Re: [net-next] seg6: add support for optional attributes during behavior construction

before this patch, each SRv6 behavior specifies a set of required
attributes that must be provided by the userspace application when the
behavior is created. If an attribute is not supplied, the creation
operation fails.
As a workaround, if an attribute is not needed by a behavior, it requires
to be set by the userspace application to a conventional skip-value. The
kernel side, that processes the creation request of a behavior, reads the
supplied attribute values and checks if it has been set to the
conventional skip-value or not. Hence, each optional attribute must have a
conventional skip-value which is known a priori and shared between
userspace applications and kernel.

Messy code and complicated tricks may arise from this approach.
On the other hand, this patch explicitly differentiates the required
mandatory attributes from the optional ones. Now, each behavior can declare
a set of required attributes and a set of optional ones. The behavior
creation fails in case a required attribute is missing, while it goes on
without generating any issue if an optional attribute is not supplied by
the userspace application.

To properly combine the required and optional attributes, a new callback
function called destroy() is used for releasing resources that have been
acquired, during the parse() operation, by a given attribute.
However, the destroy() function is optional and if an attribute does not
require resources that have to be later released, the callback can be
omitted.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
---
 net/ipv6/seg6_local.c | 226 ++++++++++++++++++++++++++++++++++++------
 1 file changed, 198 insertions(+), 28 deletions(-)

diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index 85a5447a3e8d..480f1ab35221 100644
--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c

@@ -7,6 +7,13 @@
  *  eBPF support: Mathieu Xhonneux <m.xhonneux@gmail.com>
  */

+/* Changes:
+ *
+ * Andrea Mayer <andrea.mayer@uniroma2.it>
+ *	add support for optional attributes during behavior construction
+ *
+ */