utils: switch to securejoin.MkdirAllHandle

filepath-securejoin has a bunch of extra hardening features and is very
well-tested, so we should use it instead of our own homebrew solution.

A lot of rootfs_linux.go callers pass a SecureJoin'd path, which means
we need to keep the wrapper helpers in utils, but at least the core
logic is no longer in runc. In future we will want to remove this dodgy
logic and just use file handles for everything (using libpathrs,
ideally).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

libcontainer/system/linux.go

@@ -6,9 +6,7 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"runtime"
 	"strconv"
-	"strings"
 	"syscall"
 	"unsafe"
 
@@ -216,42 +214,3 @@ func SetLinuxPersonality(personality int) error {
 	}
 	return nil
 }
-
-func prepareAt(dir *os.File, path string) (int, string) {
-	if dir == nil {
-		return unix.AT_FDCWD, path
-	}
-
-	// Rather than just filepath.Join-ing path here, do it manually so the
-	// error and handle correctly indicate cases like path=".." as being
-	// relative to the correct directory. The handle.Name() might end up being
-	// wrong but because this is (currently) only used in MkdirAllInRoot, that
-	// isn't a problem.
-	dirName := dir.Name()
-	if !strings.HasSuffix(dirName, "/") {
-		dirName += "/"
-	}
-	fullPath := dirName + path
-
-	return int(dir.Fd()), fullPath
-}
-
-func Openat(dir *os.File, path string, flags int, mode uint32) (*os.File, error) {
-	dirFd, fullPath := prepareAt(dir, path)
-	fd, err := unix.Openat(dirFd, path, flags, mode)
-	if err != nil {
-		return nil, &os.PathError{Op: "openat", Path: fullPath, Err: err}
-	}
-	runtime.KeepAlive(dir)
-	return os.NewFile(uintptr(fd), fullPath), nil
-}
-
-func Mkdirat(dir *os.File, path string, mode uint32) error {
-	dirFd, fullPath := prepareAt(dir, path)
-	err := unix.Mkdirat(dirFd, path, mode)
-	if err != nil {
-		err = &os.PathError{Op: "mkdirat", Path: fullPath, Err: err}
-	}
-	runtime.KeepAlive(dir)
-	return err
-}