[PATCH v3] selftests/mm: Fix UFFDIO_API usage with proper two-step feature negotiation

[Li Wang <liwang@redhat.com>]

The current implementation of test_unmerge_uffd_wp() explicitly sets
`uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP` before calling
UFFDIO_API. This can cause the ioctl() call to fail with EINVAL on kernels
that do not support UFFD-WP, leading the test to fail unnecessarily:

  # ------------------------------
  # running ./ksm_functional_tests
  # ------------------------------
  # TAP version 13
  # 1..9
  # # [RUN] test_unmerge
  # ok 1 Pages were unmerged
  # # [RUN] test_unmerge_zero_pages
  # ok 2 KSM zero pages were unmerged
  # # [RUN] test_unmerge_discarded
  # ok 3 Pages were unmerged
  # # [RUN] test_unmerge_uffd_wp
  # not ok 4 UFFDIO_API failed     <-----
  # # [RUN] test_prot_none
  # ok 5 Pages were unmerged
  # # [RUN] test_prctl
  # ok 6 Setting/clearing PR_SET_MEMORY_MERGE works
  # # [RUN] test_prctl_fork
  # # No pages got merged
  # # [RUN] test_prctl_fork_exec
  # ok 7 PR_SET_MEMORY_MERGE value is inherited
  # # [RUN] test_prctl_unmerge
  # ok 8 Pages were unmerged
  # Bail out! 1 out of 8 tests failed
  # # Planned tests != run tests (9 != 8)
  # # Totals: pass:7 fail:1 xfail:0 xpass:0 skip:0 error:0
  # [FAIL]

This patch improves compatibility and robustness of the UFFD-WP test
(test_unmerge_uffd_wp) by correctly implementing the UFFDIO_API
two-step handshake as recommended by the userfaultfd(2) man page.

Key changes:

1. Use features=0 in the initial UFFDIO_API call to query supported
   feature bits, rather than immediately requesting WP support.

2. Skip the test gracefully if:
   - UFFDIO_API fails with EINVAL (e.g. unsupported API version), or
   - UFFD_FEATURE_PAGEFAULT_FLAG_WP is not advertised by the kernel.

3. Close the initial userfaultfd and create a new one before enabling
   the required feature, since UFFDIO_API can only be called once per fd.

4. Improve diagnostics by distinguishing between expected and unexpected
   failures, using strerror() to report errors.

This ensures the test behaves correctly across a wider range of kernel
versions and configurations, while preserving the intended behavior on
kernels that support UFFD-WP.

Suggestted-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Li Wang <liwang@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Keith Lucas <keith.lucas@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
---
 .../selftests/mm/ksm_functional_tests.c       | 28 +++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
index b61803e36d1c..d8bd1911dfc0 100644
--- a/tools/testing/selftests/mm/ksm_functional_tests.c
+++ b/tools/testing/selftests/mm/ksm_functional_tests.c
@@ -393,9 +393,13 @@ static void test_unmerge_uffd_wp(void)
 
 	/* See if UFFD-WP is around. */
 	uffdio_api.api = UFFD_API;
-	uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP;
+	uffdio_api.features = 0;
 	if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) {
-		ksft_test_result_fail("UFFDIO_API failed\n");
+		if (errno == EINVAL)
+			ksft_test_result_skip("The API version requested is not supported\n");
+		else
+			ksft_test_result_fail("UFFDIO_API failed: %s\n", strerror(errno));
+
 		goto close_uffd;
 	}
 	if (!(uffdio_api.features & UFFD_FEATURE_PAGEFAULT_FLAG_WP)) {
@@ -403,6 +407,26 @@ static void test_unmerge_uffd_wp(void)
 		goto close_uffd;
 	}
 
+	/*
+	 * UFFDIO_API must only be called once to enable features.
+	 * So we close the old userfaultfd and create a new one to
+	 * actually enable UFFD_FEATURE_PAGEFAULT_FLAG_WP.
+	 */
+	close(uffd);
+	uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
+	if (uffd < 0) {
+		ksft_test_result_fail("__NR_userfaultfd failed\n");
+		goto unmap;
+	}
+
+	/* Now, enable it ("two-step handshake") */
+	uffdio_api.api = UFFD_API;
+	uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP;
+	if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) {
+		ksft_test_result_fail("UFFDIO_API failed: %s\n", strerror(errno));
+		goto close_uffd;
+	}
+
 	/* Register UFFD-WP, no need for an actual handler. */
 	if (uffd_register(uffd, map, size, false, true, false)) {
 		ksft_test_result_fail("UFFDIO_REGISTER_MODE_WP failed\n");
-- 
2.49.0