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// Copyright 2022 The gVisor 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 cmd
import (
"bytes"
"context"
_ "embed"
"errors"
"fmt"
"log"
"github.com/cilium/ebpf"
"github.com/google/subcommands"
"golang.org/x/sys/unix"
"gvisor.dev/gvisor/pkg/buffer"
"gvisor.dev/gvisor/pkg/tcpip/header"
"gvisor.dev/gvisor/pkg/tcpip/link/sniffer"
"gvisor.dev/gvisor/pkg/tcpip/stack"
"gvisor.dev/gvisor/pkg/xdp"
"gvisor.dev/gvisor/runsc/flag"
)
//go:embed bpf/tcpdump_ebpf.o
var tcpdumpProgram []byte
// TcpdumpCommand is a subcommand for capturing incoming packets.
type TcpdumpCommand struct {
device string
deviceIndex int
}
// Name implements subcommands.Command.Name.
func (*TcpdumpCommand) Name() string {
return "tcpdump"
}
// Synopsis implements subcommands.Command.Synopsis.
func (*TcpdumpCommand) Synopsis() string {
return "Run tcpdump-like program that blocks incoming packets."
}
// Usage implements subcommands.Command.Usage.
func (*TcpdumpCommand) Usage() string {
return "tcpdump -device <device> or -devidx <device index>"
}
// SetFlags implements subcommands.Command.SetFlags.
func (pc *TcpdumpCommand) SetFlags(fs *flag.FlagSet) {
fs.StringVar(&pc.device, "device", "", "which device to attach to")
fs.IntVar(&pc.deviceIndex, "devidx", 0, "which device to attach to")
}
// Execute implements subcommands.Command.Execute.
func (pc *TcpdumpCommand) Execute(context.Context, *flag.FlagSet, ...any) subcommands.ExitStatus {
if err := pc.execute(); err != nil {
fmt.Printf("%v", err)
return subcommands.ExitFailure
}
return subcommands.ExitSuccess
}
func (pc *TcpdumpCommand) execute() error {
iface, err := getIface(pc.device, pc.deviceIndex)
if err != nil {
return fmt.Errorf("%v", err)
}
// Load into the kernel.
spec, err := ebpf.LoadCollectionSpecFromReader(bytes.NewReader(tcpdumpProgram))
if err != nil {
return fmt.Errorf("failed to load spec: %v", err)
}
var objects struct {
Program *ebpf.Program `ebpf:"xdp_prog"`
SockMap *ebpf.Map `ebpf:"sock_map"`
}
if err := spec.LoadAndAssign(&objects, nil); err != nil {
return fmt.Errorf("failed to load program: %v", err)
}
defer func() {
if err := objects.Program.Close(); err != nil {
log.Printf("failed to close program: %v", err)
}
if err := objects.SockMap.Close(); err != nil {
log.Printf("failed to close sock map: %v", err)
}
}()
_, cleanup, err := attach(objects.Program, iface)
if err != nil {
return fmt.Errorf("failed to attach: %v", err)
}
defer cleanup()
controlBlock, err := xdp.New(
uint32(iface.Index), 0 /* queueID */, xdp.DefaultOpts())
if err != nil {
return fmt.Errorf("failed to create socket: %v", err)
}
// Insert our AF_XDP socket into the BPF map that dictates where
// packets are redirected to.
key := uint32(0)
val := controlBlock.UMEM.SockFD()
if err := objects.SockMap.Update(&key, &val, 0 /* flags */); err != nil {
return fmt.Errorf("failed to insert socket into BPF map: %v", err)
}
log.Printf("updated key %d to value %d", key, val)
// Put as many UMEM buffers into the fill queue as possible.
controlBlock.UMEM.Lock()
controlBlock.Fill.FillAll(&controlBlock.UMEM)
controlBlock.UMEM.Unlock()
go func() {
controlBlock.UMEM.Lock()
defer controlBlock.UMEM.Unlock()
for {
pfds := []unix.PollFd{{Fd: int32(controlBlock.UMEM.SockFD()), Events: unix.POLLIN}}
_, err := unix.Poll(pfds, -1)
if err != nil {
if errors.Is(err, unix.EINTR) {
continue
}
panic(fmt.Sprintf("poll failed: %v", err))
}
// How many packets did we get?
nReceived, rxIndex := controlBlock.RX.Peek()
if nReceived == 0 {
continue
}
// Keep the fill queue full.
controlBlock.Fill.FillAll(&controlBlock.UMEM)
// Read packets one-by-one and log them.
for i := uint32(0); i < nReceived; i++ {
// Wrap the packet in a PacketBuffer.
descriptor := controlBlock.RX.Get(rxIndex + i)
data := controlBlock.UMEM.Get(descriptor)
pkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
Payload: buffer.MakeWithData(data[header.EthernetMinimumSize:]),
})
sniffer.LogPacket("",
sniffer.DirectionRecv, // XDP operates only on ingress.
header.Ethernet(data).Type(),
pkt)
// NOTE: the address is always 256 bytes offset
// from a page boundary. The kernel masks the
// address to the frame size, so this isn't a
// problem.
//
// Note that this limits MTU to 4096-256 bytes.
controlBlock.UMEM.FreeFrame(descriptor.Addr)
}
controlBlock.RX.Release(nReceived)
}
}()
waitForever()
return nil
}
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