netdev - Re: [PATCH net-next 1/2] xdp: Always use a devmap for XDP

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Date:   Sat, 23 Feb 2019 13:11:02 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>, brouer@...hat.com
Subject: Re: [PATCH net-next 1/2] xdp: Always use a devmap for XDP_REDIRECT to a device

Jesper Dangaard Brouer <brouer@...hat.com> writes:

> On Fri, 22 Feb 2019 13:37:34 -0800 Jakub Kicinski <jakub.kicinski@...ronome.com> wrote:
>
>> On Fri, 22 Feb 2019 11:13:50 +0100, Toke Høiland-Jørgensen wrote:
>> > Jakub Kicinski <jakub.kicinski@...ronome.com> writes:  
>> > > On Thu, 21 Feb 2019 12:56:54 +0100, Toke Høiland-Jørgensen wrote:    
> [...]
>> > >
>> > > BPF programs don't obey by netns boundaries.  The fact the program is
>> > > verified in one ns doesn't mean this is the only ns it will be used in :(
>> > > Meaning if any program is using the redirect map you may need a secret
>> > > map in every ns.. no?    
>> > 
>> > Ah, yes, good point. Totally didn't think about the fact that load and
>> > attach are decoupled. Hmm, guess I'll just have to move the call to
>> > alloc_default_map() to the point where the program is attached to an
>> > interface, then...  
>> 
>> Possibly.. and you also need to handle the case where interface with a
>> program attached is moved, no?

Yup, alloc on attach was easy enough; the moving turns out to be the
tricky part :)

> True, we need to handle if e.g. a veth gets an XDP program attached and
> then is moved into a network namespace (as I've already explained to
> Toke in a meeting).

Yeah, I had somehow convinced myself that the XDP program was being
removed when the interface was being torn down before moving between
namespaces. Jesper pointed out that this was not in fact the case... :P

> I'm still not sure how to handle this...

There are a couple of options, I think. At least:

1. Maintain a flag on struct net_device indicating that this device
   needs the redirect map allocated, and react to that when interfaces
   are being moved.

2. Lookup the BPF program by ID (which we can get from the driver) on
   move, and react to the program flag.

3. Keep the allocation on program load, but allocate maps for all active
   namespaces (which would probably need a refcnt mechanism to
   deallocate things again).

I think I'm leaning towards #2; possibly combined with a refcnt so we
can actually deallocate the map in the root namespace when it's not
needed anymore.

-Toke