seg6: add NEXT-C-SID support for SRv6 End behavior

The NEXT-C-SID mechanism described in [1] offers the possibility of
encoding several SRv6 segments within a single 128 bit SID address. Such
a SID address is called a Compressed SID (C-SID) container. In this way,
the length of the SID List can be drastically reduced.

A SID instantiated with the NEXT-C-SID flavor considers an IPv6 address
logically structured in three main blocks: i) Locator-Block; ii)
Locator-Node Function; iii) Argument.

                        C-SID container
+------------------------------------------------------------------+
|     Locator-Block      |Loc-Node|            Argument            |
|                        |Function|                                |
+------------------------------------------------------------------+
<--------- B -----------> <- NF -> <------------- A --------------->

   (i) The Locator-Block can be any IPv6 prefix available to the provider;

  (ii) The Locator-Node Function represents the node and the function to
       be triggered when a packet is received on the node;

 (iii) The Argument carries the remaining C-SIDs in the current C-SID
       container.

The NEXT-C-SID mechanism relies on the "flavors" framework defined in
[2]. The flavors represent additional operations that can modify or
extend a subset of the existing behaviors.

This patch introduces the support for flavors in SRv6 End behavior
implementing the NEXT-C-SID one. An SRv6 End behavior with NEXT-C-SID
flavor works as an End behavior but it is capable of processing the
compressed SID List encoded in C-SID containers.

An SRv6 End behavior with NEXT-C-SID flavor can be configured to support
user-provided Locator-Block and Locator-Node Function lengths. In this
implementation, such lengths must be evenly divisible by 8 (i.e. must be
byte-aligned), otherwise the kernel informs the user about invalid
values with a meaningful error code and message through netlink_ext_ack.

If Locator-Block and/or Locator-Node Function lengths are not provided
by the user during configuration of an SRv6 End behavior instance with
NEXT-C-SID flavor, the kernel will choose their default values i.e.,
32-bit Locator-Block and 16-bit Locator-Node Function.

[1] - https://datatracker.ietf.org/doc/html/draft-ietf-spring-srv6-srh-compression
[2] - https://datatracker.ietf.org/doc/html/rfc8986

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>

include/uapi/linux/seg6_local.h

@@ -28,6 +28,7 @@ enum {
 	SEG6_LOCAL_BPF,
 	SEG6_LOCAL_VRFTABLE,
 	SEG6_LOCAL_COUNTERS,
+	SEG6_LOCAL_FLAVORS,
 	__SEG6_LOCAL_MAX,
 };
 #define SEG6_LOCAL_MAX (__SEG6_LOCAL_MAX - 1)
@@ -110,4 +111,27 @@ enum {
 
 #define SEG6_LOCAL_CNT_MAX (__SEG6_LOCAL_CNT_MAX - 1)
 
+/* SRv6 End* Flavor attributes */
+enum {
+	SEG6_LOCAL_FLV_UNSPEC,
+	SEG6_LOCAL_FLV_OPERATION,
+	SEG6_LOCAL_FLV_LCBLOCK_LEN,
+	SEG6_LOCAL_FLV_LCNODE_FN_LEN,
+	__SEG6_LOCAL_FLV_MAX,
+};
+
+#define SEG6_LOCAL_FLV_MAX (__SEG6_LOCAL_FLV_MAX - 1)
+
+/* Designed flavor operations for SRv6 End* Behavior */
+enum {
+	SEG6_LOCAL_FLV_OP_UNSPEC,
+	SEG6_LOCAL_FLV_OP_PSP,
+	SEG6_LOCAL_FLV_OP_USP,
+	SEG6_LOCAL_FLV_OP_USD,
+	SEG6_LOCAL_FLV_OP_NEXT_CSID,
+	__SEG6_LOCAL_FLV_OP_MAX
+};
+
+#define SEG6_LOCAL_FLV_OP_MAX (__SEG6_LOCAL_FLV_OP_MAX - 1)
+
 #endif