deletions(-)

drivers/net/ethernet/sfc/selftest.c +0 -0

index 3c5227afd497..74b42efe7267 100644

drivers/net/ethernet/sfc/selftest.c line 51

Quoted 10 lines

drivers/net/ethernet/sfc/selftest.c +1 -0

@@ -42,12 +42,16 @@
  * Falcon only performs RSS on TCP/UDP packets.
  */
 struct efx_loopback_payload {
+	char pad[2]; /* Ensures ip is 4-byte aligned */
 	struct ethhdr header;
 	struct iphdr ip;
 	struct udphdr udp;
 	__be16 iteration;
 	char msg[64];
 } __packed;

There should probably be an extra __aligned(4) after the __packed, so that the compiler knows the start of the structure is aligned, otherwise (depending on the architecture and compiler), any access to a member can still turn into multiple single-byte accesses.

I would also expect to still see a warning without that extra attribute. Aside from this, the patch looks good to me.

Arnd

Edward Cree

maintainer

2023-06-22 12:02 UTC #3 lore raw

Re: [PATCH net-next 1/3] sfc: use padding to fix alignment in loopback test

Quoted 17 lines · Arnd Bergmann

Quoted 16 lines · On Thu, Jun 22, 2023, at 12:18, [email protected] wrote:

 drivers/net/ethernet/sfc/selftest.c | 45 +++++++++++++++++------------
 1 file changed, 27 insertions(+), 18 deletions(-)

drivers/net/ethernet/sfc/selftest.c +1 -0

index 3c5227afd497..74b42efe7267 100644
@@ -42,12 +42,16 @@
  * Falcon only performs RSS on TCP/UDP packets.
  */
 struct efx_loopback_payload {
+	char pad[2]; /* Ensures ip is 4-byte aligned */
 	struct ethhdr header;
 	struct iphdr ip;
 	struct udphdr udp;

Quoted 8 lines

drivers/net/ethernet/sfc/selftest.c line 51
Quoted 3 lines
drivers/net/ethernet/sfc/selftest.c +0 -0
 	__be16 iteration;
 	char msg[64];
 } __packed;
There should probably be an extra __aligned(4) after the __packed, so that the compiler knows the start of the structure is aligned, otherwise (depending on the architecture and compiler), any access to a member can still turn into multiple single-byte accesses.

Ok, will add that in v2.

Quoted 2 lines

I would also expect to still see a warning without that extra attribute. Aside from this, the patch looks good to me.

My compiler (gcc 8.3.1) doesn't reproduce the original warning, so
I'm flying slightly blind here :( If you could build-test this on your setup with/without the __aligned(4), I'd be very grateful.

-ed

Arnd Bergmann

2023-06-22 12:20 UTC #4 lore raw

Re: [PATCH net-next 1/3] sfc: use padding to fix alignment in loopback test

Quoted 8 lines · Edward Cree

Quoted 6 lines · Arnd Bergmann
On Thu, Jun 22, 2023, at 12:18, [email protected] wrote:
There should probably be an extra __aligned(4) after the __packed, so that the compiler knows the start of the structure is aligned, otherwise (depending on the architecture and compiler), any access to a member can still turn into multiple single-byte accesses.

Quoted 7 lines

Ok, will add that in v2.
Quoted 2 lines
I would also expect to still see a warning without that extra attribute. Aside from this, the patch looks good to me.
My compiler (gcc 8.3.1) doesn't reproduce the original warning, so
I'm flying slightly blind here :( If you could build-test this on your setup with/without the __aligned(4), I'd be very grateful.

I can confirm that your patch addresses the warning for me with any version of clang I have installed, and that the __aligned(4) is not needed for the warning, though it is still a good idea for code generation.

Arnd

[email protected]

author

2023-06-22 10:20 UTC #5 lore raw

PATCH 2/3 sfc: siena: use padding to fix alignment in loopback test

In reply to [email protected]

From: Edward Cree <[email protected]>

Add two bytes of padding to the start of struct efx_loopback_payload,
which are not sent on the wire. This ensures the 'ip' member is
4-byte aligned, preventing the following W=1 warning:
net/ethernet/sfc/siena/selftest.c:46:15: error: field ip within 'struct efx_loopback_payload' is less aligned than 'struct iphdr' and is usually due to 'struct efx_loopback_payload' being packed, which can lead to unaligned accesses [-Werror,-Wunaligned-access]

        struct iphdr ip;

Reported-by Arnd Bergmann <[email protected]>

Signed-off-by Edward Cree <[email protected]>

---
 drivers/net/ethernet/sfc/siena/selftest.c | 45 ++++++++++++++---------
 1 file changed, 27 insertions(+), 18 deletions(-)

drivers/net/ethernet/sfc/siena/selftest.c +27 ~~-18~~

index 07715a3d6bea..c65c62dc642e 100644
@@ -42,12 +42,16 @@
  * Falcon only performs RSS on TCP/UDP packets.
  */
 struct efx_loopback_payload {
+	char pad[2]; /* Ensures ip is 4-byte aligned */
 	struct ethhdr header;
 	struct iphdr ip;
 	struct udphdr udp;
 	__be16 iteration;
 	char msg[64];
 } __packed;
+#define EFX_LOOPBACK_PAYLOAD_LEN	(sizeof(struct efx_loopback_payload) - \
+					 offsetof(struct efx_loopback_payload, \
+					 	  header))
 
 /* Loopback test source MAC address */
 static const u8 payload_source[ETH_ALEN] __aligned(2) = {

@@ -282,7 +286,7 @@ void efx_siena_loopback_rx_packet(struct efx_nic *efx,
 				  const char *buf_ptr, int pkt_len)
 {
 	struct efx_loopback_state *state = efx->loopback_selftest;
-	struct efx_loopback_payload *received;
+	struct efx_loopback_payload received;
 	struct efx_loopback_payload *payload;
 
 	BUG_ON(!buf_ptr);

@@ -293,13 +297,14 @@ void efx_siena_loopback_rx_packet(struct efx_nic *efx,
 
 	payload = &state->payload;
 
-	received = (struct efx_loopback_payload *) buf_ptr;
-	received->ip.saddr = payload->ip.saddr;
+	memcpy(&received.header, buf_ptr,
+	       min_t(int, pkt_len, EFX_LOOPBACK_PAYLOAD_LEN));
+	received.ip.saddr = payload->ip.saddr;
 	if (state->offload_csum)
-		received->ip.check = payload->ip.check;
+		received.ip.check = payload->ip.check;
 
 	/* Check that header exists */
-	if (pkt_len < sizeof(received->header)) {
+	if (pkt_len < sizeof(received.header)) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw runt RX packet (length %d) in %s loopback "
 			  "test\n", pkt_len, LOOPBACK_MODE(efx));

@@ -307,7 +312,7 @@ void efx_siena_loopback_rx_packet(struct efx_nic *efx,
 	}
 
 	/* Check that the ethernet header exists */
-	if (memcmp(&received->header, &payload->header, ETH_HLEN) != 0) {
+	if (memcmp(&received.header, &payload->header, ETH_HLEN) != 0) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw non-loopback RX packet in %s loopback test\n",
 			  LOOPBACK_MODE(efx));

@@ -315,16 +320,16 @@ void efx_siena_loopback_rx_packet(struct efx_nic *efx,
 	}
 
 	/* Check packet length */
-	if (pkt_len != sizeof(*payload)) {
+	if (pkt_len != EFX_LOOPBACK_PAYLOAD_LEN) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw incorrect RX packet length %d (wanted %d) in "
-			  "%s loopback test\n", pkt_len, (int)sizeof(*payload),
-			  LOOPBACK_MODE(efx));
+			  "%s loopback test\n", pkt_len,
+			  (int)EFX_LOOPBACK_PAYLOAD_LEN, LOOPBACK_MODE(efx));
 		goto err;
 	}
 
 	/* Check that IP header matches */
-	if (memcmp(&received->ip, &payload->ip, sizeof(payload->ip)) != 0) {
+	if (memcmp(&received.ip, &payload->ip, sizeof(payload->ip)) != 0) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw corrupted IP header in %s loopback test\n",
 			  LOOPBACK_MODE(efx));

@@ -332,7 +337,7 @@ void efx_siena_loopback_rx_packet(struct efx_nic *efx,
 	}
 
 	/* Check that msg and padding matches */
-	if (memcmp(&received->msg, &payload->msg, sizeof(received->msg)) != 0) {
+	if (memcmp(&received.msg, &payload->msg, sizeof(received.msg)) != 0) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw corrupted RX packet in %s loopback test\n",
 			  LOOPBACK_MODE(efx));

@@ -340,10 +345,10 @@ void efx_siena_loopback_rx_packet(struct efx_nic *efx,
 	}
 
 	/* Check that iteration matches */
-	if (received->iteration != payload->iteration) {
+	if (received.iteration != payload->iteration) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw RX packet from iteration %d (wanted %d) in "
-			  "%s loopback test\n", ntohs(received->iteration),
+			  "%s loopback test\n", ntohs(received.iteration),
 			  ntohs(payload->iteration), LOOPBACK_MODE(efx));
 		goto err;
 	}

@@ -363,7 +368,8 @@ void efx_siena_loopback_rx_packet(struct efx_nic *efx,
 			       buf_ptr, pkt_len, 0);
 		netif_err(efx, drv, efx->net_dev, "expected packet:\n");
 		print_hex_dump(KERN_ERR, "", DUMP_PREFIX_OFFSET, 0x10, 1,
-			       &state->payload, sizeof(state->payload), 0);
+			       &state->payload.header, EFX_LOOPBACK_PAYLOAD_LEN,
+			       0);
 	}
 #endif
 	atomic_inc(&state->rx_bad);

@@ -385,14 +391,15 @@ static void efx_iterate_state(struct efx_nic *efx)
 	payload->ip.daddr = htonl(INADDR_LOOPBACK);
 	payload->ip.ihl = 5;
 	payload->ip.check = (__force __sum16) htons(0xdead);
-	payload->ip.tot_len = htons(sizeof(*payload) - sizeof(struct ethhdr));
+	payload->ip.tot_len = htons(sizeof(*payload) -
+				    offsetof(struct efx_loopback_payload, ip));
 	payload->ip.version = IPVERSION;
 	payload->ip.protocol = IPPROTO_UDP;
 
 	/* Initialise udp header */
 	payload->udp.source = 0;
-	payload->udp.len = htons(sizeof(*payload) - sizeof(struct ethhdr) -
-				 sizeof(struct iphdr));
+	payload->udp.len = htons(sizeof(*payload) -
+				 offsetof(struct efx_loopback_payload, udp));
 	payload->udp.check = 0;	/* checksum ignored */
 
 	/* Fill out payload */

@@ -418,7 +425,7 @@ static int efx_begin_loopback(struct efx_tx_queue *tx_queue)
 	for (i = 0; i < state->packet_count; i++) {
 		/* Allocate an skb, holding an extra reference for
 		 * transmit completion counting */
-		skb = alloc_skb(sizeof(state->payload), GFP_KERNEL);
+		skb = alloc_skb(EFX_LOOPBACK_PAYLOAD_LEN, GFP_KERNEL);
 		if (!skb)
 			return -ENOMEM;
 		state->skbs[i] = skb;

@@ -429,6 +436,8 @@ static int efx_begin_loopback(struct efx_tx_queue *tx_queue)
 		payload = skb_put(skb, sizeof(state->payload));
 		memcpy(payload, &state->payload, sizeof(state->payload));
 		payload->ip.saddr = htonl(INADDR_LOOPBACK | (i << 2));
+		/* Strip off the leading padding */
+		skb_pull(skb, offsetof(struct efx_loopback_payload, header));
 
 		/* Ensure everything we've written is visible to the
 		 * interrupt handler. */

[email protected]

author

2023-06-22 10:20 UTC #6 lore raw

PATCH 3/3 sfc: falcon: use padding to fix alignment in loopback test

In reply to [email protected]

From: Edward Cree <[email protected]>

Add two bytes of padding to the start of struct ef4_loopback_payload,
which are not sent on the wire. This ensures the 'ip' member is
4-byte aligned, preventing the following W=1 warning:
net/ethernet/sfc/falcon/selftest.c:43:15: error: field ip within 'struct ef4_loopback_payload' is less aligned than 'struct iphdr' and is usually due to 'struct ef4_loopback_payload' being packed, which can lead to unaligned accesses [-Werror,-Wunaligned-access]

        struct iphdr ip;

Reported-by Arnd Bergmann <[email protected]>

Signed-off-by Edward Cree <[email protected]>

---
 drivers/net/ethernet/sfc/falcon/selftest.c | 45 +++++++++++++---------
 1 file changed, 27 insertions(+), 18 deletions(-)

drivers/net/ethernet/sfc/falcon/selftest.c +27 ~~-18~~

index 6a454ac6f876..5fa3e745a525 100644
@@ -39,12 +39,16 @@
  * Falcon only performs RSS on TCP/UDP packets.
  */
 struct ef4_loopback_payload {
+	char pad[2]; /* Ensures ip is 4-byte aligned */
 	struct ethhdr header;
 	struct iphdr ip;
 	struct udphdr udp;
 	__be16 iteration;
 	char msg[64];
 } __packed;
+#define EF4_LOOPBACK_PAYLOAD_LEN	(sizeof(struct ef4_loopback_payload) - \
+					 offsetof(struct ef4_loopback_payload, \
+					 	  header))
 
 /* Loopback test source MAC address */
 static const u8 payload_source[ETH_ALEN] __aligned(2) = {

@@ -284,7 +288,7 @@ void ef4_loopback_rx_packet(struct ef4_nic *efx,
 			    const char *buf_ptr, int pkt_len)
 {
 	struct ef4_loopback_state *state = efx->loopback_selftest;
-	struct ef4_loopback_payload *received;
+	struct ef4_loopback_payload received;
 	struct ef4_loopback_payload *payload;
 
 	BUG_ON(!buf_ptr);

@@ -295,13 +299,14 @@ void ef4_loopback_rx_packet(struct ef4_nic *efx,
 
 	payload = &state->payload;
 
-	received = (struct ef4_loopback_payload *) buf_ptr;
-	received->ip.saddr = payload->ip.saddr;
+	memcpy(&received.header, buf_ptr,
+	       min_t(int, pkt_len, EF4_LOOPBACK_PAYLOAD_LEN));
+	received.ip.saddr = payload->ip.saddr;
 	if (state->offload_csum)
-		received->ip.check = payload->ip.check;
+		received.ip.check = payload->ip.check;
 
 	/* Check that header exists */
-	if (pkt_len < sizeof(received->header)) {
+	if (pkt_len < sizeof(received.header)) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw runt RX packet (length %d) in %s loopback "
 			  "test\n", pkt_len, LOOPBACK_MODE(efx));

@@ -309,7 +314,7 @@ void ef4_loopback_rx_packet(struct ef4_nic *efx,
 	}
 
 	/* Check that the ethernet header exists */
-	if (memcmp(&received->header, &payload->header, ETH_HLEN) != 0) {
+	if (memcmp(&received.header, &payload->header, ETH_HLEN) != 0) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw non-loopback RX packet in %s loopback test\n",
 			  LOOPBACK_MODE(efx));

@@ -317,16 +322,16 @@ void ef4_loopback_rx_packet(struct ef4_nic *efx,
 	}
 
 	/* Check packet length */
-	if (pkt_len != sizeof(*payload)) {
+	if (pkt_len != EF4_LOOPBACK_PAYLOAD_LEN) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw incorrect RX packet length %d (wanted %d) in "
-			  "%s loopback test\n", pkt_len, (int)sizeof(*payload),
-			  LOOPBACK_MODE(efx));
+			  "%s loopback test\n", pkt_len,
+			  (int)EF4_LOOPBACK_PAYLOAD_LEN, LOOPBACK_MODE(efx));
 		goto err;
 	}
 
 	/* Check that IP header matches */
-	if (memcmp(&received->ip, &payload->ip, sizeof(payload->ip)) != 0) {
+	if (memcmp(&received.ip, &payload->ip, sizeof(payload->ip)) != 0) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw corrupted IP header in %s loopback test\n",
 			  LOOPBACK_MODE(efx));

@@ -334,7 +339,7 @@ void ef4_loopback_rx_packet(struct ef4_nic *efx,
 	}
 
 	/* Check that msg and padding matches */
-	if (memcmp(&received->msg, &payload->msg, sizeof(received->msg)) != 0) {
+	if (memcmp(&received.msg, &payload->msg, sizeof(received.msg)) != 0) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw corrupted RX packet in %s loopback test\n",
 			  LOOPBACK_MODE(efx));

@@ -342,10 +347,10 @@ void ef4_loopback_rx_packet(struct ef4_nic *efx,
 	}
 
 	/* Check that iteration matches */
-	if (received->iteration != payload->iteration) {
+	if (received.iteration != payload->iteration) {
 		netif_err(efx, drv, efx->net_dev,
 			  "saw RX packet from iteration %d (wanted %d) in "
-			  "%s loopback test\n", ntohs(received->iteration),
+			  "%s loopback test\n", ntohs(received.iteration),
 			  ntohs(payload->iteration), LOOPBACK_MODE(efx));
 		goto err;
 	}

@@ -365,7 +370,8 @@ void ef4_loopback_rx_packet(struct ef4_nic *efx,
 			       buf_ptr, pkt_len, 0);
 		netif_err(efx, drv, efx->net_dev, "expected packet:\n");
 		print_hex_dump(KERN_ERR, "", DUMP_PREFIX_OFFSET, 0x10, 1,
-			       &state->payload, sizeof(state->payload), 0);
+			       &state->payload.header, EF4_LOOPBACK_PAYLOAD_LEN,
+			       0);
 	}
 #endif
 	atomic_inc(&state->rx_bad);

@@ -387,14 +393,15 @@ static void ef4_iterate_state(struct ef4_nic *efx)
 	payload->ip.daddr = htonl(INADDR_LOOPBACK);
 	payload->ip.ihl = 5;
 	payload->ip.check = (__force __sum16) htons(0xdead);
-	payload->ip.tot_len = htons(sizeof(*payload) - sizeof(struct ethhdr));
+	payload->ip.tot_len = htons(sizeof(*payload) -
+				    offsetof(struct ef4_loopback_payload, ip));
 	payload->ip.version = IPVERSION;
 	payload->ip.protocol = IPPROTO_UDP;
 
 	/* Initialise udp header */
 	payload->udp.source = 0;
-	payload->udp.len = htons(sizeof(*payload) - sizeof(struct ethhdr) -
-				 sizeof(struct iphdr));
+	payload->udp.len = htons(sizeof(*payload) -
+				 offsetof(struct ef4_loopback_payload, udp));
 	payload->udp.check = 0;	/* checksum ignored */
 
 	/* Fill out payload */

@@ -420,7 +427,7 @@ static int ef4_begin_loopback(struct ef4_tx_queue *tx_queue)
 	for (i = 0; i < state->packet_count; i++) {
 		/* Allocate an skb, holding an extra reference for
 		 * transmit completion counting */
-		skb = alloc_skb(sizeof(state->payload), GFP_KERNEL);
+		skb = alloc_skb(EF4_LOOPBACK_PAYLOAD_LEN, GFP_KERNEL);
 		if (!skb)
 			return -ENOMEM;
 		state->skbs[i] = skb;

@@ -431,6 +438,8 @@ static int ef4_begin_loopback(struct ef4_tx_queue *tx_queue)
 		payload = skb_put(skb, sizeof(state->payload));
 		memcpy(payload, &state->payload, sizeof(state->payload));
 		payload->ip.saddr = htonl(INADDR_LOOPBACK | (i << 2));
+		/* Strip off the leading padding */
+		skb_pull(skb, offsetof(struct ef4_loopback_payload, header));
 
 		/* Ensure everything we've written is visible to the
 		 * interrupt handler. */