[patch 070/158] memfd: add test for COW on MAP_PRIVATE and F_SEAL_FUTURE_WRITE mappings

[patch 070/158] memfd: add test for COW on MAP_PRIVATE and F_SEAL_FUTURE_WRITE mappings

[akpm]

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Subject: memfd: add test for COW on MAP_PRIVATE and F_SEAL_FUTURE_WRITE mappings

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.

Link: http://lkml.kernel.org/r/20191107195355.80608-2-joel@joelfernandes.org
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Nicolas Geoffray <ngeoffray@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 tools/testing/selftests/memfd/memfd_test.c |   36 +++++++++++++++++++
 1 file changed, 36 insertions(+)

--- a/tools/testing/selftests/memfd/memfd_test.c~memfd-add-test-for-cow-on-map_private-and-f_seal_future_write-mappings
+++ a/tools/testing/selftests/memfd/memfd_test.c
@@ -290,6 +290,40 @@ static void mfd_assert_read_shared(int f
 	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);
_