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

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Date:   Wed, 19 May 2021 16:26:20 -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 Wed, May 19, 2021 at 2:54 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> Cong Wang wrote:
> > On Wed, May 19, 2021 at 12:06 PM John Fastabend
> > <john.fastabend@...il.com> wrote:
> > >
> > > Cong Wang wrote:
> > > > 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.
> > >
> > > The benefit of _not_ dropping it is the packet gets to the receiver
> > > side. We've spent a bit of effort to get a packet across the network,
> > > received on the stack, and then we drop it at the last point is not
> > > so friendly.
> >
> > Well, at least udp_recvmsg() could drop packets too in various
> > scenarios, for example, a copy error. So, I do not think sockmap
> > is special.
>
> OK I am at least convinced now that dropping packets is OK and likely
> a useful performance/complexity compromise.
>
> But, at this point we wont have any visibility into these drops correct?
> Looks like the pattern in UDP stack to handle this is to increment
> sk_drops and UDP_MIB_INERRORS. How about we do that here as well?

We are not dropping the packet, the packet is cloned and deliver to
user-space via sk_psock_verdict_recv(), thus, we are simply leaking
the original skb, regardless of any error. Maybe udp_read_sock()
should check desc->error, but it has nothing to do with this path which
only aims to address a memory leak. A separate patch is need to check
desc->error, if really needed.

Thanks.