netdev - Re: [PATCH RFC v2 1/8] xdp: Infrastructure to generalize XDP

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Date:   Thu, 9 Feb 2017 15:08:22 -0800
From:   Tom Herbert <tom@...bertland.com>
To:     David Miller <davem@...emloft.net>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH RFC v2 1/8] xdp: Infrastructure to generalize XDP

On Thu, Feb 9, 2017 at 2:34 PM, David Miller <davem@...emloft.net> wrote:
> From: Tom Herbert <tom@...bertland.com>
> Date: Thu, 9 Feb 2017 14:26:50 -0800
>
>> On Thu, Feb 9, 2017 at 2:17 PM, David Miller <davem@...emloft.net> wrote:
>>> From: Tom Herbert <tom@...bertland.com>
>>> Date: Wed, 8 Feb 2017 15:41:20 -0800
>>>
>>>> These hooks are also generic to allow for XDP/BPF programs as well
>>>> as non-BPF code (e.g. kernel code can be written in a module).
>>>
>>> I don't think we should even remotely consider surrendering the XDP
>>> hook to module code.
>>>
>>> We restrict it to eBPF for a reason, because that framework is
>>> restricted in what it can do, what it can access, and how it can do
>>> so.
>>>
>> Kernel modules go through extensive netdev review before they are
>> taken into the kernel, for BPF programs we just allow what any user
>> gives us without any peer review even implied.
>
> We can actually control what externally written XDP eBPF programs can
> do, for kernel modules we have no such control or influence.  This
> hook runs right in the driver and bypasses the entire stack, it has to
> execute in a hardened thing that cannot crash and it will not as long
> as BPF verifier is correct.
>
> And you're going to make it even more complicated what XDP offload in
> hardware actually means.  With eBPF it is very clearly defined what
> the necessary execution engine is.
>
> Tom I'm strongly against being allowed to run arbitrary module code
> from the XDP hook, sorry.
>
Okay, how about this... I'll add a configuration option like
XDP_ALLOW_OTHER_HOOKS. The default will be to disallow setting any
hook other than a BPF. If it is set, then we'll accept other hooks to
be run. This way mostly restrict the interface by default, but still
allow experimentation with other hook types like I need with TXDP or
maybe the netfilter guys might want to fastpath netfilter etc. When we
we bring a working robust implementation to netdev that show clear
benefits then we can add those to BPF as the "allowed" hooks at that
time. So this strictly controls the interfaces, but still also allows
room for innovation.

Tom

> It is as important as the distinction between full stack offload and
> partial offload in those nice charts in your talks. :-)
>