Reopening stdin, stdout and stderr if they are pointing to /dev/null to not have '/dev/null' from the global namespace opened inside the container.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)

mount/init.go

@@ -52,9 +52,17 @@ func InitializeMountNamespace(rootfs, console string, mountConfig *MountConfig)
 	if err := SetupPtmx(rootfs, console, mountConfig.MountLabel); err != nil {
 		return err
 	}
+
+	// stdin, stdout and stderr could be pointing to /dev/null from parent namespace.
+	// Re-open them inside this namespace.
+	if err := reOpenDevNull(rootfs); err != nil {
+		return fmt.Errorf("Failed to reopen /dev/null %s", err)
+	}
+
 	if err := setupDevSymlinks(rootfs); err != nil {
 		return fmt.Errorf("dev symlinks %s", err)
 	}
+
 	if err := syscall.Chdir(rootfs); err != nil {
 		return fmt.Errorf("chdir into %s %s", rootfs, err)
 	}
@@ -203,3 +211,29 @@ func newSystemMounts(rootfs, mountLabel string, mounts Mounts) []mount {
 
 	return systemMounts
 }
+
+// Is stdin, stdout or stderr were to be pointing to '/dev/null',
+// this method will make them point to '/dev/null' from within this namespace.
+func reOpenDevNull(rootfs string) error {
+	var stat, devNullStat syscall.Stat_t
+	file, err := os.Open(filepath.Join(rootfs, "/dev/null"))
+	if err != nil {
+		return fmt.Errorf("Failed to open /dev/null - %s", err)
+	}
+	defer file.Close()
+	if err = syscall.Fstat(int(file.Fd()), &devNullStat); err != nil {
+		return fmt.Errorf("Failed to stat /dev/null - %s", err)
+	}
+	for fd := 0; fd < 3; fd++ {
+		if err = syscall.Fstat(fd, &stat); err != nil {
+			return fmt.Errorf("Failed to stat fd %d - %s", fd, err)
+		}
+		if stat.Rdev == devNullStat.Rdev {
+			// Close and re-open the fd.
+			if err = syscall.Dup2(int(file.Fd()), fd); err != nil {
+				return fmt.Errorf("Failed to dup fd %d to fd %d - %s", file.Fd(), fd)
+			}
+		}
+	}
+	return nil
+}