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

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Date:   Tue, 18 May 2021 14:21:47 -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 Tue, May 18, 2021 at 12:56 PM John Fastabend
<john.fastabend@...il.com> wrote:
>
> Cong Wang wrote:
> > On Mon, May 17, 2021 at 10:36 PM John Fastabend
> > <john.fastabend@...il.com> wrote:
> > >
> > > Cong Wang wrote:
> > > > From: Cong Wang <cong.wang@...edance.com>
> > > >
> > > > sk_psock_verdict_recv() clones the skb and uses the clone
> > > > afterward, so udp_read_sock() should free the original skb after
> > > > done using it.
> > >
> > > The clone only happens if sk_psock_verdict_recv() returns >0.
> >
> > Sure, in case of error, no one uses the original skb either,
> > so still need to free it.
>
> But the data is going to be dropped then. I'm questioning if this
> is the best we can do or not. Its simplest sure, but could we
> do a bit more work and peek those skbs or requeue them? Otherwise
> if you cross memory limits for a bit your likely to drop these
> unnecessarily.

What are the benefits of not dropping it? When sockmap takes
over sk->sk_data_ready() it should have total control over the skb's
in the receive queue. Otherwise user-space recvmsg() would race
with sockmap when they try to read the first skb at the same time,
therefore potentially user-space could get duplicated data (one via
recvmsg(), one via sockmap). I don't see any benefits but races here.

Thanks.