Re: [PATCH net-next] ipv6: remove RTNL protection from ip6addrlbl_dump()

[David Ahern <dsahern@kernel.org>]

On 4/3/24 10:25 AM, Eric Dumazet wrote:
> On Wed, Apr 3, 2024 at 6:13 PM David Ahern <dsahern@kernel.org> wrote:
>>
>> On 4/3/24 6:39 AM, Eric Dumazet wrote:
>>> diff --git a/net/ipv6/addrlabel.c b/net/ipv6/addrlabel.c
>>> index 17ac45aa7194ce9c148ed95e14dd575d17feeb98..961e853543b64cd2060ef693ae3ad32a44780aa1 100644
>>> --- a/net/ipv6/addrlabel.c
>>> +++ b/net/ipv6/addrlabel.c
>>> @@ -234,7 +234,8 @@ static int __ip6addrlbl_add(struct net *net, struct ip6addrlbl_entry *newp,
>>>               hlist_add_head_rcu(&newp->list, &net->ipv6.ip6addrlbl_table.head);
>>>  out:
>>>       if (!ret)
>>> -             net->ipv6.ip6addrlbl_table.seq++;
>>> +             WRITE_ONCE(net->ipv6.ip6addrlbl_table.seq,
>>> +                        net->ipv6.ip6addrlbl_table.seq + 1);
>>>       return ret;
>>>  }
>>>
>>> @@ -445,7 +446,7 @@ static void ip6addrlbl_putmsg(struct nlmsghdr *nlh,
>>>  };
>>>
>>>  static int ip6addrlbl_fill(struct sk_buff *skb,
>>> -                        struct ip6addrlbl_entry *p,
>>> +                        const struct ip6addrlbl_entry *p,
>>>                          u32 lseq,
>>>                          u32 portid, u32 seq, int event,
>>>                          unsigned int flags)
>>> @@ -498,7 +499,7 @@ static int ip6addrlbl_dump(struct sk_buff *skb, struct netlink_callback *cb)
>>>       struct net *net = sock_net(skb->sk);
>>>       struct ip6addrlbl_entry *p;
>>>       int idx = 0, s_idx = cb->args[0];
>>> -     int err;
>>> +     int err = 0;
>>>
>>>       if (cb->strict_check) {
>>>               err = ip6addrlbl_valid_dump_req(nlh, cb->extack);
>>> @@ -510,7 +511,7 @@ static int ip6addrlbl_dump(struct sk_buff *skb, struct netlink_callback *cb)
>>>       hlist_for_each_entry_rcu(p, &net->ipv6.ip6addrlbl_table.head, list) {
>>>               if (idx >= s_idx) {
>>>                       err = ip6addrlbl_fill(skb, p,
>>> -                                           net->ipv6.ip6addrlbl_table.seq,
>>> +                                           READ_ONCE(net->ipv6.ip6addrlbl_table.seq),
>>
>> seems like this should be read once on entry, and the same value used
>> for all iterations.
> 
> I thought of that, but this will miss any update done concurrently (if
> all entries fit in a single skb)
> 
> It is unclear to me what user space can do with ifal_seq
> 
> There is no clear signal like :
> 
> Initial seq _before_ the dump, and seq seen _after_ the dump.

Since this a known change in behavior (however slight), it would be best
to add something in the commit message or a code comment so if someone
hits a problem and does a git bisect the problem and solution are clear.