pkg/idtools: remove CanAccess(), and move to daemon

The implementation of CanAccess() is very rudimentary, and should
not be used for anything other than a basic check (and maybe not
even for that). It's only used in a single location in the daemon,
so move it there, and un-export it to not encourage others to use
it out of context.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/daemon_unix.go

@@ -15,6 +15,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 
 	"github.com/containerd/cgroups"
@@ -1247,7 +1248,7 @@ func setupDaemonRoot(config *config.Config, rootDir string, remappedRoot idtools
 			if dirPath == "/" {
 				break
 			}
-			if !idtools.CanAccess(dirPath, remappedRoot) {
+			if !canAccess(dirPath, remappedRoot) {
 				return fmt.Errorf("a subdirectory in your graphroot path (%s) restricts access to the remapped root uid/gid; please fix by allowing 'o+x' permissions on existing directories", config.Root)
 			}
 		}
@@ -1259,6 +1260,34 @@ func setupDaemonRoot(config *config.Config, rootDir string, remappedRoot idtools
 	return nil
 }
 
+// canAccess takes a valid (existing) directory and a uid, gid pair and determines
+// if that uid, gid pair has access (execute bit) to the directory.
+//
+// Note: this is a very rudimentary check, and may not produce accurate results,
+// so should not be used for anything other than the current use, see:
+// https://github.com/moby/moby/issues/43724
+func canAccess(path string, pair idtools.Identity) bool {
+	statInfo, err := os.Stat(path)
+	if err != nil {
+		return false
+	}
+	perms := statInfo.Mode().Perm()
+	if perms&0o001 == 0o001 {
+		// world access
+		return true
+	}
+	ssi := statInfo.Sys().(*syscall.Stat_t)
+	if ssi.Uid == uint32(pair.UID) && (perms&0o100 == 0o100) {
+		// owner access.
+		return true
+	}
+	if ssi.Gid == uint32(pair.GID) && (perms&0o010 == 0o010) {
+		// group access.
+		return true
+	}
+	return false
+}
+
 func setupDaemonRootPropagation(cfg *config.Config) error {
 	rootParentMount, mountOptions, err := getSourceMount(cfg.Root)
 	if err != nil {

pkg/idtools/idtools_unix.go

@@ -79,30 +79,6 @@ func mkdirAs(path string, mode os.FileMode, owner Identity, mkAll, chownExisting
 	return nil
 }
 
-// CanAccess takes a valid (existing) directory and a uid, gid pair and determines
-// if that uid, gid pair has access (execute bit) to the directory
-func CanAccess(path string, pair Identity) bool {
-	statInfo, err := os.Stat(path)
-	if err != nil {
-		return false
-	}
-	perms := statInfo.Mode().Perm()
-	if perms&0o001 == 0o001 {
-		// world access
-		return true
-	}
-	ssi := statInfo.Sys().(*syscall.Stat_t)
-	if ssi.Uid == uint32(pair.UID) && (perms&0o100 == 0o100) {
-		// owner access.
-		return true
-	}
-	if ssi.Gid == uint32(pair.GID) && (perms&0o010 == 0o010) {
-		// group access.
-		return true
-	}
-	return false
-}
-
 // LookupUser uses traditional local system files lookup (from libcontainer/user) on a username,
 // followed by a call to `getent` for supporting host configured non-files passwd and group dbs
 func LookupUser(name string) (user.User, error) {

pkg/idtools/idtools_unix_test.go

@@ -6,8 +6,10 @@ package idtools // import "github.com/docker/docker/pkg/idtools"
 import (
 	"fmt"
 	"os"
+	"os/exec"
 	"os/user"
 	"path/filepath"
+	"syscall"
 	"testing"
 
 	"golang.org/x/sys/unix"
@@ -425,7 +427,12 @@ func TestNewIDMappings(t *testing.T) {
 
 	err = MkdirAllAndChown(dirName, 0o700, Identity{UID: rootUID, GID: rootGID})
 	assert.Check(t, err, "Couldn't change ownership of file path. Got error")
-	assert.Check(t, CanAccess(dirName, idMapping.RootPair()), "Unable to access %s directory with user UID:%d and GID:%d", dirName, rootUID, rootGID)
+	cmd := exec.Command("ls", "-la", dirName)
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Credential: &syscall.Credential{Uid: uint32(rootUID), Gid: uint32(rootGID)},
+	}
+	out, err := cmd.CombinedOutput()
+	assert.Check(t, err, "Unable to access %s directory with user UID:%d and GID:%d:\n%s", dirName, rootUID, rootGID, string(out))
 }
 
 func TestLookupUserAndGroup(t *testing.T) {