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

[Victor Nogueira <victor@mojatatu.com>]

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

Fixes: 42f39036cda8 ("net/sched: act_mirred: Allow mirred to block")
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
---
 net/sched/act_mirred.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
index 553342c55cf7..1c98e4d81ebe 100644
--- a/net/sched/act_mirred.c
+++ b/net/sched/act_mirred.c
@@ -607,7 +607,8 @@ tcf_mirred_get_dev(const struct tc_action *a,
 
 static size_t tcf_mirred_get_fill_size(const struct tc_action *act)
 {
-	return nla_total_size(sizeof(struct tc_mirred));
+	return nla_total_size(sizeof(struct tc_mirred)) /* TCA_MIRRED_PARMS */
+		+ nla_total_size(sizeof(u32)); /* TCA_MIRRED_BLOCKID */
 }
 
 static void tcf_offload_mirred_get_dev(struct flow_action_entry *entry,
-- 
2.55.0