libnet: bridge: ignore EINVAL when configuring bridge MTU

Since 964ab7158c, we explicitly set the bridge MTU if it was specified.
Unfortunately, kernel <v4.17 have a check preventing us to manually set
the MTU to anything greater than 1500 if no links is attached to the
bridge, which is how we do things -- create the bridge, set its MTU and
later on, attach veths to it.

Relevant kernel commit: 804b854d37

As we still have to support CentOS/RHEL 7 (and their old v3.10 kernels)
for a few more months, we need to ignore EINVAL if the MTU is > 1500
(but <= 65535).

Signed-off-by: Albin Kerouanton <albinker@gmail.com>

libnetwork/drivers/bridge/setup_device_linux.go

@@ -2,9 +2,11 @@ package bridge
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
+	"syscall"
 
 	"github.com/containerd/log"
 	"github.com/docker/docker/libnetwork/netutils"
@@ -47,6 +49,14 @@ func setupDevice(config *networkConfiguration, i *bridgeInterface) error {
 
 func setupMTU(config *networkConfiguration, i *bridgeInterface) error {
 	if err := i.nlh.LinkSetMTU(i.Link, config.Mtu); err != nil {
+		// Before Linux v4.17, bridges couldn't be configured "manually" with an MTU greater than 1500, although it
+		// could be autoconfigured with such a value when interfaces were added to the bridge. In that case, the
+		// bridge MTU would be set automatically by the kernel to the lowest MTU of all interfaces attached. To keep
+		// compatibility with older kernels, we need to discard -EINVAL.
+		// TODO(aker): remove this once we drop support for CentOS/RHEL 7.
+		if config.Mtu > 1500 && config.Mtu <= 0xFFFF && errors.Is(err, syscall.EINVAL) {
+			return nil
+		}
 		log.G(context.TODO()).WithError(err).Errorf("Failed to set bridge MTU %s via netlink", config.BridgeName)
 		return err
 	}

libnetwork/drivers/bridge/setup_device_linux_test.go

@@ -3,11 +3,13 @@ package bridge
 import (
 	"bytes"
 	"net"
+	"syscall"
 	"testing"
 
 	"github.com/docker/docker/internal/testutils/netnsutils"
 	"github.com/docker/docker/libnetwork/netutils"
 	"github.com/vishvananda/netlink"
+	"gotest.tools/v3/assert"
 )
 
 func TestSetupNewBridge(t *testing.T) {
@@ -92,3 +94,35 @@ func TestGenerateRandomMAC(t *testing.T) {
 		t.Fatalf("Generated twice the same MAC address %v", mac1)
 	}
 }
+
+func TestMTUBiggerThan1500(t *testing.T) {
+	defer netnsutils.SetupTestOSContext(t)()
+
+	nh, err := netlink.NewHandle()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer nh.Close()
+
+	config := &networkConfiguration{BridgeName: DefaultBridgeName, Mtu: 9000}
+	br := &bridgeInterface{nlh: nh}
+
+	assert.NilError(t, setupDevice(config, br))
+	assert.NilError(t, setupMTU(config, br))
+}
+
+func TestMTUBiggerThan64K(t *testing.T) {
+	defer netnsutils.SetupTestOSContext(t)()
+
+	nh, err := netlink.NewHandle()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer nh.Close()
+
+	config := &networkConfiguration{BridgeName: DefaultBridgeName, Mtu: 65536}
+	br := &bridgeInterface{nlh: nh}
+
+	assert.NilError(t, setupDevice(config, br))
+	assert.ErrorIs(t, setupMTU(config, br), syscall.EINVAL)
+}