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moby--moby/pkg/idtools/idtools.go
Kir Kolyshkin 516010e92d Simplify/fix MkdirAll usage
This subtle bug keeps lurking in because error checking for `Mkdir()`
and `MkdirAll()` is slightly different wrt to `EEXIST`/`IsExist`:

 - for `Mkdir()`, `IsExist` error should (usually) be ignored
   (unless you want to make sure directory was not there before)
   as it means "the destination directory was already there"

 - for `MkdirAll()`, `IsExist` error should NEVER be ignored.

Mostly, this commit just removes ignoring the IsExist error, as it
should not be ignored.

Also, there are a couple of cases then IsExist is handled as
"directory already exist" which is wrong. As a result, some code
that never worked as intended is now removed.

NOTE that `idtools.MkdirAndChown()` behaves like `os.MkdirAll()`
rather than `os.Mkdir()` -- so its description is amended accordingly,
and its usage is handled as such (i.e. IsExist error is not ignored).

For more details, a quote from my runc commit 6f82d4b (July 2015):

    TL;DR: check for IsExist(err) after a failed MkdirAll() is both
    redundant and wrong -- so two reasons to remove it.

    Quoting MkdirAll documentation:

    > MkdirAll creates a directory named path, along with any necessary
    > parents, and returns nil, or else returns an error. If path
    > is already a directory, MkdirAll does nothing and returns nil.

    This means two things:

    1. If a directory to be created already exists, no error is
    returned.

    2. If the error returned is IsExist (EEXIST), it means there exists
    a non-directory with the same name as MkdirAll need to use for
    directory. Example: we want to MkdirAll("a/b"), but file "a"
    (or "a/b") already exists, so MkdirAll fails.

    The above is a theory, based on quoted documentation and my UNIX
    knowledge.

    3. In practice, though, current MkdirAll implementation [1] returns
    ENOTDIR in most of cases described in #2, with the exception when
    there is a race between MkdirAll and someone else creating the
    last component of MkdirAll argument as a file. In this very case
    MkdirAll() will indeed return EEXIST.

    Because of #1, IsExist check after MkdirAll is not needed.

    Because of #2 and #3, ignoring IsExist error is just plain wrong,
    as directory we require is not created. It's cleaner to report
    the error now.

    Note this error is all over the tree, I guess due to copy-paste,
    or trying to follow the same usage pattern as for Mkdir(),
    or some not quite correct examples on the Internet.

    [1] https://github.com/golang/go/blob/f9ed2f75/src/os/path.go

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
2017-11-27 17:32:12 -08:00

266 lines
7.8 KiB
Go

package idtools
import (
"bufio"
"fmt"
"os"
"sort"
"strconv"
"strings"
)
// IDMap contains a single entry for user namespace range remapping. An array
// of IDMap entries represents the structure that will be provided to the Linux
// kernel for creating a user namespace.
type IDMap struct {
ContainerID int `json:"container_id"`
HostID int `json:"host_id"`
Size int `json:"size"`
}
type subIDRange struct {
Start int
Length int
}
type ranges []subIDRange
func (e ranges) Len() int { return len(e) }
func (e ranges) Swap(i, j int) { e[i], e[j] = e[j], e[i] }
func (e ranges) Less(i, j int) bool { return e[i].Start < e[j].Start }
const (
subuidFileName string = "/etc/subuid"
subgidFileName string = "/etc/subgid"
)
// MkdirAllAndChown creates a directory (include any along the path) and then modifies
// ownership to the requested uid/gid. If the directory already exists, this
// function will still change ownership to the requested uid/gid pair.
func MkdirAllAndChown(path string, mode os.FileMode, owner IDPair) error {
return mkdirAs(path, mode, owner.UID, owner.GID, true, true)
}
// MkdirAndChown creates a directory and then modifies ownership to the requested uid/gid.
// If the directory already exists, this function still changes ownership.
// Note that unlike os.Mkdir(), this function does not return IsExist error
// in case path already exists.
func MkdirAndChown(path string, mode os.FileMode, owner IDPair) error {
return mkdirAs(path, mode, owner.UID, owner.GID, false, true)
}
// MkdirAllAndChownNew creates a directory (include any along the path) and then modifies
// ownership ONLY of newly created directories to the requested uid/gid. If the
// directories along the path exist, no change of ownership will be performed
func MkdirAllAndChownNew(path string, mode os.FileMode, owner IDPair) error {
return mkdirAs(path, mode, owner.UID, owner.GID, true, false)
}
// GetRootUIDGID retrieves the remapped root uid/gid pair from the set of maps.
// If the maps are empty, then the root uid/gid will default to "real" 0/0
func GetRootUIDGID(uidMap, gidMap []IDMap) (int, int, error) {
uid, err := toHost(0, uidMap)
if err != nil {
return -1, -1, err
}
gid, err := toHost(0, gidMap)
if err != nil {
return -1, -1, err
}
return uid, gid, nil
}
// toContainer takes an id mapping, and uses it to translate a
// host ID to the remapped ID. If no map is provided, then the translation
// assumes a 1-to-1 mapping and returns the passed in id
func toContainer(hostID int, idMap []IDMap) (int, error) {
if idMap == nil {
return hostID, nil
}
for _, m := range idMap {
if (hostID >= m.HostID) && (hostID <= (m.HostID + m.Size - 1)) {
contID := m.ContainerID + (hostID - m.HostID)
return contID, nil
}
}
return -1, fmt.Errorf("Host ID %d cannot be mapped to a container ID", hostID)
}
// toHost takes an id mapping and a remapped ID, and translates the
// ID to the mapped host ID. If no map is provided, then the translation
// assumes a 1-to-1 mapping and returns the passed in id #
func toHost(contID int, idMap []IDMap) (int, error) {
if idMap == nil {
return contID, nil
}
for _, m := range idMap {
if (contID >= m.ContainerID) && (contID <= (m.ContainerID + m.Size - 1)) {
hostID := m.HostID + (contID - m.ContainerID)
return hostID, nil
}
}
return -1, fmt.Errorf("Container ID %d cannot be mapped to a host ID", contID)
}
// IDPair is a UID and GID pair
type IDPair struct {
UID int
GID int
}
// IDMappings contains a mappings of UIDs and GIDs
type IDMappings struct {
uids []IDMap
gids []IDMap
}
// NewIDMappings takes a requested user and group name and
// using the data from /etc/sub{uid,gid} ranges, creates the
// proper uid and gid remapping ranges for that user/group pair
func NewIDMappings(username, groupname string) (*IDMappings, error) {
subuidRanges, err := parseSubuid(username)
if err != nil {
return nil, err
}
subgidRanges, err := parseSubgid(groupname)
if err != nil {
return nil, err
}
if len(subuidRanges) == 0 {
return nil, fmt.Errorf("No subuid ranges found for user %q", username)
}
if len(subgidRanges) == 0 {
return nil, fmt.Errorf("No subgid ranges found for group %q", groupname)
}
return &IDMappings{
uids: createIDMap(subuidRanges),
gids: createIDMap(subgidRanges),
}, nil
}
// NewIDMappingsFromMaps creates a new mapping from two slices
// Deprecated: this is a temporary shim while transitioning to IDMapping
func NewIDMappingsFromMaps(uids []IDMap, gids []IDMap) *IDMappings {
return &IDMappings{uids: uids, gids: gids}
}
// RootPair returns a uid and gid pair for the root user. The error is ignored
// because a root user always exists, and the defaults are correct when the uid
// and gid maps are empty.
func (i *IDMappings) RootPair() IDPair {
uid, gid, _ := GetRootUIDGID(i.uids, i.gids)
return IDPair{UID: uid, GID: gid}
}
// ToHost returns the host UID and GID for the container uid, gid.
// Remapping is only performed if the ids aren't already the remapped root ids
func (i *IDMappings) ToHost(pair IDPair) (IDPair, error) {
var err error
target := i.RootPair()
if pair.UID != target.UID {
target.UID, err = toHost(pair.UID, i.uids)
if err != nil {
return target, err
}
}
if pair.GID != target.GID {
target.GID, err = toHost(pair.GID, i.gids)
}
return target, err
}
// ToContainer returns the container UID and GID for the host uid and gid
func (i *IDMappings) ToContainer(pair IDPair) (int, int, error) {
uid, err := toContainer(pair.UID, i.uids)
if err != nil {
return -1, -1, err
}
gid, err := toContainer(pair.GID, i.gids)
return uid, gid, err
}
// Empty returns true if there are no id mappings
func (i *IDMappings) Empty() bool {
return len(i.uids) == 0 && len(i.gids) == 0
}
// UIDs return the UID mapping
// TODO: remove this once everything has been refactored to use pairs
func (i *IDMappings) UIDs() []IDMap {
return i.uids
}
// GIDs return the UID mapping
// TODO: remove this once everything has been refactored to use pairs
func (i *IDMappings) GIDs() []IDMap {
return i.gids
}
func createIDMap(subidRanges ranges) []IDMap {
idMap := []IDMap{}
// sort the ranges by lowest ID first
sort.Sort(subidRanges)
containerID := 0
for _, idrange := range subidRanges {
idMap = append(idMap, IDMap{
ContainerID: containerID,
HostID: idrange.Start,
Size: idrange.Length,
})
containerID = containerID + idrange.Length
}
return idMap
}
func parseSubuid(username string) (ranges, error) {
return parseSubidFile(subuidFileName, username)
}
func parseSubgid(username string) (ranges, error) {
return parseSubidFile(subgidFileName, username)
}
// parseSubidFile will read the appropriate file (/etc/subuid or /etc/subgid)
// and return all found ranges for a specified username. If the special value
// "ALL" is supplied for username, then all ranges in the file will be returned
func parseSubidFile(path, username string) (ranges, error) {
var rangeList ranges
subidFile, err := os.Open(path)
if err != nil {
return rangeList, err
}
defer subidFile.Close()
s := bufio.NewScanner(subidFile)
for s.Scan() {
if err := s.Err(); err != nil {
return rangeList, err
}
text := strings.TrimSpace(s.Text())
if text == "" || strings.HasPrefix(text, "#") {
continue
}
parts := strings.Split(text, ":")
if len(parts) != 3 {
return rangeList, fmt.Errorf("Cannot parse subuid/gid information: Format not correct for %s file", path)
}
if parts[0] == username || username == "ALL" {
startid, err := strconv.Atoi(parts[1])
if err != nil {
return rangeList, fmt.Errorf("String to int conversion failed during subuid/gid parsing of %s: %v", path, err)
}
length, err := strconv.Atoi(parts[2])
if err != nil {
return rangeList, fmt.Errorf("String to int conversion failed during subuid/gid parsing of %s: %v", path, err)
}
rangeList = append(rangeList, subIDRange{startid, length})
}
}
return rangeList, nil
}