On Tue, Jan 21, 2020 at 04:37:31PM +0100, Toke Høiland-Jørgensen wrote:

Alexei Starovoitov [off-list ref] writes:

quoted

The last few month BPF community has been discussing an approach to call
chaining, since exiting bpt_tail_call() mechanism used in production XDP
programs has plenty of downsides. The outcome of these discussion was a
conclusion to implement dynamic re-linking of BPF programs. Where rootlet XDP
program attached to a netdevice can programmatically define a policy of
execution of other XDP programs. Such rootlet would be compiled as normal XDP
program and provide a number of placeholder global functions which later can be
replaced with future XDP programs. BPF trampoline, function by function
verification were building blocks towards that goal. The patch 1 is a final
building block. It introduces dynamic program extensions. A number of
improvements like more flexible function by function verification and better
libbpf api will be implemented in future patches.

This is great, thank you! I'll go play around with it; couldn't spot
anything obvious from eye-balling the code, except that yeah, it does
need a more flexible libbpf api :)

One thing that's not obvious to me: How can userspace tell which
programs replace which functions after they are loaded? Is this put into
prog_tags in struct bpf_prog_info, or?

good point. Would be good to extend bpf_prog_info. Since prog-to-prog
connection is unidirectional the bpf_prog_info of extension prog will be able
to say which original program it's replacing. bpftool prog show will be able to
print all this data. I think fenry/fexit progs would need the same
bpf_prog_info extension. attach_prog_id + attach_btf_id would be enough.
In the mean time I can try to hack drgn script to do the same.