Re: [Patch bpf] udp: fix a memory leak in udp_read_sock()

[John Fastabend <john.fastabend@gmail.com>]

Cong Wang wrote:
> On Wed, May 19, 2021 at 12:06 PM John Fastabend
> <john.fastabend@gmail.com> wrote:
> >
> > Cong Wang wrote:
> > > On Tue, May 18, 2021 at 12:56 PM John Fastabend
> > > <john.fastabend@gmail.com> wrote:
> > > >
> > > > Cong Wang wrote:
> > > > > On Mon, May 17, 2021 at 10:36 PM John Fastabend
> > > > > <john.fastabend@gmail.com> wrote:
> > > > > >
> > > > > > Cong Wang wrote:
> > > > > > > From: Cong Wang <cong.wang@bytedance.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?