[PATCH v2 2/2] selftests/mm: add a uffd test for re-registering during UFFDIO_COPY

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

Re-register the destination without WP while a UFFDIO_COPY_MODE_WP copy
waits for its source page, then check that the copy fails with -EINVAL
and leaves no uffd-wp bit behind. This covers all memory types.

Signed-off-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
---
Changes in v2:
- "all memory types" in the commit message
- rename copy to result

v1: https://lore.kernel.org/20261029221129.89068-3-azpijr@gmail.com

 tools/testing/selftests/mm/uffd-unit-tests.c | 105 +++++++++++++++++++
 1 file changed, 105 insertions(+)

diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index 2a97ba1d6feb..cc22b4cde621 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -375,6 +375,104 @@ static void uffd_wp_unpopulated_test(uffd_global_test_opts_t *gopts, uffd_test_a
 	uffd_test_pass();
 }
 
+struct wp_copy_args {
+	uffd_global_test_opts_t *gopts;
+	char *src;
+	__s64 result;
+};
+
+static void *wp_copy_thread(void *data)
+{
+	struct wp_copy_args *args = data;
+	struct uffdio_copy copy = {
+		.dst = (unsigned long)args->gopts->area_dst,
+		.src = (unsigned long)args->src,
+		.len = args->gopts->page_size,
+		.mode = UFFDIO_COPY_MODE_WP,
+	};
+
+	ioctl(args->gopts->uffd, UFFDIO_COPY, &copy);
+	args->result = copy.copy;
+
+	return NULL;
+}
+
+/*
+ * UFFDIO_COPY drops its locks to fault in the source page.  Re-register the
+ * destination without WP while it waits there: the MODE_WP copy must then
+ * fail instead of leaving a uffd-wp bit in a VMA that no longer has WP.
+ */
+static void uffd_wp_copy_reregister_test(uffd_global_test_opts_t *gopts,
+					 uffd_test_args_t *args)
+{
+	struct uffdio_api api = { .api = UFFD_API };
+	struct wp_copy_args copy_args = { .gopts = gopts };
+	unsigned long len = gopts->page_size;
+	struct uffdio_zeropage zeropage;
+	struct uffd_msg msg;
+	struct pollfd pfd;
+	pthread_t thread;
+	uint64_t value;
+	int pagemap_fd;
+	int src_uffd;
+
+	/* The source fault comes from the kernel, so no UFFD_USER_MODE_ONLY */
+	src_uffd = uffd_open(O_CLOEXEC);
+	if (src_uffd < 0) {
+		uffd_test_skip("cannot open a kernel-mode userfaultfd");
+		return;
+	}
+	if (ioctl(src_uffd, UFFDIO_API, &api))
+		err("UFFDIO_API failed");
+
+	copy_args.src = mmap(NULL, len, PROT_READ | PROT_WRITE,
+			     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	if (copy_args.src == MAP_FAILED)
+		err("mmap() failed");
+
+	if (uffd_register(src_uffd, copy_args.src, len, true, false, false))
+		err("register src failed");
+	if (uffd_register(gopts->uffd, gopts->area_dst, len, true, true, false))
+		err("register dst failed");
+
+	if (pthread_create(&thread, NULL, wp_copy_thread, &copy_args))
+		err("pthread_create() failed");
+
+	/* Wait until UFFDIO_COPY blocks on the source page */
+	pfd = (struct pollfd){ .fd = src_uffd, .events = POLLIN };
+	if (poll(&pfd, 1, 10000) != 1)
+		err("no fault on the source page");
+	if (read(src_uffd, &msg, sizeof(msg)) != sizeof(msg))
+		err("read() failed");
+
+	if (uffd_unregister(gopts->uffd, gopts->area_dst, len))
+		err("unregister dst failed");
+	if (uffd_register(gopts->uffd, gopts->area_dst, len, true, false, false))
+		err("re-register dst failed");
+
+	zeropage.range.start = (unsigned long)copy_args.src;
+	zeropage.range.len = len;
+	zeropage.mode = 0;
+	if (ioctl(src_uffd, UFFDIO_ZEROPAGE, &zeropage))
+		err("UFFDIO_ZEROPAGE failed");
+
+	if (pthread_join(thread, NULL))
+		err("pthread_join() failed");
+
+	if (copy_args.result != -EINVAL)
+		err("UFFDIO_COPY returned %lld, expected %d",
+		    (long long)copy_args.result, -EINVAL);
+
+	pagemap_fd = pagemap_open();
+	value = pagemap_get_entry(pagemap_fd, gopts->area_dst);
+	pagemap_check_wp(value, false);
+	close(pagemap_fd);
+
+	munmap(copy_args.src, len);
+	close(src_uffd);
+	uffd_test_pass();
+}
+
 static void uffd_wp_fork_test_common(uffd_global_test_opts_t *gopts, uffd_test_args_t *args,
 				     bool with_event)
 {
@@ -2403,6 +2501,13 @@ uffd_test_case_t uffd_tests[] = {
 		.uffd_feature_required =
 		UFFD_FEATURE_PAGEFAULT_FLAG_WP | UFFD_FEATURE_WP_UNPOPULATED,
 	},
+	{
+		.name = "wp-copy-reregister",
+		.uffd_fn = uffd_wp_copy_reregister_test,
+		.mem_targets = MEM_ALL,
+		.uffd_feature_required = UFFD_FEATURE_PAGEFAULT_FLAG_WP |
+		UFFD_FEATURE_WP_HUGETLBFS_SHMEM,
+	},
 	{
 		.name = "minor",
 		.uffd_fn = uffd_minor_test,
-- 
2.56.0