Devlink Trap¶

Background¶

Devices capable of offloading the kernel’s datapath and perform functions such as bridging and routing must also be able to send specific packets to the kernel (i.e., the CPU) for processing.

For example, a device acting as a multicast-aware bridge must be able to send IGMP membership reports to the kernel for processing by the bridge module. Without processing such packets, the bridge module could never populate its MDB.

As another example, consider a device acting as router which has received an IP packet with a TTL of 1. Upon routing the packet the device must send it to the kernel so that it will route it as well and generate an ICMP Time Exceeded error datagram. Without letting the kernel route such packets itself, utilities such as traceroute could never work.

The fundamental ability of sending certain packets to the kernel for processing is called “packet trapping”.

Overview¶

The devlink-trap mechanism allows capable device drivers to register their supported packet traps with devlink and report trapped packets to devlink for further analysis.

Upon receiving trapped packets, devlink will perform a per-trap packets and bytes accounting and potentially report the packet to user space via a netlink event along with all the provided metadata (e.g., trap reason, timestamp, input port). This is especially useful for drop traps (see Trap Types) as it allows users to obtain further visibility into packet drops that would otherwise be invisible.

The following diagram provides a general overview of devlink-trap:

                            Netlink event: Packet w/ metadata
                                           Or a summary of recent drops
                          ^
                          |
 Userspace                |
+---------------------------------------------------+
 Kernel                   |
                          |
                  +-------+--------+
                  |                |
                  |  drop_monitor  |
                  |                |
                  +-------^--------+
                          |
                          |
                          |
                     +----+----+
                     |         |      Kernel's Rx path
                     | devlink |      (non-drop traps)
                     |         |
                     +----^----+      ^
                          |           |
                          +-----------+
                          |
                  +-------+-------+
                  |               |
                  | Device driver |
                  |               |
                  +-------^-------+
 Kernel                   |
+---------------------------------------------------+
 Hardware                 |
                          | Trapped packet
                          |
                       +--+---+
                       |      |
                       | ASIC |
                       |      |
                       +------+

Trap Types¶

The devlink-trap mechanism supports the following packet trap types:

drop: Trapped packets were dropped by the underlying device. Packets are only processed by devlink and not injected to the kernel’s Rx path. The trap action (see Trap Actions) can be changed.

exception: Trapped packets were not forwarded as intended by the underlying device due to an exception (e.g., TTL error, missing neighbour entry) and trapped to the control plane for resolution. Packets are processed by devlink and injected to the kernel’s Rx path. Changing the action of such traps is not allowed, as it can easily break the control plane.

Trap Actions¶

The devlink-trap mechanism supports the following packet trap actions:

trap: The sole copy of the packet is sent to the CPU.

drop: The packet is dropped by the underlying device and a copy is not sent to the CPU.

Generic Packet Traps¶

Generic packet traps are used to describe traps that trap well-defined packets or packets that are trapped due to well-defined conditions (e.g., TTL error). Such traps can be shared by multiple device drivers and their description must be added to the following table:

List of Generic Packet Traps¶
Name	Type	Description
`source_mac_is_multicast`	`drop`	Traps incoming packets that the device decided to drop because of a multicast source MAC
`vlan_tag_mismatch`	`drop`	Traps incoming packets that the device decided to drop in case of VLAN tag mismatch: The ingress bridge port is not configured with a PVID and the packet is untagged or prio-tagged
`ingress_vlan_filter`	`drop`	Traps incoming packets that the device decided to drop in case they are tagged with a VLAN that is not configured on the ingress bridge port
`ingress_spanning_tree_filter`	`drop`	Traps incoming packets that the device decided to drop in case the STP state of the ingress bridge port is not “forwarding”
`port_list_is_empty`	`drop`	Traps packets that the device decided to drop in case they need to be flooded (e.g., unknown unicast, unregistered multicast) and there are no ports the packets should be flooded to
`port_loopback_filter`	`drop`	Traps packets that the device decided to drop in case after layer 2 forwarding the only port from which they should be transmitted through is the port from which they were received
`blackhole_route`	`drop`	Traps packets that the device decided to drop in case they hit a blackhole route
`ttl_value_is_too_small`	`exception`	Traps unicast packets that should be forwarded by the device whose TTL was decremented to 0 or less
`tail_drop`	`drop`	Traps packets that the device decided to drop because they could not be enqueued to a transmission queue which is full

Driver-specific Packet Traps¶

Device drivers can register driver-specific packet traps, but these must be clearly documented. Such traps can correspond to device-specific exceptions and help debug packet drops caused by these exceptions. The following list includes links to the description of driver-specific traps registered by various device drivers:

/devlink-trap-netdevsim

Generic Packet Trap Groups¶

Generic packet trap groups are used to aggregate logically related packet traps. These groups allow the user to batch operations such as setting the trap action of all member traps. In addition, devlink-trap can report aggregated per-group packets and bytes statistics, in case per-trap statistics are too narrow. The description of these groups must be added to the following table:

List of Generic Packet Trap Groups¶
Name	Description
`l2_drops`	Contains packet traps for packets that were dropped by the device during layer 2 forwarding (i.e., bridge)
`l3_drops`	Contains packet traps for packets that were dropped by the device or hit an exception (e.g., TTL error) during layer 3 forwarding
`buffer_drops`	Contains packet traps for packets that were dropped by the device due to an enqueue decision

Testing¶

See tools/testing/selftests/drivers/net/netdevsim/devlink_trap.sh for a test covering the core infrastructure. Test cases should be added for any new functionality.

Device drivers should focus their tests on device-specific functionality, such as the triggering of supported packet traps.