netdev - Re: cBPF socket filters failing

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Message-ID: <CAFSh4UzG=WpnLr5ZYCK16t3M22HV9gf-UuvM4Un+8sQBm2uSVw@mail.gmail.com>
Date:   Fri, 15 Jan 2021 22:45:59 +0000
From:   Tom Cook <tom.k.cook@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: cBPF socket filters failing - inexplicably?

On Fri, Jan 15, 2021 at 2:16 PM Eric Dumazet <edumazet@...gle.com> wrote:
[snip]
> > My wild guess is that as soon as socket got created:
> > socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
> > the packets were already queued to it.
> > So later setsockopt() is too late to filter.
> >
> > Eric, thoughts?
>
> Exactly, this is what happens.

I understand.  Thanks for the explanation.

> I do not know how tcpdump and other programs deal with this.
>
> Maybe by setting a small buffer size, or draining the queue.

libpcap has its own cBPF implementation which it applies after it
receives the packets from the queue.

Thanks again,
Tom Cook