selftests/memfd: add tests for F_SEAL_FUTURE_WRITE seal

Add tests to verify sealing memfds with the F_SEAL_FUTURE_WRITE works as
expected.

Link: http://lkml.kernel.org/r/20190112203816.85534-3-joel@joelfernandes.org
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Shuah Khan <shuah@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: J. Bruce Fields <bfields@fieldses.org>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Marc-Andr Lureau <marcandre.lureau@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/memfd/memfd_test.c

@@ -54,6 +54,22 @@ static int mfd_assert_new(const char *name, loff_t sz, unsigned int flags)
 	return fd;
 }
 
+static int mfd_assert_reopen_fd(int fd_in)
+{
+	int r, fd;
+	char path[100];
+
+	sprintf(path, "/proc/self/fd/%d", fd_in);
+
+	fd = open(path, O_RDWR);
+	if (fd < 0) {
+		printf("re-open of existing fd %d failed\n", fd_in);
+		abort();
+	}
+
+	return fd;
+}
+
 static void mfd_fail_new(const char *name, unsigned int flags)
 {
 	int r;
@@ -255,6 +271,25 @@ static void mfd_assert_read(int fd)
 	munmap(p, mfd_def_size);
 }
 
+/* Test that PROT_READ + MAP_SHARED mappings work. */
+static void mfd_assert_read_shared(int fd)
+{
+	void *p;
+
+	/* verify PROT_READ and MAP_SHARED *is* allowed */
+	p = mmap(NULL,
+		 mfd_def_size,
+		 PROT_READ,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+	munmap(p, mfd_def_size);
+}
+
 static void mfd_assert_write(int fd)
 {
 	ssize_t l;
@@ -692,6 +727,44 @@ static void test_seal_write(void)
 	close(fd);
 }
 
+/*
+ * Test SEAL_FUTURE_WRITE
+ * Test whether SEAL_FUTURE_WRITE actually prevents modifications.
+ */
+static void test_seal_future_write(void)
+{
+	int fd, fd2;
+	void *p;
+
+	printf("%s SEAL-FUTURE-WRITE\n", memfd_str);
+
+	fd = mfd_assert_new("kern_memfd_seal_future_write",
+			    mfd_def_size,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+
+	p = mfd_assert_mmap_shared(fd);
+
+	mfd_assert_has_seals(fd, 0);
+
+	mfd_assert_add_seals(fd, F_SEAL_FUTURE_WRITE);
+	mfd_assert_has_seals(fd, F_SEAL_FUTURE_WRITE);
+
+	/* read should pass, writes should fail */
+	mfd_assert_read(fd);
+	mfd_assert_read_shared(fd);
+	mfd_fail_write(fd);
+
+	fd2 = mfd_assert_reopen_fd(fd);
+	/* read should pass, writes should still fail */
+	mfd_assert_read(fd2);
+	mfd_assert_read_shared(fd2);
+	mfd_fail_write(fd2);
+
+	munmap(p, mfd_def_size);
+	close(fd2);
+	close(fd);
+}
+
 /*
  * Test SEAL_SHRINK
  * Test whether SEAL_SHRINK actually prevents shrinking
@@ -945,6 +1018,7 @@ int main(int argc, char **argv)
 	test_basic();
 
 	test_seal_write();
+	test_seal_future_write();
 	test_seal_shrink();
 	test_seal_grow();
 	test_seal_resize();