[PATCH 2/2] memfd: Add test for COW on MAP_PRIVATE and F_SEAL_FUTURE_WRITE mappings

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

In this test, the parent and child both have writable private mappings.
The test shows that without the patch in this series, the parent and
child shared the same memory which is incorrect. In other words, COW
needs to be triggered so any writes to child's copy stays local to the
child.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 tools/testing/selftests/memfd/memfd_test.c | 36 ++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/tools/testing/selftests/memfd/memfd_test.c b/tools/testing/selftests/memfd/memfd_test.c
index c67d32eeb668..334a7eea2004 100644
--- a/tools/testing/selftests/memfd/memfd_test.c
+++ b/tools/testing/selftests/memfd/memfd_test.c
@@ -290,6 +290,40 @@ static void mfd_assert_read_shared(int fd)
 	munmap(p, mfd_def_size);
 }
 
+static void mfd_assert_fork_private_write(int fd)
+{
+	int *p;
+	pid_t pid;
+
+	p = mmap(NULL,
+		 mfd_def_size,
+		 PROT_READ | PROT_WRITE,
+		 MAP_PRIVATE,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+
+	p[0] = 22;
+
+	pid = fork();
+	if (pid == 0) {
+		p[0] = 33;
+		exit(0);
+	} else {
+		waitpid(pid, NULL, 0);
+
+		if (p[0] != 22) {
+			printf("MAP_PRIVATE copy-on-write failed: %m\n");
+			abort();
+		}
+	}
+
+	munmap(p, mfd_def_size);
+}
+
 static void mfd_assert_write(int fd)
 {
 	ssize_t l;
@@ -760,6 +794,8 @@ static void test_seal_future_write(void)
 	mfd_assert_read_shared(fd2);
 	mfd_fail_write(fd2);
 
+	mfd_assert_fork_private_write(fd);
+
 	munmap(p, mfd_def_size);
 	close(fd2);
 	close(fd);
-- 
2.24.0.rc1.363.gb1bccd3e3d-goog