selftests/memfd: add test for mapping write-sealed memfd read-only

Now we have reinstated the ability to map F_SEAL_WRITE mappings read-only,
assert that we are able to do this in a test to ensure that we do not
regress this again.

Link: https://lkml.kernel.org/r/a6377ec470b14c0539b4600cf8fa24bf2e4858ae.1732804776.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Julian Orth <ju.orth@gmail.com>
Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: lorenzo-stoakes

tools/testing/selftests/memfd/memfd_test.c

@@ -282,6 +282,24 @@ static void *mfd_assert_mmap_shared(int fd)
 	return p;
 }
 
+static void *mfd_assert_mmap_read_shared(int fd)
+{
+	void *p;
+
+	p = mmap(NULL,
+		 mfd_def_size,
+		 PROT_READ,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+
+	return p;
+}
+
 static void *mfd_assert_mmap_private(int fd)
 {
 	void *p;
@@ -980,6 +998,30 @@ static void test_seal_future_write(void)
 	close(fd);
 }
 
+static void test_seal_write_map_read_shared(void)
+{
+	int fd;
+	void *p;
+
+	printf("%s SEAL-WRITE-MAP-READ\n", memfd_str);
+
+	fd = mfd_assert_new("kern_memfd_seal_write_map_read",
+			    mfd_def_size,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+
+	mfd_assert_add_seals(fd, F_SEAL_WRITE);
+	mfd_assert_has_seals(fd, F_SEAL_WRITE);
+
+	p = mfd_assert_mmap_read_shared(fd);
+
+	mfd_assert_read(fd);
+	mfd_assert_read_shared(fd);
+	mfd_fail_write(fd);
+
+	munmap(p, mfd_def_size);
+	close(fd);
+}
+
 /*
  * Test SEAL_SHRINK
  * Test whether SEAL_SHRINK actually prevents shrinking
@@ -1593,6 +1635,7 @@ int main(int argc, char **argv)
 
 	test_seal_write();
 	test_seal_future_write();
+	test_seal_write_map_read_shared();
 	test_seal_shrink();
 	test_seal_grow();
 	test_seal_resize();