Re: [PATCH v4] af_netlink: ensure that NLMSG_DONE never fails in dumps

[David Miller <davem@davemloft.net>]

From: Johannes Berg <johannes@sipsolutions.net>
Date: Sat, 11 Nov 2017 15:15:21 +0100

> On Sat, 2017-11-11 at 23:09 +0900, David Miller wrote:
>> From: "Jason A. Donenfeld" <Jason@zx2c4.com>
>> Date: Thu,  9 Nov 2017 13:04:44 +0900
>> 
>> > @@ -2195,13 +2197,15 @@ static int netlink_dump(struct sock *sk)
>> >               return 0;
>> >       }
>> >  
>> > -     nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
>> > -     if (!nlh)
>> > +     nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE,
>> > +                            sizeof(nlk->dump_done_errno), NLM_F_MULTI);
>> > +     if (WARN_ON(!nlh))
>> >               goto errout_skb;
>> 
>> If you're handling this by forcing another read() to procude the
>> NLMSG_DONE, then you have no reason to WARN_ON() here.
>> 
>> In fact you are adding a WARN_ON() which is trivially triggerable by
>> any user.
> 
> I added this in my suggestion for how this could work, but I don't
> think you're right, since we previously check if there's enough space.
> The patch is missing the full context, but this is:
 ...
> So unless the nlmsg_total_size() vs. nlmsg_put_answer() suddenly gets a
> different idea of how much space is needed, nlh shouldn't ever be NULL
> once we get here.

Aha, that's what I missed.  Indeed, it cannot happen.

My bad.