test(uffd): unregister UFFD range before fd close in cross-process cleanup (#2476)

Add an unregister() helper around UFFDIO_UNREGISTER and call it from the
late cmd.Wait() cleanup in configureCrossProcessTest, before the early
uffdFd.close() cleanup runs (cleanups are LIFO).

Today this is a no-op: no test enables UFFD_FEATURE_EVENT_REMOVE, so the
kernel never queues REMOVE events that could keep munmap blocked on
un-acked events against a registered range. Pulling this out as its own
small change so the upcoming PR that does enable REMOVE events doesn't
silently bring along a behavioural test-cleanup change too.

Author: ValentaTomas

packages/orchestrator/pkg/sandbox/uffd/userfaultfd/cross_process_helpers_test.go

@@ -187,6 +187,14 @@ func configureCrossProcessTest(t *testing.T, tt testConfig) (*testHandler, error
 				waitErr == nil,
 			"unexpected error: %v", waitErr,
 		)
+
+		// Tear down the UFFD registration before the early uffdFd.close()
+		// cleanup runs. Today this is a no-op (no test enables
+		// UFFD_FEATURE_EVENT_REMOVE) but a follow-up that does will
+		// otherwise see munmap block on un-acked REMOVE events queued
+		// against the still-registered range. Cleanups run LIFO, so
+		// this fires before the close registered earlier.
+		assert.NoError(t, unregister(uffdFd, memoryStart, uint64(size)))
 	})
 
 	// pageStatesOnce asks the serving process for a snapshot of its pageTracker

packages/orchestrator/pkg/sandbox/uffd/userfaultfd/fd_helpers_test.go

@@ -41,6 +41,21 @@ func configureApi(f Fd, pagesize uint64) error {
 	return nil
 }
 
+// unregister tears down the UFFD registration over [addr, addr+size).
+// Used in test cleanup so that any in-flight REMOVE events the kernel
+// may have queued (once UFFD_FEATURE_EVENT_REMOVE is enabled in a
+// follow-up) don't keep munmap blocked on un-acked events.
+func unregister(f Fd, addr uintptr, size uint64) error {
+	r := newUffdioRange(CULong(addr), CULong(size))
+
+	ret, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(f), UFFDIO_UNREGISTER, uintptr(unsafe.Pointer(&r)))
+	if errno != 0 {
+		return fmt.Errorf("UFFDIO_UNREGISTER ioctl failed: %w (ret=%d)", errno, ret)
+	}
+
+	return nil
+}
+
 // mode: UFFDIO_REGISTER_MODE_WP|UFFDIO_REGISTER_MODE_MISSING
 // This is already called by the FC, but only with the UFFDIO_REGISTER_MODE_MISSING
 // We need to call it with UFFDIO_REGISTER_MODE_WP when we use both missing and wp