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

[Eric B Munson <emunson@akamai.com>]

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On Fri, 01 May 2015, Eric Dumazet wrote:

> On Fri, 2015-05-01 at 16:14 -0400, Eric B Munson wrote:
> > On Fri, 01 May 2015, Tom Herbert wrote:
> > 
> > > On Fri, May 1, 2015 at 11:42 AM, Eric Dumazet <eric.dumazet@gmail.com> 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@akamai.com>
> > > >> Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
> > > >> Cc: James Morris <jmorris@namei.org>
> > > >> Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
> > > >> Cc: Patrick McHardy <kaber@trash.net>
> > > >> Cc: netdev@vger.kernel.org
> > > >> Cc: linux-api@vger.kernel.org
> > > >> Cc: linux-kernel@vger.kernel.org
> > > >> ---
> > > >
> > > > 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
> > > 
> > 
> > I cached the skb largely to take advantage of the built in reference
> > counting and avoid having to manage allocating memory and ownership of
> > said memory.  For V2, how about I keep the skb reference in the request
> > structure and kmalloc() a buffer, to be owned by the tcp sock structure,
> > when the new tcp socket is created?  This would also simplify the
> > getsockopt() so that the data was available to all callers until the
> > socket is closed.
> 
> Please do not keep a reference on skb. This has a too big cost.
> 
> Have you read that we plan to have up to 4 or 10 million request socks ?
> 
> skb also holds a dst.
> 
> We can upstream our implementation (based on Tom prior patch), we have
> been using it more than 2 years with success.
> 
> 

As long as your implementation provides the IP and TCP headers, I would
be happy with that.  I am also happy to rework my implementation to
extract and cache information when the request structure is built.  If
you all have an implementation that you want to post, I will add my ack
if it meets our needs as well.

Eric

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