netdev - Re: [PATCH net] uapi: ioam6: adjust the maximum size of a schema

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Message-ID: <69709e39-a6e2-4289-9202-f5776109f42e@uliege.be>
Date: Fri, 21 Nov 2025 10:23:51 +0100
From: Justin Iurman <justin.iurman@...ege.be>
To: davem@...emloft.net
Cc: netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH net] uapi: ioam6: adjust the maximum size of a schema

On 11/20/25 17:33, Justin Iurman wrote:
> With IPv6, the maximum size of the IOAM Pre-allocated Trace is 244 bytes
> (see IOAM6_TRACE_DATA_SIZE_MAX), due to IPv6 Options length constraint.
> However, RFC9197 defines the Opaque State Snapshot (i.e., a data field
> that may be added by IOAM nodes in the pre-allocated trace) as follows:
> 
>      0                   1                   2                   3
>      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
>     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>     |   Length      |                     Schema ID                 |
>     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>     |                                                               |
>     ~                        Opaque data                            ~
>     |                                                               |
>     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> 
>     Length:
>        1-octet unsigned integer.  It is the length in multiples of 4
>        octets of the Opaque data field that follows Schema ID.
> 
>     Schema ID:
>        3-octet unsigned integer identifying the schema of Opaque data.
> 
>     Opaque data:
>        Variable-length field.  This field is interpreted as specified by
>        the schema identified by the Schema ID.
> 
> Based on that, IOAM6_MAX_SCHEMA_DATA_LEN was initially set to 1020 bytes
> (i.e., 255 * 4), which is already bigger than what is allowed in a
> pre-allocated trace. As a result, if the Opaque State Snapshot (i.e.,
> schema) configured on an IOAM node exceeds 244 bytes, it would just
> skip the insertion of its data. This patch sets a more realistic
> boundary to avoid any confusion. Now, IOAM6_MAX_SCHEMA_DATA_LEN is set
> to 240 bytes (i.e., IOAM6_TRACE_DATA_SIZE_MAX - 4, to account for its
> 4-byte header).
> 
> Fixes: 8c6f6fa67726 ("ipv6: ioam: IOAM Generic Netlink API")
> Signed-off-by: Justin Iurman <justin.iurman@...ege.be>
> ---
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Paolo Abeni <pabeni@...hat.com>
> ---
>   include/uapi/linux/ioam6_genl.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/ioam6_genl.h b/include/uapi/linux/ioam6_genl.h
> index 1733fbc51fb5..ce3d8bdabd3b 100644
> --- a/include/uapi/linux/ioam6_genl.h
> +++ b/include/uapi/linux/ioam6_genl.h
> @@ -19,7 +19,7 @@ enum {
>   	IOAM6_ATTR_NS_DATA,	/* u32 */
>   	IOAM6_ATTR_NS_DATA_WIDE,/* u64 */
>   
> -#define IOAM6_MAX_SCHEMA_DATA_LEN (255 * 4)
> +#define IOAM6_MAX_SCHEMA_DATA_LEN 240
>   	IOAM6_ATTR_SC_ID,	/* u32 */
>   	IOAM6_ATTR_SC_DATA,	/* Binary */
>   	IOAM6_ATTR_SC_NONE,	/* Flag */

So, just to clarify, I know such changes in the uapi are generally 
prohibited. However, in this specific context, I don't believe it breaks 
backward compatibility (IMO). But I may be wrong...

Adding David to get his opinion from iproute2's point of view.

If we don't want to go that way, the alternative solution is to submit a 
patch to net-next that adds a new constant IOAM6_MAX_SCHEMA_DATA_LEN_NEW 
(=240) and a comment "/* Deprecated */" next to 
IOAM6_MAX_SCHEMA_DATA_LEN. Then, submit a patch to iproute2-next to use 
IOAM6_MAX_SCHEMA_DATA_LEN_NEW instead of IOAM6_MAX_SCHEMA_DATA_LEN.