netdev - Re: [PATCH net-next 0/6] kcm: Kernel Connection Multiplexor (KCM)

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Message-ID: <CALx6S36qHkUhz5VN+xvsMhszTQnCUUhQ+Doja86wLGWJEe_u_A@mail.gmail.com>
Date:	Tue, 24 Nov 2015 09:43:18 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Florian Westphal <fw@...len.de>
Cc:	David Miller <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Kernel Team <kernel-team@...com>, davejwatson@...com,
	Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH net-next 0/6] kcm: Kernel Connection Multiplexor (KCM)

On Tue, Nov 24, 2015 at 9:16 AM, Florian Westphal <fw@...len.de> wrote:
> Tom Herbert <tom@...bertland.com> wrote:
>> No one is being forced to use any of this.
>
> Right.  But it will need to be maintained.
> Lets ignore ktls for the time being and focus on KCM.
>
> I'm currently trying to figure out how memory handling in KCM
> is supposed to work.
>
> say we have following record framing:
>
> struct record {
>         u32 len;
>         char data[];
> };
>
> And I have a epbf filter that returns record->len within KCM.
> Now this program says 'length 128mbyte' (or whatever).
>
> If this was userspace, things are simple, userspace can either
> decide to hang up or start to read this in chunks as data arrives.
>
> AFAICS, with KCM, the kernel now has to keep 128mb of allocated
> memory around, rmem limits are ignored.
>
> Is that correct?  What if next record claims 4g in size?
> I don't really see how we can make any guarantees wrt.
> kernel stability...
>
> Am I missing something?

Message size limits can be enforced in BPF or we could add a limit
enforced by KCM. For instance, the message size limit in http/2 is
16M. If it's needed, it wouldn't be much trouble to add a streaming
interface for large messages.

>
> Thanks,
> Florian
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