Re: [PATCH RFC 1/4] net: sched: fix action overwrite reference counting

On Sat 03 Apr 2021 at 01:13, Cong Wang [off-list ref] wrote:

On Wed, Mar 31, 2021 at 9:41 AM Vlad Buslov [off-list ref] wrote:

quoted

Action init code increments reference counter when it changes an action.
This is the desired behavior for cls API which needs to obtain action
reference for every classifier that points to action. However, act API just
needs to change the action and releases the reference before returning.
This sequence breaks when the requested action doesn't exist, which causes
act API init code to create new action with specified index, but action is
still released before returning and is deleted (unless it was referenced
concurrently by cls API).

Please also add a summary of how you fix it. From what I understand,
it seems you just skip the refcnt put of successful cases?

Oops, I didn't regenerate patches after amending the commit message.
This should include the following paragraph:

Extend tcf_action_init() to accept 'init_res' array and initialize it with
action->ops->init() result. Refactor tcf_action_put_many() to also accept
such array and only put actions for which init result match provided value.
Modify tcf_action_add() to only put actions with init_res==0 instead of
unconditionally putting all actions when user set NLM_F_REPLACE netlink
message flag.

One comment below.

quoted

Fixes: cae422f379f3 ("net: sched: use reference counting action init")
Reported-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Vlad Buslov <redacted>
---
 include/net/act_api.h |  5 +++--
 net/sched/act_api.c   | 27 +++++++++++++++++----------
 net/sched/cls_api.c   |  9 +++++----
 3 files changed, 25 insertions(+), 16 deletions(-)

diff --git a/include/net/act_api.h b/include/net/act_api.h
index 2bf3092ae7ec..312f0f6554a0 100644
--- a/include/net/act_api.h
+++ b/include/net/act_api.h

@@ -185,7 +185,7 @@ int tcf_action_exec(struct sk_buff *skb, struct tc_action **actions,
                    int nr_actions, struct tcf_result *res);
 int tcf_action_init(struct net *net, struct tcf_proto *tp, struct nlattr *nla,
                    struct nlattr *est, char *name, int ovr, int bind,
-                   struct tc_action *actions[], size_t *attr_size,
+                   struct tc_action *actions[], int init_res[], size_t *attr_size,
                    bool rtnl_held, struct netlink_ext_ack *extack);
 struct tc_action_ops *tc_action_load_ops(char *name, struct nlattr *nla,
                                         bool rtnl_held,

@@ -193,7 +193,8 @@ struct tc_action_ops *tc_action_load_ops(char *name, struct nlattr *nla,
 struct tc_action *tcf_action_init_1(struct net *net, struct tcf_proto *tp,
                                    struct nlattr *nla, struct nlattr *est,
                                    char *name, int ovr, int bind,
-                                   struct tc_action_ops *ops, bool rtnl_held,
+                                   struct tc_action_ops *a_o, int *init_res,
+                                   bool rtnl_held,
                                    struct netlink_ext_ack *extack);
 int tcf_action_dump(struct sk_buff *skb, struct tc_action *actions[], int bind,
                    int ref, bool terse);

diff --git a/net/sched/act_api.c b/net/sched/act_api.c
index b919826939e0..eb20a75796d5 100644
--- a/net/sched/act_api.c
+++ b/net/sched/act_api.c

@@ -777,8 +777,11 @@ static int tcf_action_put(struct tc_action *p)
        return __tcf_action_put(p, false);
 }

-/* Put all actions in this array, skip those NULL's. */
-static void tcf_action_put_many(struct tc_action *actions[])
+/* Put all actions in this array, skip those NULL's. If cond array is provided
+ * by caller, then only put actions that match.
+ */
+static void tcf_action_put_many(struct tc_action *actions[], int *cond,
+                               int match)
 {
        int i;

@@ -786,7 +789,7 @@ static void tcf_action_put_many(struct tc_action *actions[])
                struct tc_action *a = actions[i];
                const struct tc_action_ops *ops;

-               if (!a)
+               if (!a || (cond && cond[i] != match))

This looks a bit odd. How about passing an array of action pointers which
only contains those that need to be put?

I wanted to make it extensible with cond array instead of make every
user manually filter the action array before calling
tcf_action_put_many(). But I guess there is currently no need for that
and just extending tcf_action_add() with a loop to zero-out the pointers
for newly created actions will be clearer. Will change it in V2.

Thanks.