Best userspace programming API for XDP features query to kernel?


Daniel Borkmann
 

On 04/05/2018 10:51 PM, Jesper Dangaard Brouer wrote:
On Thu, 5 Apr 2018 12:37:19 +0200
Daniel Borkmann <daniel@...> wrote:

On 04/04/2018 02:28 PM, Jesper Dangaard Brouer via iovisor-dev wrote:
Hi Suricata people,

When Eric Leblond (and I helped) integrated XDP in Suricata, we ran
into the issue, that at Suricata load/start time, we cannot determine
if the chosen XDP config options, like xdp-cpu-redirect[1], is valid on
this HW (e.g require driver XDP_REDIRECT support and bpf cpumap).

We would have liked a way to report that suricata.yaml config was
invalid for this hardware/setup. Now, it just loads, and packets gets
silently dropped by XDP (well a WARN_ONCE and catchable via tracepoints).

My question to suricata developers: (Q1) Do you already have code that
query the kernel or drivers for features?

At the IOvisor call (2 weeks ago), we discussed two options of exposing
XDP features avail in a given driver.

Option#1: Extend existing ethtool -k/-K "offload and other features"
with some XDP features, that userspace can query. (Do you already query
offloads, regarding Q1)

Option#2: Invent a new 'ip link set xdp' netlink msg with a query option.
I don't really mind if you go via ethtool, as long as we handle this
generically from there and e.g. call the dev's ndo_bpf handler such that
we keep all the information in one place. This can be a new ndo_bpf command
e.g. XDP_QUERY_FEATURES or such.
Just to be clear: notice as Victor points out[2], they are programmable
going though the IOCTL (SIOCETHTOOL) and not using cmdline tools.
Sure, that was perfectly clear. (But at the same time if you extend the
ioctl, it's obvious to also add support to actual ethtool cmdline tool.)

[2] https://github.com/OISF/suricata/blob/master/src/util-ioctl.c#L326

If you want everything to go through the drivers ndo_bpf call anyway
(which userspace API is netlink based) then at what point to you
Not really, that's the front end. ndo_bpf itself is a plain netdev op
and has no real tie to netlink.

want drivers to call their own ndo_bpf, when activated though their
ethtool_ops ? (Sorry, but I don't follow the flow you are proposing)

Notice, I'm not directly against using the drivers ndo_bpf call. I can
see it does provide kernel more flexibility than the ethtool IOCTL.
What I was saying is that even if you go via ethtool ioctl api, where
you end up in dev_ethtool() and have some new ETHTOOL_* query command,
then instead of adding a new ethtool_ops callback, we can and should
reuse ndo_bpf from there.

[...]
Here, I want to discuss how drivers expose/tell userspace that they
support a given feature: Specifically a bit for: XDP_REDIRECT action
support.

Same for meta data,
Well, not really. It would be a "nice-to-have", but not strictly
needed as a feature bit. XDP meta-data is controlled via a helper.
And the BPF-prog can detect/see runtime, that the helper bpf_xdp_adjust_meta
returns -ENOTSUPP (and need to check the ret value anyhow). Thus,
there is that not much gained by exposing this to be detected setup
time, as all drivers should eventually support this, and we can detect
it runtime.

The missing XDP_REDIRECT action features bit it different, as the
BPF-prog cannot detect runtime that this is an unsupported action.
Plus, setup time we cannot query the driver for supported XDP actions.
Ok, so with the example of meta data, you're arguing that it's okay
to load a native XDP program onto a driver, and run actual traffic on
the NIC in order probe for the availability of the feature when you're
saying that it "can detect/see [at] runtime". I totally agree with you
that all drivers should eventually support this (same with XDP_REDIRECT),
but today there are even differences in drivers on bpf_xdp_adjust_meta()/
bpf_xdp_adjust_head() with regards to how much headroom they have available,
etc (e.g. some of them have none), so right now you can either go and
read the code or do a runtime test with running actual traffic through
the NIC to check whether your BPF prog is supported or not. Theoretically,
you can do the same runtime test with XDP_REDIRECT (taking the warn in
bpf_warn_invalid_xdp_action() aside for a moment), but you do have the
trace_xdp_exception() tracepoint to figure it out, yes, it's a painful
hassle, but overall, it's not that different as you were trying to argue
here. For /both/ cases it would be nice to know at setup time whether
this would be supported or not. Hence, such query is not just limited to
XDP_REDIRECT alone. Eventually once such interface is agreed upon,
undoubtedly the list of feature bits will grow is what I'm trying to say;
only arguing on the XDP_REDIRECT here would be short term.

[...]
What about keeping this high level to users? E.g. say you have 2 options
that drivers can expose as netdev_features_strings 'xdp-native-full' or
'xdp-native-partial'. If a driver truly supports all XDP features for a
given kernel e.g. v4.16, then a query like 'ethtool -k foo' will say
'xdp-native-full', if at least one feature is missing to be feature complete
from e.g. above list, then ethtool will tell 'xdp-native-partial', and if
not even ndo_bpf callback exists then no 'xdp-native-*' is reported.
I use-to-be, an advocate for this. I even think I send patches
implementing this. Later, I've realized that this model is flawed.

When e.g. suricata loads it need to look at both "xdp-native-full" and
the kernel version, to determine if XDP_REDIRECT action is avail.
Later when a new kernel version gets released, the driver is missing a
new XDP feature. Then suricata, which doesn't use/need the new
feature, need to be updated, to check that kernel below this version,
with 'xdp-native-partial' and this NIC driver is still okay. Can you
see the problem?

Even if Suricate goes though the pain of keeping track of kernel
version vs drivers vs xdp-native-full/partial. Then, they also want to
run their product on distro kernels. They might convince some distro,
to backport some XDP features they need. So, now they also need to
keep track of distro kernel minor versions... and all they really
wanted as a single feature bit saying if the running NIC driver
supports the XDP_REDIRECT action code.
Yep, agree it's not pretty, not claiming any of this is. You kind of
need to be aware of the underlying kernel, similar to the tracing case.
The underlying problem is effectively the decoupling of program verification
that doesn't have/know the context of where it is being attached to in
this case. Thinking out loud for a sec on couple of other options aside
from feature bits, what about i) providing the target ifindex to the
verifier for XDP programs, such that at verification time you have the
full context similar to nfp offload case today, or ii) populating some
XDP specific auxillary data to the BPF program at verification time such
that the driver can check at program attach time whether the requested
features are possible and if not it will reject and respond with netlink
extack message to the user (as we do in various XDP attach cases already
through XDP_SETUP_PROG{,_HW}).

This would, for example, avoid the need for feature bits, and do actual
rejection of the program while retaining flexibility (and avoiding to
expose bits that over time hopefully will deprecate anyway due to all
XDP aware drivers implementing them). For both cases i) and ii), it
would mean we make the verifier a bit smarter with regards to keeping
track of driver related (XDP) requirements. Program return code checking
is already present in the verifier's check_return_code() and we could
extend it for XDP as well, for example. Seems cleaner and more extensible
than feature bits, imho.

Thanks,
Daniel

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