selftests: openat2: add RESOLVE_ conflict test

Now that we reject conflicting RESOLVE_ flags, add a selftest to avoid
regressions.
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/r/20201027235044.5240-3-cyphar@cyphar.comSigned-off-by: Christian Brauner <christian.brauner@ubuntu.com>

tools/testing/selftests/openat2/openat2_test.c

@@ -155,7 +155,7 @@ struct flag_test {
 	int err;
 };
 
-#define NUM_OPENAT2_FLAG_TESTS 23
+#define NUM_OPENAT2_FLAG_TESTS 24
 
 void test_openat2_flags(void)
 {
@@ -210,6 +210,12 @@ void test_openat2_flags(void)
 		  .how.flags = O_TMPFILE | O_RDWR,
 		  .how.mode = 0x0000A00000000000ULL, .err = -EINVAL },
 
+		/* ->resolve flags must not conflict. */
+		{ .name = "incompatible resolve flags (BENEATH | IN_ROOT)",
+		  .how.flags = O_RDONLY,
+		  .how.resolve = RESOLVE_BENEATH | RESOLVE_IN_ROOT,
+		  .err = -EINVAL },
+
 		/* ->resolve must only contain RESOLVE_* flags. */
 		{ .name = "invalid how.resolve and O_RDONLY",
 		  .how.flags = O_RDONLY,