Java heap file on daxfs should be more secure

[Florian Weimer]

* Yasumasa Suenaga:

> HotSpot allocates Java heap on daxfs if we pass -XX:AllocateHeapAt.
> It performs open(2) and unlink(2) on daxfs, and it is used via mmap'ed
> address.
>
> mmap(2) would be called with MAP_SHARED, and it is not atomically
> between open(2) and unlink(2). If malicious user open Java heap file
> before unlink(2), it might be exposed.

The existing code uses mkstemp, so the content is not exposed to other
users.  The same user can still access the file through /proc, with and
without O_TMPFILE.

> So I think we can use open(2) with O_TMPFILE instead of mkstemp(3) as below.
>
>   http://cr.openjdk.java.net/~ysuenaga/dax/

Old systems have neither __O_TMPFILE nor O_TMPFILE, so this will result
in a compilation error there.

Thanks,
Florian