os: disallow Root.Remove(".") on Plan 9, js, and Windows

Windows already forbids this, since removing the root causes a
sharing violation (can't delete the directory while the os.Root
has a handle open to it), but add a more explicit check for
attempts to delete "." and return EINVAL.

Note that this change to Windows doesn't affect operations like
Root.Remove("dir/."), since the path is cleaned into just "dir"
before attempting the deletion.

Fixes #73863

Change-Id: I0f45ccb6c9f171d3a52831632c134150388d77b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/679377
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/syscall/windows/at_windows.go

@@ -192,6 +192,11 @@ func Mkdirat(dirfd syscall.Handle, name string, mode uint32) error {
 }
 
 func Deleteat(dirfd syscall.Handle, name string, options uint32) error {
+	if name == "." {
+		// NtOpenFile's documentation isn't explicit about what happens when deleting ".".
+		// Make this an error consistent with that of POSIX.
+		return syscall.EINVAL
+	}
 	objAttrs := &OBJECT_ATTRIBUTES{}
 	if err := objAttrs.init(dirfd, name); err != nil {
 		return err

src/os/root_noopenat.go

@@ -8,6 +8,7 @@ package os
 
 import (
 	"errors"
+	"internal/filepathlite"
 	"internal/stringslite"
 	"sync/atomic"
 	"syscall"
@@ -173,6 +174,12 @@ func rootRemove(r *Root, name string) error {
 	if err := checkPathEscapesLstat(r, name); err != nil {
 		return &PathError{Op: "removeat", Path: name, Err: err}
 	}
+	if endsWithDot(name) {
+		// We don't want to permit removing the root itself, so check for that.
+		if filepathlite.Clean(name) == "." {
+			return &PathError{Op: "removeat", Path: name, Err: errPathEscapes}
+		}
+	}
 	if err := Remove(joinPath(r.root.name, name)); err != nil {
 		return &PathError{Op: "removeat", Path: name, Err: underlyingError(err)}
 	}