netdev - Re: [Patch net v3] tipc: call start and done ops directly in __tipc_nl_compat

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Date:   Tue, 4 Sep 2018 22:49:49 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Ying Xue <ying.xue@...driver.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        tipc-discussion@...ts.sourceforge.net,
        Jon Maloy <jon.maloy@...csson.com>
Subject: Re: [Patch net v3] tipc: call start and done ops directly in __tipc_nl_compat_dumpit()

On Tue, Sep 4, 2018 at 8:25 PM Ying Xue <ying.xue@...driver.com> wrote:
>
> On 09/05/2018 05:54 AM, Cong Wang wrote:
> > __tipc_nl_compat_dumpit() uses a netlink_callback on stack,
> > so the only way to align it with other ->dumpit() call path
> > is calling tipc_dump_start() and tipc_dump_done() directly
> > inside it. Otherwise ->dumpit() would always get NULL from
> > cb->args[].
> >
> > But tipc_dump_start() uses sock_net(cb->skb->sk) to retrieve
> > net pointer, the cb->skb here doesn't set skb->sk, the net pointer
> > is saved in msg->net instead, so introduce a helper function
> > __tipc_dump_start() to pass in msg->net.
> >
> > Ying pointed out cb->args[0...3] are already used by other
> > callbacks on this call path, so we can't use cb->args[0] any
> > more, use cb->args[4] instead.
>
> It's a common mechanism to save rhashtable iterator pointer in cb->args
> after tipc_dump_start() and tipc_dump_done() are introduced. Someday
> probably we will involve new dumpit function. In order to lower the risk
> that rhashtable iterator pointer saved is overwritten, it's better to
> use the last slot, ie, cb->args[5].

I don't understand, currently only cb->args[0..3] are used at most,
therefore cb->args[4] is pretty safe in current code base, there is
no reason to be so defensive to use cb->args[5].

If you really worry about future, you probably want to extend cb->args
from 6 to whatever larger, rather than just skipping cb->args[4].

I don't see any reason to do so.