BugLink: https://launchpad.net/bugs/1721676
BugLink: https://launchpad.net/bugs/1567597

Userspace needs to be able to reliably detect the support of a filter
flag. A good way of doing that is by attempting to enter filter mode,
with the flag bit(s) in question set, and a NULL pointer for the args
parameter of seccomp(2). EFAULT indicates that the flag is valid and
EINVAL indicates that the flag is invalid.

This patch adds a selftest that can be used to test this method of
detection in userspace.
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
(cherry picked from commit 2b7ea5b5)
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>