selftests/filesystems: add open() test for anonymous inodes

Test that anonymous inodes cannot be open()ed.

Link: https://lore.kernel.org/20250407-work-anon_inode-v1-9-53a44c20d44e@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

tools/testing/selftests/filesystems/anon_inode_test.c

@@ -48,5 +48,22 @@ TEST(anon_inode_no_exec)
 	EXPECT_EQ(close(fd_context), 0);
 }
 
+TEST(anon_inode_no_open)
+{
+	int fd_context;
+
+	fd_context = sys_fsopen("tmpfs", 0);
+	ASSERT_GE(fd_context, 0);
+
+	ASSERT_GE(dup2(fd_context, 500), 0);
+	ASSERT_EQ(close(fd_context), 0);
+	fd_context = 500;
+
+	ASSERT_LT(open("/proc/self/fd/500", 0), 0);
+	ASSERT_EQ(errno, ENXIO);
+
+	EXPECT_EQ(close(fd_context), 0);
+}
+
 TEST_HARNESS_MAIN