init_linux.go

/*
   Copyright The containerd Authors.

   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 rootfs

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

const (
	defaultInitializer = "linux-init"
)

func init() {
	initializers[defaultInitializer] = initFS
}

func createDirectory(name string, uid, gid int) initializerFunc {
	return func(root string) error {
		dname := filepath.Join(root, name)
		st, err := os.Stat(dname)
		if err != nil && !os.IsNotExist(err) {
			return err
		} else if err == nil {
			if st.IsDir() {
				stat := st.Sys().(*syscall.Stat_t)
				if int(stat.Gid) == gid && int(stat.Uid) == uid {
					return nil
				}
			} else {
				if err := os.Remove(dname); err != nil {
					return err
				}
				if err := os.Mkdir(dname, 0755); err != nil {
					return err
				}
			}
		} else {
			if err := os.Mkdir(dname, 0755); err != nil {
				return err
			}
		}

		return os.Chown(dname, uid, gid)
	}
}

func touchFile(name string, uid, gid int) initializerFunc {
	return func(root string) error {
		fname := filepath.Join(root, name)

		st, err := os.Stat(fname)
		if err != nil && !os.IsNotExist(err) {
			return err
		} else if err == nil {
			stat := st.Sys().(*syscall.Stat_t)
			if int(stat.Gid) == gid && int(stat.Uid) == uid {
				return nil
			}
			return os.Chown(fname, uid, gid)
		}

		f, err := os.OpenFile(fname, os.O_CREATE, 0644)
		if err != nil {
			return err
		}
		defer f.Close()

		return f.Chown(uid, gid)
	}
}

func symlink(oldname, newname string) initializerFunc {
	return func(root string) error {
		linkName := filepath.Join(root, newname)
		if _, err := os.Stat(linkName); err != nil && !os.IsNotExist(err) {
			return err
		} else if err == nil {
			return nil
		}
		return os.Symlink(oldname, linkName)
	}
}

func initFS(root string) error {
	st, err := os.Stat(root)
	if err != nil {
		return err
	}
	stat := st.Sys().(*syscall.Stat_t)
	uid := int(stat.Uid)
	gid := int(stat.Gid)

	initFuncs := []initializerFunc{
		createDirectory("/dev", uid, gid),
		createDirectory("/dev/pts", uid, gid),
		createDirectory("/dev/shm", uid, gid),
		touchFile("/dev/console", uid, gid),
		createDirectory("/proc", uid, gid),
		createDirectory("/sys", uid, gid),
		createDirectory("/etc", uid, gid),
		touchFile("/etc/resolv.conf", uid, gid),
		touchFile("/etc/hosts", uid, gid),
		touchFile("/etc/hostname", uid, gid),
		symlink("/proc/mounts", "/etc/mtab"),
	}

	for _, fn := range initFuncs {
		if err := fn(root); err != nil {
			return err
		}
	}

	return nil
}