[PATCH] net/mlx5: remove nonsensical flow action class_id checks

[Dariusz Sosnowski]

On Wed, May 20, 2026 at 03:25:32PM +0200, Adrian Schollmeyer wrote:
> From: Michael Pfeiffer <m.pfeiffer at syseleven.de>
> 
> For a MODIFY_FIELD action, flow_hw_validate_action_modify_field() is
> invoked and enforces class_id == 0 in the action's source and
> destination, if the modified field is none of
> RTE_FLOW_FIELD_GENEVE_OPT_*, as the value is used solely for GENEVE
> fields.
> 
> However, this check is flawed due to the way rte_flow_field_data is
> initialized. As it consists of unions and anonymous structs as members,
> empty initialization of this struct or initializing just the tag_index
> only guarantees initialization of the first union member, while the
> remaining member's default initialization behavior is unspecified.
> Therefore, depending on the compiler type, version and configuration,
> the remaining members may either be default-initialized as well or
> contain bytes from uninitialized memory. This causes the check to fail
> depending on how the struct is initialized wherever it is used.
> 
> For example, rte_flow_configure() sometimes fails on mlx5 under these
> circumstances with an error "destination class id is not supported"
> during creation of representor tagging rules, as these internally use
> MODIFY_FIELD actions in the following call stack:
> 
>   1. rte_flow_configure
>   2. mlx5_flow_port_configure
>   3. flow_hw_configure
>   4. __flow_hw_configure
>   5. flow_hw_setup_tx_repr_tagging
>   6. flow_hw_create_tx_repr_tag_jump_acts_tmpl
>      --> various rte_flow_action_modify_field are initialized here, but
>          class_id remains uninitialized
>   7. __flow_hw_actions_template_create
>   8. mlx5_flow_hw_actions_validate
>   9. flow_hw_validate_action_modify_field
>      --> invoked with class_id containing uninitialized bytes and
>          non-GENEVE field type
> 
> Remove the two checks for class_id in the non-GENEVE case, as this field
> is unused for these actions and avoids additional implicit dependencies
> on the correct ordering of union members.
> 
> Fixes: 1caa89ec1891 ("net/mlx5: support GENEVE options modification")
> Cc: stable at dpdk.org
> 
> Signed-off-by: Michael Pfeiffer <m.pfeiffer at syseleven.de>
> Signed-off-by: Adrian Schollmeyer <a.schollmeyer at syseleven.de>

Acked-by: Dariusz Sosnowski <dsosnowski at nvidia.com>

Thank you for the contribution.

Best regards,
Dariusz Sosnowski