Update restrict.Restrict to both show the error message when failing to mount /dev/null over /proc/kcore, and to ignore "not exists" errors while doing so (for when CONFIG_PROC_KCORE=n in the kernel)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

Author: tianon

security/restrict/restrict.go

@@ -4,6 +4,7 @@ package restrict
 
 import (
 	"fmt"
+	"os"
 	"syscall"
 
 	"github.com/dotcloud/docker/pkg/system"
@@ -18,8 +19,8 @@ func Restrict(mounts ...string) error {
 			return fmt.Errorf("unable to remount %s readonly: %s", dest, err)
 		}
 	}
-	if err := system.Mount("/dev/null", "/proc/kcore", "", syscall.MS_BIND, ""); err != nil {
-		return fmt.Errorf("unable to bind-mount /dev/null over /proc/kcore")
+	if err := system.Mount("/dev/null", "/proc/kcore", "", syscall.MS_BIND, ""); err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("unable to bind-mount /dev/null over /proc/kcore: %s", err)
 	}
 	return nil
 }