On 11/13/22 10:27 AM, John Fastabend wrote:
Yonghong Song wrote:
On 11/10/22 3:11 PM, John Fastabend wrote:
John Fastabend wrote:
Yonghong Song wrote:
On 11/9/22 6:17 PM, John Fastabend wrote:
Yonghong Song wrote:
On 11/9/22 1:52 PM, John Fastabend wrote:
Allow xdp progs to read the net_device structure. Its useful to extract
info from the dev itself. Currently, our tracing tooling uses kprobes
to capture statistics and information about running net devices. We use
kprobes instead of other hooks tc/xdp because we need to collect
information about the interface not exposed through the xdp_md structures.
This has some down sides that we want to avoid by moving these into the
XDP hook itself. First, placing the kprobes in a generic function in
the kernel is after XDP so we miss redirects and such done by the
XDP networking program. And its needless overhead because we are
already paying the cost for calling the XDP program, calling yet
another prog is a waste. Better to do everything in one hook from
performance side.
Of course we could one-off each one of these fields, but that would
explode the xdp_md struct and then require writing convert_ctx_access
writers for each field. By using BTF we avoid writing field specific
convertion logic, BTF just knows how to read the fields, we don't
have to add many fields to xdp_md, and I don't have to get every
field we will use in the future correct.
For reference current examples in our code base use the ifindex,
ifname, qdisc stats, net_ns fields, among others. With this
patch we can now do the following,
dev = ctx->rx_dev;
net = dev->nd_net.net;
uid.ifindex = dev->ifindex;
memcpy(uid.ifname, dev->ifname, NAME);
if (net)
uid.inum = net->ns.inum;
to report the name, index and ns.inum which identifies an
interface in our system.
[...]
Yep.
I'm fine doing it with bpf_get_kern_ctx() did you want me to code it
the rest of the way up and test it?
.John
Related I think. We also want to get kernel variable net_namespace_list,
this points to the network namespace lists. Based on above should
we do something like,
void *bpf_get_kern_var(enum var_id);
then,
net_ns_list = bpf_get_kern_var(__btf_net_namesapce_list);
would get us a ptr to the list? The other thought was to put it in the
xdp_md but from above seems better idea to get it through helper.
Sounds great. I guess my new proposed bpf_get_kern_btf_id() kfunc could
cover such a use case as well.
Yes I think this should be good. The only catch is that we need to
get the kernel global var pointer net_namespace_list.
Currently, the kernel supports percpu variable, but
not other global var like net_namespace_list. Currently, there is
an effort to add global var to BTF:
https://lore.kernel.org/bpf/20221104231103.752040-1-stephen.s.brennan@xxxxxxxxxx/
Then we can write iterators on network namespaces and net_devices
without having to do anything else. The usecase is to iterate
the network namespace and collect some subset of netdevices. Populate
a map with these and then keep it in sync from XDP with stats. We
already hook create/destroy paths so have built up maps that track
this and have some XDP stats but not everything we would want.
the net_namespace_list is defined as:
struct list_head net_namespace_list;
So it is still difficult to iterate with bpf program. But we
could have a bpf_iter (similar to task, task_file, etc.)
for net namespaces and it can provide enough context
for the bpf program for each namespace to satisfy your
above need.
You can also with a bounded loop to traverse net_namespace_list
in the bpf program, but it may incur complicated codes...
The other piece I would like to get out of the xdp ctx is the
rx descriptor of the device. I want to use this to pull out info
about the received buffer for debug mostly, but could also grab
some fields that are useful for us to track. That we can likely
do this,
ctx->rxdesc
I think it is possible. Adding rxdesc to xdp_buff as
unsigned char *rxdesc;
or
void *rxdesc;
and using bpf_get_kern_btf_id(kctx->rxdesc, expected_btf_id)
to get a btf id for rxdesc. Here we assume there is
a struct available for rxdesc in vmlinux.h.
Then you can trace through rxdesc with direct memory
access.
I have a RFC patch
https://lore.kernel.org/bpf/20221114162328.622665-1-yhs@xxxxxx/
please help take a look.
Recently had to debug an ugly hardware/driver bug where this would
have been very useful.
.John
