Two racing tc filter add requests on the same chain/prio of an unlocked classifier both run change() on the shared proto and both can fail: the winner's tcf_chain_tp_delete_empty() attempt gives up because the loser's handle is still in the idr, and the loser's error path drops only its own reference without a second reclamation attempt. The empty proto stays linked in the chain, holding the chain reference, a block reference and the classifier module reference until the chain or block is torn down. Reclaim the proto on the error path of any failed request that holds a proto reference. The reclamation is emptiness-gated: tcf_chain_tp_delete_empty() unlinks the proto only when delete_empty() admits it is empty, so a live shared proto is never unlinked. A proto the request created is reclaimed unconditionally - it is the only owner, so marking it for deletion is safe even without a delete_empty callback. Classifiers without one (the check marks the proto unconditionally) are rtnl-serialized, so the raced window this guard closes cannot arise for them. This is a follow-up to commit d4e359b3608a ("net/sched: cls_api: fix teardown of an adopted proto on insert-race loss"), which stopped the loser of the insert race from unlinking the winner's live proto but left the empty-proto residual in place. Conditions to recreate: - CONFIG_NET_CLS_FLOWER=y; veth pair - tc qdisc add dev veth0 ingress - two concurrent `tc filter add dev veth0 ingress protocol ip pref 1 flower skip_sw ... action drop` (both fail in fl_hw_replace_filter after publishing their handle in the idr); repeat in a loop - an empty flower tp stays linked after both requests fail; visible as a bare `filter protocol ip pref 1 flower chain 0` header in `tc filter show` with no filter entries - CAP_NET_ADMIN (namespace-local via unshare -Urn suffices) Fixes: 8b64678e0af8 ("net: sched: refactor tp insert/delete for concurrent execution") Reported-by: Sashiko (nipa) Closes: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260805134049.927864-1-victor@mojatatu.com Tested-by: hybris Signed-off-by: Jamal Hadi Salim --- net/sched/cls_api.c | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c index c47d2ee13641..a9f54988561f 100644 --- a/net/sched/cls_api.c +++ b/net/sched/cls_api.c @@ -2463,7 +2463,20 @@ static int tc_new_tfilter(struct sk_buff *skb, struct nlmsghdr *n, } errout: - if (err && tp_state == TP_CREATED) + if (err && !IS_ERR_OR_NULL(tp) && + (tp_state == TP_CREATED || tp->ops->delete_empty)) + /* + * The request is dropping its reference to tp. If it was + * the last user (the idr is empty now), reclaim the proto. + * A tp this request created is reclaimed unconditionally: + * it is the only owner, so marking it for deletion is + * safe. Otherwise only classifiers with a delete_empty + * callback are reclaimed -- the callback admits an empty + * proto only, so a live shared proto is never unlinked. + * Classifiers without one (deleting is set + * unconditionally) are rtnl-serialized, so the raced + * window this guard closes cannot arise for them. + */ tcf_chain_tp_delete_empty(chain, tp, rtnl_held, NULL); errout_tp: if (chain) { -- 2.43.0