firecracker-netns/internal/netns/nshandle.go

package netns

// inspired from https://github.com/vishvananda/netns/blob/master/nshandle_linux.go#L11

import (
	"fmt"
	"golang.org/x/sys/unix"
	"os"
	"path"
)

const bindMountPath = "/run/netns"

// NsHandle is a handle to a network namespace. It can be cast directly
// to an int and used as a file descriptor.
type NsHandle struct {
	Fd   int
	Name string
}

// persist and bind mount net namespace to `/run/netns`
func (ns *NsHandle) persist() error {
	if _, err := os.Stat(bindMountPath); os.IsNotExist(err) {
		err = os.MkdirAll(bindMountPath, 0o755)
		if err != nil {
			return err
		}
	}

	namedPath := path.Join(bindMountPath, ns.Name)

	f, err := os.OpenFile(namedPath, os.O_CREATE|os.O_EXCL, 0o444)
	if err != nil {
		return err
	}
	err = f.Close()
	if err != nil {
		return err
	}

	err = unix.Mount(GetPath(), namedPath, "bind", unix.MS_BIND, "")
	if err != nil {
		return err
	}

	return nil
}

// Equal determines if two network handles refer to the same network
// namespace. This is done by comparing the device and inode that the
// file descriptors point to.
func (ns *NsHandle) Equal(other NsHandle) bool {
	if *ns == other {
		return true
	}
	var s1, s2 unix.Stat_t
	if err := unix.Fstat(ns.Fd, &s1); err != nil {
		return false
	}
	if err := unix.Fstat(other.Fd, &s2); err != nil {
		return false
	}
	return (s1.Dev == s2.Dev) && (s1.Ino == s2.Ino)
}

// String shows the file descriptor number and its dev and inode.
func (ns *NsHandle) String() string {
	if ns.Fd == -1 {
		return "NS(none)"
	}
	var s unix.Stat_t
	if err := unix.Fstat(ns.Fd, &s); err != nil {
		return fmt.Sprintf("NS(%d: unknown)", ns.Fd)
	}
	return fmt.Sprintf("NS(%d: %d, %d)", ns.Fd, s.Dev, s.Ino)
}

// IsOpen returns true if Close() has not been called.
func (ns *NsHandle) IsOpen() bool {
	return ns.Fd != -1
}

// Close closes the NsHandle and resets its file descriptor to -1.
// It is not safe to use an NsHandle after Close() is called.
func (ns *NsHandle) Close() error {
	if err := unix.Close(ns.Fd); err != nil {
		return err
	}
	ns.Fd = -1
	return nil
}

// None gets an empty (closed) NsHandle.
func None() NsHandle {
	return NsHandle{
		Fd: -1,
	}
}