On 05/20/2017 09:53 AM, Jesper Dangaard Brouer wrote:
On Fri, 19 May 2017 19:13:29 +0200
Daniel Borkmann <dan...@iogearbox.net> wrote:
On 05/18/2017 05:41 PM, Jesper Dangaard Brouer wrote:
There is a fundamental difference between normal eBPF programs
and (XDP) eBPF programs getting attached in a driver. For normal
eBPF programs it is easy to add a new bpf feature, like a bpf
helper, because is it strongly tied to the feature being
available in the current core kernel code. When drivers invoke a
bpf_prog, then it is not sufficient to simply relying on whether
a bpf_helper exists or not. When a driver haven't implemented a
given feature yet, then it is possible to expose uninitialized
parts of xdp_buff. The driver pass in a pointer to xdp_buff,
usually "allocated" on the stack, which must not be exposed.
When xdp_buff is being extended, then we should at least zero
initialize all in-tree users that don't support or populate this
field, thus that it's not uninitialized memory. Better would be
to have a way to reject the prog in the first place until it's
implemented (but further comments on feature bits below).
Going down a path where we need to zero out the xdp_buff looks a lot
like the sk_buff zeroing, which is the top perf cost associated with
SKBs see[1]. XDP is is about not repeating the same issue we had with
the SKB...
But if we agree on implementing a certain feature that requires to
add a new member, then basic requirement should be that it needs to
be implementable by all XDP enabled drivers, so that users eventually
don't need to worry which driver they run to access a XDP feature.
In that case, zeroing that member until it gets properly implemented
is just temporary (and could be an incentive perhaps).
[1]
https://prototype-kernel.readthedocs.io/en/latest/blogposts/xdp25_eval_generic_xdp_tx.html#analyzing-build-skb-and-memset
Only two user visible NETIF_F_XDP_* net_device feature flags are
exposed via ethtool (-k) seen as "xdp" and "xdp-partial".
The "xdp-partial" is detected when there is not feature equality
between kernel and driver, and a netdev_warn is given.
I think having something like a NETIF_F_XDP_BIT for ethtool to
indicate support as "xdp" is quite useful. Avoids having to grep
the kernel tree for ndo_xdp callback. ;) A "xdp-partial" would
still be unclear/confusing to the user whether his program loads
or doesn't which is the only thing a user (or some loading infra)
cares about eventually, so one still needs to go trying to load
the XDP code to see whether that fails for the native case.
Good that we agree on usefulness of the NETIF_F_XDP_BIT. The
"xdp-partial" or "xdp-challenged" is an early indication to the user
that they should complain to the vendor. I tried to keep it simple
towards the user. Do you think every feature bit should be exposed to
userspace?
That would potentially require us to go down that path and expose
feature bits for everything, even something as silly as new flags
for a specific helper that requires some sort of additional support.
We probably rather want to keep such thing in the kernel for now
and potentially reject loads instead.
The idea is that XDP_DRV_* feature bits define a contract between
the driver and the kernel, giving a reliable way to know that XDP
features a driver promised to implement. Thus, knowing what bpf
side features are safe to allow.
There are 3 levels of features: "required", "devel" and "optional".
The motivation is pushing driver vendors forward to support all
the new XDP features. Once a given feature bit is moved into
the "required" features, the kernel will reject loading XDP
program if feature isn't implemented by driver. Features under
developement, require help from the bpf infrastrucure to detect
when a given helper or direct-access is used, using a bpf_prog
bit to mark a need for the feature, and pulling in this bit in
the xdp_features_check(). When all drivers have implemented
a "devel" feature, it can be moved to the "required" feature and
The problem is that once you add bits markers to bpf_prog like we
used to do in the past, then as you do in patch 4/5 with the
xdp_rxhash_needed bit, they will need to be turned /on/ unconditionally
when a prog has tail calls.
Yes, with tail calls, we have to enable all features. But that is a
good thing, as it forces vendors to quickly implement all features.
And it is no different from moving a feature into the "required" bits,
once all drivers implement it. It is only a limitation for tail calls,
and something we can fix later (for handling this for tail calls).
But the issue I pointed out with this approach is that XDP programs
using tail calls will suddenly break and get rejected from one
version to another.
That's basically breaking the contract we have today where it must
be guaranteed that older programs keep working on newer kernels,
exactly the same with uapi. Breaking user programs is a no-go,
not a good thing at all (it will hurt users and they move on with
something else). So it's not something we can 'fix' at some later
point in time; such feature detection would need to consider this
from day 1.
BPF have some nice features of evaluating the input program
"load-time", which is what I'm taking advantage of as an optimization
here (let use this nice bpf property). It is only tail calls that
cannot evaluate this "load-time". Thus, if you care about tail calls,
supporting intermediate features, we could later fix that by adding a
runtime feature check in the case of tail calls.
