Avoid fanout group collisions with best effort

Running multiple instances of netstack in the same network namespace can
cause collisions when enabling packet fanout for fdbased endpoints. The
only bulletproof fix is to run in different network namespaces, but by
using `getpid()` instead of 0 as the fanout ID starting point we can
avoid collisions in the common case, particularly when
testing/experimenting.

Addresses #6124

1 files changed, 6 insertions, 8 deletions

diff --git a/pkg/tcpip/link/fdbased/endpoint.go b/pkg/tcpip/link/fdbased/endpoint.go
index bddb1d0a2..735c28da1 100644
--- a/pkg/tcpip/link/fdbased/endpoint.go
+++ b/pkg/tcpip/link/fdbased/endpoint.go
@@ -41,7 +41,6 @@ package fdbased
 
 import (
 	"fmt"
-	"math"
 	"sync/atomic"
 
 	"golang.org/x/sys/unix"
@@ -196,8 +195,12 @@ type Options struct {
 // option for an FD with a fanoutID already in use by another FD for a different
 // NIC will return an EINVAL.
 //
+// Since fanoutID must be unique within the network namespace, we start with
+// the PID to avoid collisions. The only way to be sure of avoiding collisions
+// is to run in a new network namespace.
+//
 // Must be accessed using atomic operations.
-var fanoutID int32 = 0
+var fanoutID int32 = int32(unix.Getpid())
 
 // New creates a new fd-based endpoint.
 //
@@ -292,11 +295,6 @@ func createInboundDispatcher(e *endpoint, fd int, isSocket bool, fID int32) (lin
 		}
 		switch sa.(type) {
 		case *unix.SockaddrLinklayer:
-			// See: PACKET_FANOUT_MAX in net/packet/internal.h
-			const packetFanoutMax = 1 << 16
-			if fID > packetFanoutMax {
-				return nil, fmt.Errorf("host fanoutID limit exceeded, fanoutID must be <= %d", math.MaxUint16)
-			}
 			// 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
@@ -317,7 +315,7 @@ func createInboundDispatcher(e *endpoint, fd int, isSocket bool, fID int32) (lin
 			//
 			// See: https://github.com/torvalds/linux/blob/7acac4b3196caee5e21fb5ea53f8bc124e6a16fc/net/packet/af_packet.c#L3881
 			const fanoutType = unix.PACKET_FANOUT_HASH
-			fanoutArg := int(fID) | fanoutType<<16
+			fanoutArg := (int(fID) & 0xffff) | fanoutType<<16
 			if err := unix.SetsockoptInt(fd, unix.SOL_PACKET, unix.PACKET_FANOUT, fanoutArg); err != nil {
 				return nil, fmt.Errorf("failed to enable PACKET_FANOUT option: %v", err)
 			}