151 lines
3.9 KiB
Go
151 lines
3.9 KiB
Go
//go:build linux
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// +build linux
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package socket
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import (
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"errors"
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"fmt"
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"os"
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"runtime"
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"golang.org/x/sync/errgroup"
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"golang.org/x/sys/unix"
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)
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// errNetNSDisabled is returned when network namespaces are unavailable on
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// a given system.
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var errNetNSDisabled = errors.New("socket: Linux network namespaces are not enabled on this system")
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// withNetNS invokes fn within the context of the network namespace specified by
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// fd, while also managing the logic required to safely do so by manipulating
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// thread-local state.
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func withNetNS(fd int, fn func() (*Conn, error)) (*Conn, error) {
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var (
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eg errgroup.Group
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conn *Conn
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)
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eg.Go(func() error {
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// Retrieve and store the calling OS thread's network namespace so the
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// thread can be reassigned to it after creating a socket in another network
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// namespace.
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runtime.LockOSThread()
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ns, err := threadNetNS()
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if err != nil {
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// No thread-local manipulation, unlock.
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runtime.UnlockOSThread()
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return err
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}
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defer ns.Close()
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// Beyond this point, the thread's network namespace is poisoned. Do not
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// unlock the OS thread until all network namespace manipulation completes
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// to avoid returning to the caller with altered thread-local state.
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// Assign the current OS thread the goroutine is locked to to the given
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// network namespace.
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if err := ns.Set(fd); err != nil {
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return err
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}
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// Attempt Conn creation and unconditionally restore the original namespace.
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c, err := fn()
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if nerr := ns.Restore(); nerr != nil {
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// Failed to restore original namespace. Return an error and allow the
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// runtime to terminate the thread.
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if err == nil {
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_ = c.Close()
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}
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return nerr
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}
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// No more thread-local state manipulation; return the new Conn.
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runtime.UnlockOSThread()
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conn = c
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return nil
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})
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if err := eg.Wait(); err != nil {
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return nil, err
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}
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return conn, nil
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}
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// A netNS is a handle that can manipulate network namespaces.
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//
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// Operations performed on a netNS must use runtime.LockOSThread before
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// manipulating any network namespaces.
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type netNS struct {
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// The handle to a network namespace.
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f *os.File
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// Indicates if network namespaces are disabled on this system, and thus
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// operations should become a no-op or return errors.
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disabled bool
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}
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// threadNetNS constructs a netNS using the network namespace of the calling
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// thread. If the namespace is not the default namespace, runtime.LockOSThread
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// should be invoked first.
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func threadNetNS() (*netNS, error) {
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return fileNetNS(fmt.Sprintf("/proc/self/task/%d/ns/net", unix.Gettid()))
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}
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// fileNetNS opens file and creates a netNS. fileNetNS should only be called
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// directly in tests.
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func fileNetNS(file string) (*netNS, error) {
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f, err := os.Open(file)
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switch {
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case err == nil:
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return &netNS{f: f}, nil
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case os.IsNotExist(err):
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// Network namespaces are not enabled on this system. Use this signal
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// to return errors elsewhere if the caller explicitly asks for a
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// network namespace to be set.
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return &netNS{disabled: true}, nil
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default:
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return nil, err
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}
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}
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// Close releases the handle to a network namespace.
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func (n *netNS) Close() error {
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return n.do(func() error { return n.f.Close() })
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}
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// FD returns a file descriptor which represents the network namespace.
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func (n *netNS) FD() int {
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if n.disabled {
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// No reasonable file descriptor value in this case, so specify a
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// non-existent one.
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return -1
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}
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return int(n.f.Fd())
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}
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// Restore restores the original network namespace for the calling thread.
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func (n *netNS) Restore() error {
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return n.do(func() error { return n.Set(n.FD()) })
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}
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// Set sets a new network namespace for the current thread using fd.
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func (n *netNS) Set(fd int) error {
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return n.do(func() error {
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return os.NewSyscallError("setns", unix.Setns(fd, unix.CLONE_NEWNET))
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})
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}
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// do runs fn if network namespaces are enabled on this system.
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func (n *netNS) do(fn func() error) error {
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if n.disabled {
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return errNetNSDisabled
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}
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return fn()
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}
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