Re: [PATCH bpf-next v5 00/17] Introduce eBPF support for HID devices

Alexei Starovoitov
2022-05-21T00:18:41Z

On Thu, May 19, 2022 at 4:56 AM Benjamin Tissoires
<benjamin.tissoires@redhat.com> wrote:
>
> As Greg mentioned in his reply, report descriptors fixups don't do
> much besides changing a memory buffer at probe time. So we can either
> have udev load the program, pin it and forget about it, or we can also
> have the kernel do that for us.
>
> So I envision the distribution to be hybrid:
> - for plain fixups where no userspace is required, we should
> distribute those programs in the kernel itself, in-tree.
> This series already implements pre-loading of BPF programs for the
> core part of HID-BPF, but I plan on working on some automation of
> pre-loading of these programs from the kernel itself when we need to
> do so.
>
> Ideally, the process would be:
> * user reports a bug
> * developer produces an eBPF program (and maybe compile it if the user
> doesn't have LLVM)
> * user tests/validates the fix without having to recompile anything
> * developer drops the program in-tree
> * some automated magic happens (still unclear exactly how to define
> which HID device needs which eBPF program ATM)
> * when the kernel sees this exact same device (BUS/VID/PID/INTERFACE)
> it loads the fixup
>
> - the other part of the hybrid solution is for when userspace is
> heavily involved (because it exports a new dbus interface for that
> particular feature on this device). We can not really automatically
> preload the BPF program because we might not have the user in front of
> it.
> So in that case, the program would be hosted alongside the
> application, out-of-the-tree, but given that to be able to call kernel
> functions you need to be GPL, some public distribution of the sources
> is required.

Agree with everything you've said earlier.
Just one additional comment:
By default the source code is embedded in bpf objects.
Here is an example.
$ bpftool prog dump jited id 3927008|head -50
void cwnd_event(long long unsigned int * ctx):
bpf_prog_9b9adc0a36a25303_cwnd_event:
; void BPF_STRUCT_OPS(cwnd_event, struct sock* sk, enum tcp_ca_event ev) {
0: nopl 0x0(%rax,%rax,1)
5: xchg %ax,%ax
...
; switch (ev) {
25: mov %r14d,%edi
28: add $0xfffffffc,%edi
...
; ca->loss_cwnd = tp->snd_cwnd;
4a: mov %edi,0x18(%r13)
4e: mov $0x2,%edi
; tp->snd_ssthresh = max(tp->snd_cwnd >> 1U, 2U);
53: test %rbx,%rbx
56: jne 0x000000000000005c

It's not the full source, of course, but good enough in practice
for a person to figure out what program is doing.