[PATCH iproute2-next 0/2] tc: add NLM_F_ECHO support for actions and filters

[Victor Nogueira <victor@mojatatu.com>]

Continuing on what Hangbin Liu started [1], this patch set adds support for
the NLM_F_ECHO flag for tc actions and filters. For qdiscs it will require
some kernel surgery, and we'll send it soon after this surgery is merged.

When user space configures the kernel with netlink messages, it can set
NLM_F_ECHO flag to request the kernel to send the applied configuration
back to the caller. This allows user space to receive back configuration
information that is populated by the kernel. Often because there are
parameters that can only be set by the kernel which become visible with the
echo, or because user space lets the kernel choose a default value.

To illustrate a use case where the kernel will give us a default value,
the example below shows the user not specifying the action index:

    tc -echo actions add action mirred egress mirror dev lo
  
    total acts 0
    Added action
          action order 1: mirred (Egress Mirror to device lo) pipe
          index 1 ref 1 bind 0
          not_in_hw

Note that the echoed response indicates that the kernel gave us a value
of index 1

[1] https://lore.kernel.org/netdev/20220916033428.400131-2-liuhangbin@gmail.com/

Victor Nogueira (2):
  tc: add NLM_F_ECHO support for actions
  tc: Add NLM_F_ECHO support for filters

 man/man8/tc.8  |  6 +++++-
 tc/m_action.c  | 25 ++++++++++++++++++++++---
 tc/tc.c        |  6 +++++-
 tc/tc_filter.c |  8 +++++++-
 4 files changed, 39 insertions(+), 6 deletions(-)

-- 
2.25.1