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package main
import (
"fmt"
"os"
"strings"
"syscall"
"time"
"golang.org/x/sys/unix"
)
func status(keys ...string) string {
raw, _ := os.ReadFile("/proc/self/status")
var out []string
for _, l := range strings.Split(string(raw), "\n") {
for _, k := range keys {
if strings.HasPrefix(l, k+":") {
out = append(out, strings.Join(strings.Fields(l), " "))
}
}
}
return strings.Join(out, " | ")
}
func die(step string, err error) {
fmt.Fprintf(os.Stderr, "tapexec: %s: %v\n", step, err)
os.Exit(1)
}
func setIff(name string, flags uint16) (int, error) {
fd, err := unix.Open("/dev/net/tun", unix.O_RDWR|unix.O_CLOEXEC, 0)
if err != nil {
return -1, fmt.Errorf("open /dev/net/tun: %w", err)
}
ifr, err := unix.NewIfreq(name)
if err != nil {
return -1, err
}
ifr.SetUint16(flags)
if err := unix.IoctlIfreq(fd, unix.TUNSETIFF, ifr); err != nil {
unix.Close(fd)
return -1, fmt.Errorf("TUNSETIFF: %w", err)
}
return fd, nil
}
func linkUp(name string) error {
s, err := unix.Socket(unix.AF_INET, unix.SOCK_DGRAM|unix.SOCK_CLOEXEC, 0)
if err != nil {
return err
}
defer unix.Close(s)
ifr, err := unix.NewIfreq(name)
if err != nil {
return err
}
if err := unix.IoctlIfreq(s, unix.SIOCGIFFLAGS, ifr); err != nil {
return fmt.Errorf("SIOCGIFFLAGS: %w", err)
}
ifr.SetUint16(ifr.Uint16() | unix.IFF_UP)
if err := unix.IoctlIfreq(s, unix.SIOCSIFFLAGS, ifr); err != nil {
return fmt.Errorf("SIOCSIFFLAGS: %w", err)
}
return nil
}
func main() {
if len(os.Args) >= 3 && os.Args[1] == "attach" {
name := os.Args[2]
fd, err := setIff(name, unix.IFF_TAP|unix.IFF_NO_PI|unix.IFF_VNET_HDR)
if err != nil {
die("attach "+name, err)
}
fmt.Printf("attach: pid=%d opened existing %s with IFF_TAP|IFF_NO_PI|IFF_VNET_HDR fd=%d (as firecracker does for host_dev_name)\n", os.Getpid(), name, fd)
for {
time.Sleep(time.Hour)
}
}
if len(os.Args) < 3 {
die("usage", fmt.Errorf("tapexec <tap> <argv...> | tapexec attach <tap>"))
}
name := os.Args[1]
fmt.Printf("tapexec: pid=%d ppid=%d %s\n", os.Getpid(), os.Getppid(), status("Uid", "CapEff"))
uidmap, _ := os.ReadFile("/proc/self/uid_map")
fmt.Printf("tapexec: uid_map=%q\n", strings.TrimSpace(string(uidmap)))
fd, err := setIff(name, unix.IFF_TAP|unix.IFF_NO_PI)
if err != nil {
die("create "+name, err)
}
if err := unix.IoctlSetInt(fd, unix.TUNSETPERSIST, 1); err != nil {
die("TUNSETPERSIST", err)
}
unix.Close(fd)
if err := linkUp(name); err != nil {
die("link up", err)
}
fmt.Printf("tapexec: created persistent %s, up; execve %v\n", name, os.Args[2:])
if err := syscall.Exec(os.Args[2], os.Args[2:], os.Environ()); err != nil {
die("execve", err)
}
}
