selftests: openat2: Add missing dependency in Makefile

Add a dependency on header helpers.h to the main target; while at that add
to helpers.h also a missing include for bool types.

Cc: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

selftests: openat2: Print also errno in failure messages

In E_func() macro, on error, print also errno in order to aid debugging.

Cc: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

selftests: add openat2(2) selftests

Test all of the various openat2(2) flags. A small stress-test of a
symlink-rename attack is included to show that the protections against
".."-based attacks are sufficient.

The main things these self-tests are enforcing are:

* The struct+usize ABI for openat2(2) and copy_struct_from_user() to
ensure that upgrades will be handled gracefully (in addition,
ensuring that misaligned structures are also handled correctly).

* The -EINVAL checks for openat2(2) are all correctly handled to avoid
userspace passing unknown or conflicting flag sets (most
importantly, ensuring that invalid flag combinations are checked).

* All of the RESOLVE_* semantics (including errno values) are
correctly handled with various combinations of paths and flags.

* RESOLVE_IN_ROOT correctly protects against the symlink rename(2)
attack that has been responsible for several CVEs (and likely will
be responsible for several more).

Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>