util.go

/*
 * util.go - Various helpers used throughout fscrypt
 *
 * Copyright 2017 Google Inc.
 * Author: Joe Richey (joerichey@google.com)
 *
 * 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 util contains useful components for simplifying Go code.
//
// The package contains common error types (errors.go) and functions for
// converting arrays to pointers.
package util

import (
	"bufio"
	"fmt"
	"log"
	"os"
	"os/user"
	"strconv"
	"unsafe"

	"golang.org/x/sys/unix"
)

// Ptr converts a Go byte array to a pointer to the start of the array.
func Ptr(slice []byte) unsafe.Pointer {
	if len(slice) == 0 {
		return nil
	}
	return unsafe.Pointer(&slice[0])
}

// ByteSlice takes a pointer to some data and views it as a slice of bytes.
// Note, indexing into this slice is unsafe.
func ByteSlice(ptr unsafe.Pointer) []byte {
	// Slice must fit in the smallest address space go supports.
	return (*[1 << 30]byte)(ptr)[:]
}

// PointerSlice takes a pointer to an array of pointers and views it as a slice
// of pointers. Note, indexing into this slice is unsafe.
func PointerSlice(ptr unsafe.Pointer) []unsafe.Pointer {
	// Slice must fit in the smallest address space go supports.
	return (*[1 << 28]unsafe.Pointer)(ptr)[:]
}

// Index returns the first index i such that inVal == inArray[i].
// ok is true if we find a match, false otherwise.
func Index(inVal int64, inArray []int64) (index int, ok bool) {
	for index, val := range inArray {
		if val == inVal {
			return index, true
		}
	}
	return 0, false
}

// Lookup finds inVal in inArray and returns the corresponding element in
// outArray. Specifically, if inVal == inArray[i], outVal == outArray[i].
// ok is true if we find a match, false otherwise.
func Lookup(inVal int64, inArray, outArray []int64) (outVal int64, ok bool) {
	index, ok := Index(inVal, inArray)
	if !ok {
		return 0, false
	}
	return outArray[index], true
}

// MinInt returns the lesser of a and b.
func MinInt(a, b int) int {
	if a < b {
		return a
	}
	return b
}

// MaxInt returns the greater of a and b.
func MaxInt(a, b int) int {
	if a > b {
		return a
	}
	return b
}

// MinInt64 returns the lesser of a and b.
func MinInt64(a, b int64) int64 {
	if a < b {
		return a
	}
	return b
}

// ReadLine returns a line of input from standard input. An empty string is
// returned if the user didn't insert anything or on error.
func ReadLine() (string, error) {
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Scan()
	return scanner.Text(), scanner.Err()
}

// AtoiOrPanic converts a string to an int or it panics. Should only be used in
// situations where the input MUST be a decimal number.
func AtoiOrPanic(input string) int {
	i, err := strconv.Atoi(input)
	if err != nil {
		panic(err)
	}
	return i
}

// UserFromUID returns the User corresponding to the given user id.
func UserFromUID(uid int64) (*user.User, error) {
	return user.LookupId(strconv.FormatInt(uid, 10))
}

// EffectiveUser returns the user entry corresponding to the effective user.
func EffectiveUser() (*user.User, error) {
	return UserFromUID(int64(os.Geteuid()))
}

// IsUserRoot checks if the effective user is root.
func IsUserRoot() bool {
	return os.Geteuid() == 0
}

// Chown changes the owner of a File to a User.
func Chown(file *os.File, user *user.User) error {
	uid := AtoiOrPanic(user.Uid)
	gid := AtoiOrPanic(user.Gid)
	return file.Chown(uid, gid)
}

// IsKernelVersionAtLeast returns true if the Linux kernel version is at least
// major.minor. If something goes wrong it assumes false.
func IsKernelVersionAtLeast(major, minor int) bool {
	var uname unix.Utsname
	if err := unix.Uname(&uname); err != nil {
		log.Printf("Uname failed [%v], assuming old kernel", err)
		return false
	}
	release := string(uname.Release[:])
	log.Printf("Kernel version is %s", release)
	var actualMajor, actualMinor int
	if n, _ := fmt.Sscanf(release, "%d.%d", &actualMajor, &actualMinor); n != 2 {
		log.Printf("Unrecognized uname format %q, assuming old kernel", release)
		return false
	}
	return actualMajor > major ||
		(actualMajor == major && actualMinor >= minor)
}