Re: [PATCH] Allow TCP connections to cache SYN packet for userspace inspection

[Tom Herbert]

On Fri, May 1, 2015 at 11:42 AM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> On Fri, 2015-05-01 at 13:43 -0400, Eric B Munson wrote:
>> In order to enable policy decisions in userspace, the data contained in
>> the SYN packet would be useful for tracking or identifying connections.
>> Only parts of this data are available to userspace after the hand shake
>> is completed.  This patch exposes a new setsockopt() option that will,
>> when used with a listening socket, ask the kernel to cache the skb
>> holding the SYN packet for retrieval later.  The SYN skbs will not be
>> saved while the kernel is in syn cookie mode.
>>
>> The same option will ask the kernel for the packet headers when used
>> with getsockopt() with the socket returned from accept().  The cached
>> packet will only be available for the first getsockopt() call, the skb
>> is consumed after the requested data is copied to userspace.  Subsequent
>> calls will return -ENOENT.  Because of this behavior, getsockopt() will
>> return -E2BIG if the caller supplied a buffer that is too small to hold
>> the skb header.
>>
>> Signed-off-by: Eric B Munson <emunson@xxxxxxxxxx>
>> Cc: Alexey Kuznetsov <kuznet@xxxxxxxxxxxxx>
>> Cc: James Morris <jmorris@xxxxxxxxx>
>> Cc: Hideaki YOSHIFUJI <yoshfuji@xxxxxxxxxxxxxx>
>> Cc: Patrick McHardy <kaber@xxxxxxxxx>
>> Cc: netdev@xxxxxxxxxxxxxxx
>> Cc: linux-api@xxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> ---
>
> We have a similar patch here at Google, but we do not hold one skb and
> dst per saved syn. That can be ~4KB for some drivers.
>
> Only a kmalloc() with the needed part (headers), usually less than 128
> bytes. We store the length in first byte of this allocation.
>
> This has a huge difference if you want to have ~4 million request socks.
>
+1 on kmalloc solution. I posted a similar patch a couple of years ago
https://patchwork.ozlabs.org/patch/146034/. There was pushback on
memory usage and this having to narrow of a use case.

Tom

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