// Copyright 2019 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package cmd

import (
	"fmt"
	"os"
	"path/filepath"
	"syscall"

	"gvisor.googlesource.com/gvisor/pkg/log"
	"gvisor.googlesource.com/gvisor/runsc/specutils"
)

// chrootBinPath is the location inside the chroot where the runsc binary will
// be mounted.
const chrootBinPath = "/runsc"

// mountInChroot creates the destination mount point in the given chroot and
// mounts the source.
func mountInChroot(chroot, src, dst, typ string, flags uint32) error {
	chrootDst := filepath.Join(chroot, dst)
	log.Infof("Mounting %q at %q", src, chrootDst)

	if err := specutils.Mount(src, chrootDst, typ, flags); err != nil {
		return fmt.Errorf("error mounting %q at %q: %v", src, chrootDst, err)
	}
	return nil
}

// setUpChroot creates an empty directory with runsc mounted at /runsc and proc
// mounted at /proc.
func setUpChroot() error {
	// We are a new mount namespace, so we can use /tmp as a directory to
	// construct a new root.
	chroot := os.TempDir()

	log.Infof("Setting up sandbox chroot in %q", chroot)

	// Convert all shared mounts into slave to be sure that nothing will be
	// propagated outside of our namespace.
	if err := syscall.Mount("", "/", "", syscall.MS_SLAVE|syscall.MS_REC, ""); err != nil {
		return fmt.Errorf("error converting mounts: %v", err)
	}

	if err := syscall.Mount("runsc-root", chroot, "tmpfs", syscall.MS_NOSUID|syscall.MS_NODEV|syscall.MS_NOEXEC, ""); err != nil {
		return fmt.Errorf("error mounting tmpfs in choot: %v", err)
	}

	if err := mountInChroot(chroot, "/proc", "/proc", "bind", syscall.MS_BIND|syscall.MS_RDONLY|syscall.MS_REC); err != nil {
		return fmt.Errorf("error mounting proc in chroot: %v", err)
	}

	if err := mountInChroot(chroot, specutils.ExePath, chrootBinPath, "bind", syscall.MS_BIND|syscall.MS_RDONLY); err != nil {
		return fmt.Errorf("error mounting runsc in chroot: %v", err)
	}

	if err := os.Chdir(chroot); err != nil {
		return fmt.Errorf("error changing working directory: %v", err)
	}

	if err := syscall.Mount("", chroot, "", syscall.MS_REMOUNT|syscall.MS_RDONLY|syscall.MS_BIND, ""); err != nil {
		return fmt.Errorf("error remounting chroot in read-only: %v", err)
	}
	// pivot_root(new_root, put_old) moves the root filesystem (old_root)
	// of the calling process to the directory put_old and makes new_root
	// the new root filesystem of the calling process.
	//
	// pivot_root(".", ".") makes a mount of the working directory the new
	// root filesystem, so it will be moved in "/" and then the old_root
	// will be moved to "/" too. The parent mount of the old_root will be
	// new_root, so after umounting the old_root, we will see only
	// the new_root in "/".
	if err := syscall.PivotRoot(".", "."); err != nil {
		return fmt.Errorf("error changing root filesystem: %v", err)
	}

	if err := syscall.Unmount(".", syscall.MNT_DETACH); err != nil {
		return fmt.Errorf("error umounting the old root file system: %v", err)
	}

	return nil
}