fix preserve-fds flag may cause runc hang

Signed-off-by: lifubang <lifubang@acmcoder.com>

utils_linux.go

@@ -3,7 +3,6 @@
 package main
 
 import (
-	"errors"
 	"fmt"
 	"net"
 	"os"
@@ -20,6 +19,7 @@ import (
 	"github.com/opencontainers/runtime-spec/specs-go"
 
 	"github.com/coreos/go-systemd/activation"
+	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 	"github.com/urfave/cli"
 	"golang.org/x/sys/unix"
@@ -281,6 +281,11 @@ func (r *runner) run(config *specs.Process) (int, error) {
 	}
 	baseFd := 3 + len(process.ExtraFiles)
 	for i := baseFd; i < baseFd+r.preserveFDs; i++ {
+		_, err := os.Stat(fmt.Sprintf("/proc/self/fd/%d", i))
+		if err != nil {
+			r.destroy()
+			return -1, errors.Wrapf(err, "please check that preserved-fd %d (of %d) is present", i-baseFd, r.preserveFDs)
+		}
 		process.ExtraFiles = append(process.ExtraFiles, os.NewFile(uintptr(i), "PreserveFD:"+strconv.Itoa(i)))
 	}
 	rootuid, err := r.container.Config().HostRootUID()