ensure iptables chain creation is idempotent

Concurrent use of the `portmap` and `firewall` plugins can result in
errors during iptables chain creation:

- The `portmap` plugin has a time-of-check-time-of-use race where it
  checks for existence of the chain but the operation isn't atomic.
- The `firewall` plugin doesn't check for existing chains and just
  returns an error.

This commit makes both operations idempotent by creating the chain and
then discarding the error if it's caused by the chain already
existing. It also factors the chain creation out into `pkg/utils` as a
site for future refactoring work.

Signed-off-by: Tim Gross <tim@0x74696d.com>

pkg/utils/iptables.go

@@ -0,0 +1,64 @@
+// Copyright 2017 CNI authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package utils
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/coreos/go-iptables/iptables"
+)
+
+const statusChainExists = 1
+
+// EnsureChain idempotently creates the iptables chain. It does not
+// return an error if the chain already exists.
+func EnsureChain(ipt *iptables.IPTables, table, chain string) error {
+	if ipt == nil {
+		return errors.New("failed to ensure iptable chain: IPTables was nil")
+	}
+	exists, err := ChainExists(ipt, table, chain)
+	if err != nil {
+		return fmt.Errorf("failed to list iptables chains: %v", err)
+	}
+	if !exists {
+		err = ipt.NewChain(table, chain)
+		if err != nil {
+			eerr, eok := err.(*iptables.Error)
+			if eok && eerr.ExitStatus() != statusChainExists {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+// ChainExists checks whether an iptables chain exists.
+func ChainExists(ipt *iptables.IPTables, table, chain string) (bool, error) {
+	if ipt == nil {
+		return false, errors.New("failed to check iptable chain: IPTables was nil")
+	}
+	chains, err := ipt.ListChains(table)
+	if err != nil {
+		return false, err
+	}
+
+	for _, ch := range chains {
+		if ch == chain {
+			return true, nil
+		}
+	}
+	return false, nil
+}

pkg/utils/iptables_test.go

@@ -0,0 +1,78 @@
+// Copyright 2017-2018 CNI authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package utils
+
+import (
+	"fmt"
+	"math/rand"
+	"runtime"
+
+	"github.com/containernetworking/plugins/pkg/ns"
+	"github.com/containernetworking/plugins/pkg/testutils"
+	"github.com/coreos/go-iptables/iptables"
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+const TABLE = "filter" // We'll monkey around here
+
+var _ = Describe("chain tests", func() {
+	var testChain string
+	var ipt *iptables.IPTables
+	var cleanup func()
+
+	BeforeEach(func() {
+
+		// Save a reference to the original namespace,
+		// Add a new NS
+		currNs, err := ns.GetCurrentNS()
+		Expect(err).NotTo(HaveOccurred())
+
+		testNs, err := testutils.NewNS()
+		Expect(err).NotTo(HaveOccurred())
+
+		testChain = fmt.Sprintf("cni-test-%d", rand.Intn(10000000))
+
+		ipt, err = iptables.NewWithProtocol(iptables.ProtocolIPv4)
+		Expect(err).NotTo(HaveOccurred())
+
+		runtime.LockOSThread()
+		err = testNs.Set()
+		Expect(err).NotTo(HaveOccurred())
+
+		cleanup = func() {
+			if ipt == nil {
+				return
+			}
+			ipt.ClearChain(TABLE, testChain)
+			ipt.DeleteChain(TABLE, testChain)
+			currNs.Set()
+		}
+
+	})
+
+	AfterEach(func() {
+		cleanup()
+	})
+
+	It("creates chains idempotently", func() {
+		err := EnsureChain(ipt, TABLE, testChain)
+		Expect(err).NotTo(HaveOccurred())
+
+		// Create it again!
+		err = EnsureChain(ipt, TABLE, testChain)
+		Expect(err).NotTo(HaveOccurred())
+	})
+})

plugins/meta/firewall/firewall_iptables_test.go

@@ -270,6 +270,13 @@ var _ = Describe("firewall plugin iptables backend", func() {
 			Expect(err).NotTo(HaveOccurred())
 
 			validateFullRuleset(fullConf)
+
+			// ensure creation is idempotent
+			_, _, err = testutils.CmdAdd(targetNS.Path(), args.ContainerID, IFNAME, fullConf, func() error {
+				return cmdAdd(args)
+			})
+			Expect(err).NotTo(HaveOccurred())
+
 			return nil
 		})
 		Expect(err).NotTo(HaveOccurred())

plugins/meta/firewall/iptables.go

@@ -22,6 +22,7 @@ import (
 	"net"
 
 	"github.com/containernetworking/cni/pkg/types/current"
+	"github.com/containernetworking/plugins/pkg/utils"
 	"github.com/coreos/go-iptables/iptables"
 )
 
@@ -32,20 +33,6 @@ func getPrivChainRules(ip string) [][]string {
 	return rules
 }
 
-func ensureChain(ipt *iptables.IPTables, table, chain string) error {
-	chains, err := ipt.ListChains(table)
-	if err != nil {
-		return fmt.Errorf("failed to list iptables chains: %v", err)
-	}
-	for _, ch := range chains {
-		if ch == chain {
-			return nil
-		}
-	}
-
-	return ipt.NewChain(table, chain)
-}
-
 func generateFilterRule(privChainName string) []string {
 	return []string{"-m", "comment", "--comment", "CNI firewall plugin rules", "-j", privChainName}
 }
@@ -73,10 +60,10 @@ func (ib *iptablesBackend) setupChains(ipt *iptables.IPTables) error {
 	adminRule := generateFilterRule(ib.adminChainName)
 
 	// Ensure our private chains exist
-	if err := ensureChain(ipt, "filter", ib.privChainName); err != nil {
+	if err := utils.EnsureChain(ipt, "filter", ib.privChainName); err != nil {
 		return err
 	}
-	if err := ensureChain(ipt, "filter", ib.adminChainName); err != nil {
+	if err := utils.EnsureChain(ipt, "filter", ib.adminChainName); err != nil {
 		return err
 	}
 
@@ -160,10 +147,10 @@ func (ib *iptablesBackend) checkRules(conf *FirewallNetConf, result *current.Res
 	}
 
 	// Ensure our private chains exist
-	if err := ensureChain(ipt, "filter", ib.privChainName); err != nil {
+	if err := utils.EnsureChain(ipt, "filter", ib.privChainName); err != nil {
 		return err
 	}
-	if err := ensureChain(ipt, "filter", ib.adminChainName); err != nil {
+	if err := utils.EnsureChain(ipt, "filter", ib.adminChainName); err != nil {
 		return err
 	}
 

plugins/meta/portmap/chain.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"strings"
 
+	"github.com/containernetworking/plugins/pkg/utils"
 	"github.com/coreos/go-iptables/iptables"
 	"github.com/mattn/go-shellwords"
 )
@@ -35,16 +36,11 @@ type chain struct {
 
 // setup idempotently creates the chain. It will not error if the chain exists.
 func (c *chain) setup(ipt *iptables.IPTables) error {
-	// create the chain
-	exists, err := chainExists(ipt, c.table, c.name)
+
+	err := utils.EnsureChain(ipt, c.table, c.name)
 	if err != nil {
 		return err
 	}
-	if !exists {
-		if err := ipt.NewChain(c.table, c.name); err != nil {
-			return err
-		}
-	}
 
 	// Add the rules to the chain
 	for _, rule := range c.rules {
@@ -125,24 +121,10 @@ func insertUnique(ipt *iptables.IPTables, table, chain string, prepend bool, rul
 	}
 }
 
-func chainExists(ipt *iptables.IPTables, tableName, chainName string) (bool, error) {
-	chains, err := ipt.ListChains(tableName)
-	if err != nil {
-		return false, err
-	}
-
-	for _, ch := range chains {
-		if ch == chainName {
-			return true, nil
-		}
-	}
-	return false, nil
-}
-
 // check the chain.
 func (c *chain) check(ipt *iptables.IPTables) error {
 
-	exists, err := chainExists(ipt, c.table, c.name)
+	exists, err := utils.ChainExists(ipt, c.table, c.name)
 	if err != nil {
 		return err
 	}

plugins/meta/portmap/portmap.go

@@ -124,7 +124,7 @@ func checkPorts(config *PortMapConf, containerIP net.IP) error {
 	}
 
 	if ip4t != nil {
-		exists, err := chainExists(ip4t, dnatChain.table, dnatChain.name)
+		exists, err := utils.ChainExists(ip4t, dnatChain.table, dnatChain.name)
 		if err != nil {
 			return err
 		}
@@ -137,7 +137,7 @@ func checkPorts(config *PortMapConf, containerIP net.IP) error {
 	}
 
 	if ip6t != nil {
-		exists, err := chainExists(ip6t, dnatChain.table, dnatChain.name)
+		exists, err := utils.ChainExists(ip6t, dnatChain.table, dnatChain.name)
 		if err != nil {
 			return err
 		}