nic/packet_allocator: align allocations to 2 bytes

Override 'try_alloc/free' because ethernet frame headers are 14 bytes
(src/dst mac (12) + ethertype (2)) causing the IP header to be 2 byte
aligned, leading to problems on platforms that require load/store
operations to be naturally aligned when reading, for example, 4 byte IP
addresses. Therefore, we align the allocation to 2 bytes, so the IP
header is aligned to 4.

issue #4312

repos/os/include/nic/packet_allocator.h

@@ -17,16 +17,37 @@
 #define _INCLUDE__NIC__PACKET_ALLOCATOR__
 
 #include <os/packet_allocator.h>
+#include <base/log.h>
 
 namespace Nic { struct Packet_allocator; }
 
 
 /**
  * Packet allocator used for packet streaming in nic sessions.
+ *
+ * We override the allocator interface to align the IP packet to a 32-bit
+ * address. The ethernet frame header contains src/dst mac (12) + ethertype (2)
+ * causing the IP header to be at offset 14 in the packet. This leads to
+ * problems on platforms that require load/store operations to be naturally
+ * aligned when reading, for example, 4 byte IP addresses. Therefore, we
+ * allocate packet size plus OFFSET and offset the returned packet allocation
+ * at 2 bytes, which effectively aligns the IP header to 4 bytes.
+ *
+ * Note, this tweak reduces the usable bytes in the allocated packets to
+ * DEFAULT_PACKET_SIZE - OFFSET and assumes word-aligned allocations in the
+ * Genode::Packet_allocator. As DEFAULT_PACKET_SIZE is used for the
+ * transmission-buffer calculation we could not change it without breaking the
+ * API. OFFSET_PACKET_SIZE reflects the actual (usable) packet-buffer size.
  */
 struct Nic::Packet_allocator : Genode::Packet_allocator
 {
-	enum { DEFAULT_PACKET_SIZE = 1600 };
+	enum {
+		DEFAULT_PACKET_SIZE = 1600,
+		OFFSET = 2,
+		OFFSET_PACKET_SIZE = DEFAULT_PACKET_SIZE - OFFSET,
+	};
+
+	typedef Genode::size_t size_t;
 
 	/**
 	 * Constructor
@@ -35,6 +56,44 @@ struct Nic::Packet_allocator : Genode::Packet_allocator
 	 */
 	Packet_allocator(Genode::Allocator *md_alloc)
 	: Genode::Packet_allocator(md_alloc, DEFAULT_PACKET_SIZE) {}
+
+	Alloc_result try_alloc(size_t size) override
+	{
+		if (!size || size > OFFSET_PACKET_SIZE) {
+			Genode::error("unsupported NIC packet size ", size);
+			return Alloc_result { Alloc_error::DENIED };
+		}
+
+		Alloc_result result = Genode::Packet_allocator::try_alloc(size + OFFSET);
+
+		result.with_result(
+			[&] (void *content) {
+				/* assume word-aligned packet buffer and offset packet by 2 bytes */
+				if ((Genode::addr_t)content & 0b11) {
+					Genode::error("NIC packet allocation not word-aligned");
+					result = { Alloc_error::DENIED };
+				} else {
+					result = Alloc_result {
+						reinterpret_cast<void *>((Genode::uint8_t *)content + OFFSET) };
+				}
+			},
+			[] (Alloc_error) { }
+		);
+
+		return result;
+	}
+
+	void free(void *addr, size_t size) override
+	{
+		if (!size || size > OFFSET_PACKET_SIZE) {
+			Genode::error("unsupported NIC packet size ", size);
+			return;
+		}
+
+		Genode::Packet_allocator::free((Genode::uint8_t *)addr - OFFSET, size + OFFSET);
+	}
+
+
 };
 
 #endif /* _INCLUDE__NIC__PACKET_ALLOCATOR__ */

repos/os/src/drivers/nic/spec/linux/main.cc

@@ -122,15 +122,15 @@ class Uplink_client : public Uplink_client_base
 
 				progress = false;
 				size_t const max_pkt_size {
-					Nic::Packet_allocator::DEFAULT_PACKET_SIZE };
+					Nic::Packet_allocator::OFFSET_PACKET_SIZE };
 
 				_drv_rx_handle_pkt(
 					max_pkt_size,
 					[&] (void   *conn_tx_pkt_base,
 					     size_t &adjusted_conn_tx_pkt_size)
 				{
-					long int const read_result {
-						read(_tap_fd, conn_tx_pkt_base, max_pkt_size) };
+					ssize_t const read_result {
+						::read(_tap_fd, conn_tx_pkt_base, max_pkt_size) };
 
 					if (read_result <= 0) {
 

repos/os/src/lib/genode_c_api/uplink.cc

@@ -98,7 +98,7 @@ struct genode_uplink : private Noncopyable, private Interface
 			typedef Uplink::Packet_descriptor Packet_descriptor;
 
 			Packet_descriptor packet { };
-			size_t const max_bytes = Nic::Packet_allocator::DEFAULT_PACKET_SIZE;
+			size_t const max_bytes = Nic::Packet_allocator::OFFSET_PACKET_SIZE;
 
 			try { packet = tx_source.alloc_packet(max_bytes); }
 			catch (Uplink::Session::Tx::Source::Packet_alloc_failed) {