Disable host reassembly for fragments.

fdbased endpoint was enabling fragment reassembly on the host AF_PACKET socket
to ensure that fragments are delivered inorder to the right dispatcher. But this
prevents fragments from being delivered to gvisor at all and makes testing of
gvisor's fragment reassembly code impossible.

The potential impact from this is minimal since IP Fragmentation is not really
that prevelant and in cases where we do get fragments we may deliver the
fragment out of order to the TCP layer as multiple network dispatchers may
process the fragments and deliver a reassembled fragment after the next packet
has been delivered to the TCP endpoint. While not desirable I believe the impact
from this is minimal due to low prevalence of fragmentation.

Also removed PktType and Hatype fields when binding the socket as these are not
used when binding. Its just confusing to have them specified.

See: https://man7.org/linux/man-pages/man7/packet.7.html
"Fields used for binding are
       sll_family (should be AF_PACKET), sll_protocol, and sll_ifindex."

Fixes #5055

PiperOrigin-RevId: 346919439

1 files changed, 6 insertions, 3 deletions

diff --git a/pkg/tcpip/link/fdbased/endpoint.go b/pkg/tcpip/link/fdbased/endpoint.go
index 9f2084eae..cb94cbea6 100644
--- a/pkg/tcpip/link/fdbased/endpoint.go
+++ b/pkg/tcpip/link/fdbased/endpoint.go
@@ -284,9 +284,12 @@ func createInboundDispatcher(e *endpoint, fd int, isSocket bool) (linkDispatcher
 		}
 		switch sa.(type) {
 		case *unix.SockaddrLinklayer:
-			// enable PACKET_FANOUT mode is the underlying socket is
-			// of type AF_PACKET.
-			const fanoutType = 0x8000 // PACKET_FANOUT_HASH | PACKET_FANOUT_FLAG_DEFRAG
+			// Enable PACKET_FANOUT mode if the underlying socket is of type
+			// AF_PACKET. We do not enable PACKET_FANOUT_FLAG_DEFRAG as that will
+			// prevent gvisor from receiving fragmented packets and the host does the
+			// reassembly on our behalf before delivering the fragments. This makes it
+			// hard to test fragmentation reassembly code in Netstack.
+			const fanoutType = unix.PACKET_FANOUT_HASH
 			fanoutArg := fanoutID | fanoutType<<16
 			if err := syscall.SetsockoptInt(fd, syscall.SOL_PACKET, unix.PACKET_FANOUT, fanoutArg); err != nil {
 				return nil, fmt.Errorf("failed to enable PACKET_FANOUT option: %v", err)