[PATCH net-next] tc: Return an error if filters try to attach too many actions

["Toke Høiland-Jørgensen" <toke@redhat.com>]

While developing the fix for the buffer sizing issue in [0], I noticed
that the kernel will happily accept a long list of actions for a filter,
and then just silently truncate that list down to a maximum of 32
actions.

That seems less than ideal, so this patch changes the action parsing to
return an error message and refuse to create the filter in this case.
This results in an error like:

 # ip link add type veth
 # tc qdisc replace dev veth0 root handle 1: fq_codel
 # tc -echo filter add dev veth0 parent 1: u32 match u32 0 0 $(for i in $(seq 33); do echo action pedit munge ip dport set 22; done)
Error: Only 32 actions supported per filter.
We have an error talking to the kernel

Instead of just creating a filter with 32 actions and dropping the last
one.

This is obviously a change in UAPI. But seeing as creating more than 32
filters has never actually *worked*, it seems that returning an explicit
error is better, and any use cases that get broken by this were already
broken just in more subtle ways.

[0] https://lore.kernel.org/r/20250407105542.16601-1-toke@redhat.com

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 net/sched/act_api.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/net/sched/act_api.c b/net/sched/act_api.c
index 839790043256..057e20cef375 100644
--- a/net/sched/act_api.c
+++ b/net/sched/act_api.c
@@ -1461,17 +1461,29 @@ int tcf_action_init(struct net *net, struct tcf_proto *tp, struct nlattr *nla,
 		    struct netlink_ext_ack *extack)
 {
 	struct tc_action_ops *ops[TCA_ACT_MAX_PRIO] = {};
-	struct nlattr *tb[TCA_ACT_MAX_PRIO + 1];
+	struct nlattr *tb[TCA_ACT_MAX_PRIO + 2];
 	struct tc_action *act;
 	size_t sz = 0;
 	int err;
 	int i;
 
-	err = nla_parse_nested_deprecated(tb, TCA_ACT_MAX_PRIO, nla, NULL,
+	err = nla_parse_nested_deprecated(tb, TCA_ACT_MAX_PRIO + 1, nla, NULL,
 					  extack);
 	if (err < 0)
 		return err;
 
+	/* The nested attributes are parsed as types, but they are really an
+	 * array of actions. So we parse one more than we can handle, and return
+	 * an error if the last one is set (as that indicates that the request
+	 * contained more than the maximum number of actions).
+	 */
+	if (tb[TCA_ACT_MAX_PRIO + 1]) {
+		NL_SET_ERR_MSG_FMT(extack,
+				   "Only %d actions supported per filter",
+				   TCA_ACT_MAX_PRIO);
+		return -EINVAL;
+	}
+
 	for (i = 1; i <= TCA_ACT_MAX_PRIO && tb[i]; i++) {
 		struct tc_action_ops *a_o;
 
-- 
2.49.0