netdev - Re: [PATCH v2 net 2/2] net sched actions: decrement module refcount earlier

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Date:   Wed, 19 Apr 2017 10:09:48 +0200 (CEST)
From:   Wolfgang Bumiller <w.bumiller@...xmox.com>
To:     Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v2 net 2/2] net sched actions: decrement module refcount
 earlier


> On April 19, 2017 at 8:23 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
> 
> 
> On Tue, Apr 18, 2017 at 7:21 PM, Jamal Hadi Salim <jhs@...atatu.com> wrote:
> > Indeed. Allocate the cookie before init? That way, we fail early
> > and dont need to worry about restoring anything.
> 
> No, a->act_cookie needs an action pointer first. ;)
> 
> > In the case of a replace, do you really want to call tcf_hash_release?
> >
> 
> Good point. Probably no, we already call it inside ->init().

Right, doing this (before your change, and after patching tc to allow
sending overlong cookies):

    # tc actions add action ok index 500
    # tc actions change action ok index 500 cookie 112233445566778899aabb
    RTNETLINK answers: Invalid argument
    We have an error talking to the kernel

results in the action disappearing (`tc actions ls action gact` is empty)

And doing this in a loop keeps bumping the module refcount:

    # lsmod |grep act_gact
    act_gact               16384  8023

If I guard the tcf_hash_release() the way you suggested the action
doesn't disappear with the `change` command, and the module refcount
doesn't grow either.

Tested with the adapted version below (since we still need a second
variable due to err changing):

diff --git a/net/sched/act_api.c b/net/sched/act_api.c
index 8cc883c063f0..c7e5e437b847 100644
--- a/net/sched/act_api.c
+++ b/net/sched/act_api.c
@@ -555,6 +555,7 @@ struct tc_action *tcf_action_init_1(struct net *net, struct nlattr *nla,
 	struct nlattr *tb[TCA_ACT_MAX + 1];
 	struct nlattr *kind;
 	int err;
+	bool created;
 
 	if (name == NULL) {
 		err = nla_parse_nested(tb, TCA_ACT_MAX, nla, NULL);
@@ -603,20 +604,19 @@ struct tc_action *tcf_action_init_1(struct net *net, struct nlattr *nla,
 		err = a_o->init(net, nla, est, &a, ovr, bind);
 	if (err < 0)
 		goto err_mod;
+	created = err == ACT_P_CREATED;
 
 	if (name == NULL && tb[TCA_ACT_COOKIE]) {
 		int cklen = nla_len(tb[TCA_ACT_COOKIE]);
 
 		if (cklen > TC_COOKIE_MAX_SIZE) {
 			err = -EINVAL;
-			tcf_hash_release(a, bind);
-			goto err_mod;
+			goto err_release;
 		}
 
 		if (nla_memdup_cookie(a, tb) < 0) {
 			err = -ENOMEM;
-			tcf_hash_release(a, bind);
-			goto err_mod;
+			goto err_release;
 		}
 	}
 
@@ -624,11 +624,14 @@ struct tc_action *tcf_action_init_1(struct net *net, struct nlattr *nla,
 	 * if it exists and is only bound to in a_o->init() then
 	 * ACT_P_CREATED is not returned (a zero is).
 	 */
-	if (err != ACT_P_CREATED)
+	if (!created)
 		module_put(a_o->owner);
 
 	return a;
 
+err_release:
+	if (created)
+		tcf_hash_release(a, bind);
 err_mod:
 	module_put(a_o->owner);
 err_out: