Re: [PATCH net 4/4] net/sched: act_mirred: account for TCA_MIRRED_BLOCKID in get_fill_size

[Paolo Abeni <pabeni@redhat.com>]

On 8/24/26 5:39 PM, Victor Nogueira wrote:
> tcf_mirred_get_fill_size() only budgets TCA_MIRRED_PARMS, but
> tcf_mirred_dump() also emits TCA_MIRRED_BLOCKID whenever the action was
> created with a block instead of a device. So tcf_mirred_get_fill_size is
> missing 8 bytes in its accounting.
> 
> Fix this issue by accounting for TCA_MIRRED_BLOCKID unconditionally:
> it costs 8 bytes for device-mirred actions and avoids having to read
> tcfm_blockid outside tcf_lock, where a concurrent replace could change
> it between sizing and dumping.
> 
> Note: Dumping mirred with blocks currently works in most use cases, but
> would break in some corner cases, for example, if there are 20 blockcast
> mirred actions in one request, each with a non-ANY hw_stats and a
> non-zero user flag, with a listener on RTNLGRP_TC (or NLM_F_ECHO):
> 
> budget 180 B/action -> attr_size = 20*180 + 24 = 3624
>                     -> alloc_skb(3776) = 3776, tailroom exactly 3776
> emitted 196 B/action -> 20*196 + 24 = 3944 > 3776
>                      -> tca_get_fill() fails

Sashiko also noted that the above should not be reachable in practice on
top of the previous patches. If so, would it make sense to divert this
patch to net-next with a slightly different changelog (and no fixes tag)?

I think the first 3 patches are good as is (i.e. I agree with no need
for accounting in patch 2/4, ATM).

/P