Re: [net-next v2 2/3] seg6: add NEXT-C-SID support for SRv6 End behavior

[Paolo Abeni <pabeni@redhat.com>]

On Mon, 2022-09-12 at 19:16 +0200, Andrea Mayer wrote:
> 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 |  24 +++
>  net/ipv6/seg6_local.c           | 335 +++++++++++++++++++++++++++++++-
>  2 files changed, 356 insertions(+), 3 deletions(-)
> 
> diff --git a/include/uapi/linux/seg6_local.h b/include/uapi/linux/seg6_local.h
> index 332b18f318f8..4fdc424c9cb3 100644
> --- a/include/uapi/linux/seg6_local.h
> +++ b/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_BITS,
> +	SEG6_LOCAL_FLV_LCNODE_FN_BITS,
> +	__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
> diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
> index f43e6f0baac1..8370726ae7bf 100644
> --- a/net/ipv6/seg6_local.c
> +++ b/net/ipv6/seg6_local.c
> @@ -73,6 +73,55 @@ struct bpf_lwt_prog {
>  	char *name;
>  };
>  
> +/* default length values (expressed in bits) for both Locator-Block and
> + * Locator-Node Function.
> + *
> + * Both SEG6_LOCAL_LCBLOCK_DBITS and SEG6_LOCAL_LCNODE_FN_DBITS *must* be:
> + *    i) greater than 0;
> + *   ii) evenly divisible by 8. In other terms, the lengths of the
> + *	 Locator-Block and Locator-Node Function must be byte-aligned (we can
> + *	 relax this constraint in the future if really needed).
> + *
> + * Moreover, a third condition must hold:
> + *  iii) SEG6_LOCAL_LCBLOCK_DBITS + SEG6_LOCAL_LCNODE_FN_DBITS <= 128.
> + *
> + * The correctness of SEG6_LOCAL_LCBLOCK_DBITS and SEG6_LOCAL_LCNODE_FN_DBITS
> + * values are checked during the kernel compilation. If the compilation stops,
> + * check the value of these parameters to see if they meet conditions (i), (ii)
> + * and (iii).
> + */
> +#define SEG6_LOCAL_LCBLOCK_DBITS	32
> +#define SEG6_LOCAL_LCNODE_FN_DBITS	16
> +
> +/* The following next_csid_chk_{cntr,lcblock,lcblock_fn}_bits macros can be
> + * used directly to check whether the lengths (in bits) of Locator-Block and
> + * Locator-Node Function are valid according to (i), (ii), (iii).
> + */
> +#define next_csid_chk_cntr_bits(blen, flen)		\
> +	((blen) + (flen) > 128)
> +
> +#define next_csid_chk_lcblock_bits(blen)		\
> +({							\
> +	typeof(blen) __tmp = blen;			\
> +	(!__tmp || __tmp > 120 || (__tmp & 0x07));	\

Just a note to mention that in theory a caller could pass a signed
argument, so the above check does not ensure that __tmp is acutally >=
8.

All the current callers use unsigned arguements, so it still looks
safe. 

Cheers,

Paolo