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From: John Fastabend <john.fastabend@gmail.com>
To: "Jesper Dangaard Brouer" <brouer@redhat.com>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>
Cc: John Fastabend <john.fastabend@gmail.com>,
	Alan Maguire <alan.maguire@oracle.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Alexei Starovoitov <ast@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	Marek Majkowski <marek@cloudflare.com>,
	Lorenz Bauer <lmb@cloudflare.com>,
	David Miller <davem@davemloft.net>,
	netdev@vger.kernel.org, bpf@vger.kernel.org, brouer@redhat.com
Subject: Re: [PATCH bpf-next 0/9] xdp: Support multiple programs on a single interface through chain calls
Date: Thu, 03 Oct 2019 12:45:11 -0700	[thread overview]
Message-ID: <5d964fc7c247b_55732aec43fe05c45@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <20191003120923.2a8ec190@carbon>

Jesper Dangaard Brouer wrote:
> On Thu, 03 Oct 2019 09:48:22 +0200
> Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> 
> > John Fastabend <john.fastabend@gmail.com> writes:
> > 
> > > Toke Høiland-Jørgensen wrote:  
> > >> John Fastabend <john.fastabend@gmail.com> writes:
> > >>   
> > >> > Toke Høiland-Jørgensen wrote:  
> > >> >> Alan Maguire <alan.maguire@oracle.com> writes:
> > >> >>   
> > >> >> > On Wed, 2 Oct 2019, Toke Høiland-Jørgensen wrote:
> > >> >> >  
> > >> >> >> This series adds support for executing multiple XDP programs on a single
> > >> >> >> interface in sequence, through the use of chain calls, as discussed at the Linux
> > >> >> >> Plumbers Conference last month:
> > >> >> >> 
> > >> >> >> https://linuxplumbersconf.org/event/4/contributions/460/
> > >> >> >> 
> > >> >> >> # HIGH-LEVEL IDEA
> > >> >> >> 
> > >> >> >> The basic idea is to express the chain call sequence through a special map type,
> > >> >> >> which contains a mapping from a (program, return code) tuple to another program
> > >> >> >> to run in next in the sequence. Userspace can populate this map to express
> > >> >> >> arbitrary call sequences, and update the sequence by updating or replacing the
> > >> >> >> map.
> > >> >> >> 
> > >> >> >> The actual execution of the program sequence is done in bpf_prog_run_xdp(),
> > >> >> >> which will lookup the chain sequence map, and if found, will loop through calls
> > >> >> >> to BPF_PROG_RUN, looking up the next XDP program in the sequence based on the
> > >> >> >> previous program ID and return code.
> > >> >> >> 
> > >> >> >> An XDP chain call map can be installed on an interface by means of a new netlink
> > >> >> >> attribute containing an fd pointing to a chain call map. This can be supplied
> > >> >> >> along with the XDP prog fd, so that a chain map is always installed together
> > >> >> >> with an XDP program.
> > >> >> >>   
> > >> >> >
> > >> >> > This is great stuff Toke!  
> > >> >> 
> > >> >> Thanks! :)
> > >> >>   
> > >> >> > One thing that wasn't immediately clear to me - and this may be just
> > >> >> > me - is the relationship between program behaviour for the XDP_DROP
> > >> >> > case and chain call execution. My initial thought was that a program
> > >> >> > in the chain XDP_DROP'ping the packet would terminate the call chain,
> > >> >> > but on looking at patch #4 it seems that the only way the call chain
> > >> >> > execution is terminated is if
> > >> >> >
> > >> >> > - XDP_ABORTED is returned from a program in the call chain; or  
> > >> >> 
> > >> >> Yes. Not actually sure about this one...
> > >> >>   
> > >> >> > - the map entry for the next program (determined by the return value
> > >> >> >   of the current program) is empty; or  
> > >> >> 
> > >> >> This will be the common exit condition, I expect
> > >> >>   
> > >> >> > - we run out of entries in the map  
> > >> >> 
> > >> >> You mean if we run the iteration counter to zero, right?
> > >> >>   
> > >> >> > The return value of the last-executed program in the chain seems to be
> > >> >> > what determines packet processing behaviour after executing the chain
> > >> >> > (_DROP, _TX, _PASS, etc). So there's no way to both XDP_PASS and
> > >> >> > XDP_TX a packet from the same chain, right? Just want to make sure
> > >> >> > I've got the semantics correct. Thanks!  
> > >> >> 
> > >> >> Yeah, you've got all this right. The chain call mechanism itself doesn't
> > >> >> change any of the underlying fundamentals of XDP. I.e., each packet gets
> > >> >> exactly one verdict.
> > >> >> 
> > >> >> For chaining actual XDP programs that do different things to the packet,
> > >> >> I expect that the most common use case will be to only run the next
> > >> >> program if the previous one returns XDP_PASS. That will make the most
> > >> >> semantic sense I think.
> > >> >> 
> > >> >> But there are also use cases where one would want to match on the other
> > >> >> return codes; such as packet capture, for instance, where one might
> > >> >> install a capture program that would carry forward the previous return
> > >> >> code, but do something to the packet (throw it out to userspace) first.
> > >> >> 
> > >> >> For the latter use case, the question is if we need to expose the
> > >> >> previous return code to the program when it runs. You can do things
> > >> >> without it (by just using a different program per return code), but it
> > >> >> may simplify things if we just expose the return code. However, since
> > >> >> this will also change the semantics for running programs, I decided to
> > >> >> leave that off for now.
> > >> >> 
> > >> >> -Toke  
> > >> >
> > >> > In other cases where programs (e.g. cgroups) are run in an array the
> > >> > return codes are 'AND'ed together so that we get
> > >> >
> > >> >    result1 & result2 & ... & resultN  
> 
> But the XDP return codes are not bit values, so AND operation doesn't
> make sense to me.
> 
> > >> 
> > >> How would that work with multiple programs, though? PASS -> DROP seems
> > >> obvious, but what if the first program returns TX? Also, programs may
> > >> want to be able to actually override return codes (e.g., say you want to
> > >> turn DROPs into REDIRECTs, to get all your dropped packets mirrored to
> > >> your IDS or something).  
> > >
> > > In general I think either you hard code a precedence that will have to
> > > be overly conservative because if one program (your firewall) tells
> > > XDP to drop the packet and some other program redirects it, passes,
> > > etc. that seems incorrect to me. Or you get creative with the
> > > precedence rules and they become complex and difficult to manage,
> > > where a drop will drop a packet unless a previous/preceding program
> > > redirects it, etc. I think any hard coded precedence you come up with
> > > will make some one happy and some other user annoyed. Defeating the
> > > programability of BPF.  
> > 
> > Yeah, exactly. That's basically why I punted on that completely.
> > Besides, technically you can get this by just installing different
> > programs in each slot if you really need it.
> 
> I would really like to avoid hard coding precedence.  I know it is
> "challenging" that we want to allow overruling any XDP return code, but
> I think it makes sense and it is the most flexible solution.
> 
> 
> > > Better if its programmable. I would prefer to pass the context into
> > > the next program then programs can build their own semantics. Then
> > > leave the & of return codes so any program can if needed really drop a
> > > packet. The context could be pushed into a shared memory region and
> > > then it doesn't even need to be part of the program signature.  
> > 
> > Since it seems I'll be going down the rabbit hole of baking this into
> > the BPF execution environment itself, I guess I'll keep this in mind as
> > well. Either by stuffing the previous program return code into the
> > context object(s), or by adding a new helper to retrieve it.
> 
> I would like to see the ability to retrieve previous program return
> code, and a new helper would be the simplest approach.  As this could
> potentially simplify and compact the data-structure.

But I'm a bit lost here. I think similar to Alexei comment.

Sounds like you want 'something' that implements a graph of functions,
where depending on the return code of the function you call the next
function in the graph. And you want the edges on that graph to be
programmable so that depending on the administrators configuration
the graph may be built differently?

The more I think about this it seems that you want a BPF program
that is generated from a configuration file to chain together whatever
the administrator asks for. Then you don't need any kernel bits
other than what we have. Of course we can optimize with shared libs
and such as we go but I'm not seeing the missing piece. Use proper
calls instead of tail calls and you can get return codes to handle
however you like. It requires a bit of cooperation from the ids
writers to build you a function to call but that should be simply
their normal xdp hook. Maybe the verifier needs a bit of extra
smarts to follow ctx types into calls I'm not sure.


User says via some fancy gui or yaml or ... build this


                                      ----- XDP_PASS
                                      |
                                      |
 XDP_firefwall --- pass ---> XDP_IDS ------ XDP_DROP
                |
                -- drop ------------------> XDP_DROP

generate the file and load it. This seems to be the advantage of
BPF here you can build whatever you like, make the rules however
complex you like, and build any precedent on the return codes you
like.

.John


> 
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer



  reply	other threads:[~2019-10-03 19:45 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-02 13:30 [PATCH bpf-next 0/9] xdp: Support multiple programs on a single interface through chain calls Toke Høiland-Jørgensen
2019-10-02 13:30 ` [PATCH bpf-next 1/9] hashtab: Add new bpf_map_fd_put_value op Toke Høiland-Jørgensen
2019-10-02 13:30 ` [PATCH bpf-next 2/9] xdp: Add new xdp_chain_map type for specifying XDP call sequences Toke Høiland-Jørgensen
2019-10-02 15:50   ` Lorenz Bauer
2019-10-02 18:25     ` Toke Høiland-Jørgensen
2019-10-02 13:30 ` [PATCH bpf-next 3/9] xdp: Support setting and getting device chain map Toke Høiland-Jørgensen
2019-10-02 15:50   ` Lorenz Bauer
2019-10-02 18:32     ` Toke Høiland-Jørgensen
2019-10-02 18:07   ` kbuild test robot
2019-10-02 18:29   ` kbuild test robot
2019-10-02 13:30 ` [PATCH bpf-next 4/9] xdp: Implement chain call logic to support multiple programs on one interface Toke Høiland-Jørgensen
2019-10-02 17:33   ` kbuild test robot
2019-10-02 17:53   ` kbuild test robot
2019-10-02 13:30 ` [PATCH bpf-next 5/9] tools/include/uapi: Add XDP chain map definitions Toke Høiland-Jørgensen
2019-10-02 13:30 ` [PATCH bpf-next 6/9] tools/libbpf_probes: Add support for xdp_chain map type Toke Høiland-Jørgensen
2019-10-02 13:30 ` [PATCH bpf-next 7/9] bpftool: Add definitions " Toke Høiland-Jørgensen
2019-10-02 13:30 ` [PATCH bpf-next 8/9] libbpf: Add support for setting and getting XDP chain maps Toke Høiland-Jørgensen
2019-10-02 13:30 ` [PATCH bpf-next 9/9] selftests: Add tests for XDP chain calls Toke Høiland-Jørgensen
2019-10-02 15:10 ` [PATCH bpf-next 0/9] xdp: Support multiple programs on a single interface through " Alan Maguire
2019-10-02 15:33   ` Toke Høiland-Jørgensen
2019-10-02 16:34     ` John Fastabend
2019-10-02 18:33       ` Toke Høiland-Jørgensen
2019-10-02 20:34         ` John Fastabend
2019-10-03  7:48           ` Toke Høiland-Jørgensen
2019-10-03 10:09             ` Jesper Dangaard Brouer
2019-10-03 19:45               ` John Fastabend [this message]
2019-10-02 16:35 ` Lorenz Bauer
2019-10-02 18:54   ` Toke Høiland-Jørgensen
2019-10-02 16:43 ` John Fastabend
2019-10-02 19:09   ` Toke Høiland-Jørgensen
2019-10-02 19:15   ` Daniel Borkmann
2019-10-02 19:29     ` Toke Høiland-Jørgensen
2019-10-02 19:46     ` Alexei Starovoitov
2019-10-03  7:58       ` Toke Høiland-Jørgensen
2019-10-02 18:38 ` Song Liu
2019-10-02 18:54   ` Song Liu
2019-10-02 19:25     ` Toke Høiland-Jørgensen
2019-10-03  8:53       ` Jesper Dangaard Brouer
2019-10-03 14:03         ` Alexei Starovoitov
2019-10-03 14:33           ` Toke Høiland-Jørgensen
2019-10-03 14:53             ` Edward Cree
2019-10-03 18:49               ` Jesper Dangaard Brouer
2019-10-03 19:35               ` John Fastabend
2019-10-04  8:09                 ` Toke Høiland-Jørgensen
2019-10-04 10:34                   ` Edward Cree
2019-10-04 15:58                     ` Lorenz Bauer
2019-10-07 16:43                       ` Edward Cree
2019-10-07 17:12                         ` Lorenz Bauer
2019-10-07 19:21                           ` Edward Cree
2019-10-07 21:01                         ` Alexei Starovoitov
2019-10-02 19:23   ` Toke Høiland-Jørgensen
2019-10-02 19:49     ` Song Liu
2019-10-03  7:59       ` Toke Høiland-Jørgensen

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