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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/tools/testing/selftests/memfd/memfd_test.c b/tools/testing/selftests/memfd/memfd_test.c
index c67d32eeb668e3a145e4eacbb5c7add68ee8b894..334a7eea200428b07d8964555783f9959d028aa0 100644 (file)
--- 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);