netdev - Re: [Patch bpf] udp: fix a memory leak in udp_read

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Date:   Fri, 21 May 2021 16:39:54 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Cong Wang <cong.wang@...edance.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [Patch bpf] udp: fix a memory leak in udp_read_sock()

On Fri, May 21, 2021 at 3:09 PM John Fastabend <john.fastabend@...il.com> wrote:
> OK either add the counters in this patch or as a series of two
> patches so we get a complete fix in one series. Without it some
> box out there will randomly drop UDP packets, probably DNS
> packets for extra fun, and we will have no way of knowing other
> than sporadic packet loss. Unless your arguing against having the
> counters or that the counters don't make sense for some reason?

I never object increasing any counter here, My argument is it
belongs to a separate patch for 3 reasons:

1) TCP does not have one either, hence needs to fix together;

2) A patch should fix one bug, not two or more bugs together;

3) It is not the only one place which needs to increase the
counter, all of these kfree_skb()'s need, for example, this one
inside sk_psock_verdict_recv():

        psock = sk_psock(sk);
        if (unlikely(!psock)) {
                len = 0;
                kfree_skb(skb);
                goto out;
        }

This example also shows it is harder to do so, because
sk_psock_verdict_recv() is independent of any protocol, it is
hard to increase a protocol-specific counter there.

(Another one is in sk_psock_verdict_apply().)

>
> > counters either, yet another reason it deserves a separate patch
> > to address both.
>
> TCP case is different if we drop packets in a TCP error case
> thats not a 'lets increment the counters' problem the drop needs
> to be fixed we can't let data fall out of a TCP stream because
> no one will retransmit it. We've learned this the hard way.

I think TCP always increases some counter when dropping
a packet despite of retransmission, for example:

static void tcp_drop(struct sock *sk, struct sk_buff *skb)
{
        sk_drops_add(sk, skb);
        __kfree_skb(skb);
}

Thanks.