netdev - bpf bounded loops. Was: [flamebait] xdp

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Message-ID: <20161202183903.GC54949@ast-mbp.thefacebook.com>
Date:   Fri, 2 Dec 2016 10:39:04 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:     Tom Herbert <tom@...bertland.com>, Thomas Graf <tgraf@...g.ch>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>
Subject: bpf bounded loops. Was: [flamebait] xdp

On Thu, Dec 01, 2016 at 10:27:12PM +0100, Hannes Frederic Sowa wrote:
> like") and the problematic of parsing DNS packets in XDP due to string
> processing and looping inside eBPF.

Hannes,
Not too long ago you proposed a very interesting idea to add
support for bounded loops without adding any new bpf instructions and
changing llvm (which was way better than my 'rep' like instructions
I was experimenting with). I thought systemtap guys also wanted bounded
loops and you were cooperating on the design, so I gave up on my work and
was expecting an imminent patch from you. I guess it sounds like you know
believe that bounded loops are impossible or I misunderstand your statement ?

As far as pattern search for DNS packets...
it was requested by Cloudflare guys back in March:
https://github.com/iovisor/bcc/issues/471
and it is useful for several tracing use cases as well.
Unfortunately no one had time to implement it yet.