Re: [External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add support to record and check the accessed args

[梦龙董 <dongmenglong.8@bytedance.com>]

On Wed, Mar 13, 2024 at 12:42 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Mon, Mar 11, 2024 at 7:42 PM 梦龙董 <dongmenglong.8@bytedance.com> wrote:
> >
[......]
>
> I see.
> I thought you're sharing the trampoline across attachments.
> (since bpf prog is the same).

That seems to be a good idea, which I hadn't thought before.

> But above approach cannot possibly work with a shared trampoline.
> You need to create individual trampoline for all attachment
> and point them to single bpf prog.
>
> tbh I'm less excited about this feature now, since sharing
> the prog across different attachments is nice, but it won't scale
> to thousands of attachments.
> I assumed that there will be a single trampoline with max(argno)
> across attachments and attach/detach will scale to thousands.
>
> With individual trampoline this will work for up to a hundred
> attachments max.

What does "a hundred attachments max" means? Can't I
trace thousands of kernel functions with a bpf program of
tracing multi-link?

>
> Let's step back.
> What is the exact use case you're trying to solve?
> Not an artificial one as selftest in patch 9, but the real use case?

I have a tool, which is used to diagnose network problems,
and its name is "nettrace". It will trace many kernel functions, whose
function args contain "skb", like this:

./nettrace -p icmp
begin trace...
***************** ffff889be8fbd500,ffff889be8fbcd00 ***************
[1272349.614564] [dev_gro_receive     ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614579] [__netif_receive_skb_core] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614585] [ip_rcv              ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614592] [ip_rcv_core         ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614599] [skb_clone           ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614616] [nf_hook_slow        ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614629] [nft_do_chain        ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614635] [ip_rcv_finish       ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614643] [ip_route_input_slow ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614647] [fib_validate_source ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614652] [ip_local_deliver    ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614658] [nf_hook_slow        ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614663] [ip_local_deliver_finish] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614666] [icmp_rcv            ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614671] [icmp_echo           ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614675] [icmp_reply          ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614715] [consume_skb         ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614722] [packet_rcv          ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220
[1272349.614725] [consume_skb         ] ICMP: 169.254.128.15 ->
172.27.0.6 ping request, seq: 48220

For now, I have to create a bpf program for every kernel
function that I want to trace, which is up to 200.

With this multi-link, I only need to create 5 bpf program,
like this:

int BPF_PROG(trace_skb_1, struct *skb);
int BPF_PROG(trace_skb_2, u64 arg0, struct *skb);
int BPF_PROG(trace_skb_3, u64 arg0, u64 arg1, struct *skb);
int BPF_PROG(trace_skb_4, u64 arg0, u64 arg1, u64 arg2, struct *skb);
int BPF_PROG(trace_skb_5, u64 arg0, u64 arg1, u64 arg2, u64 arg3, struct *skb);

Then, I can attach trace_skb_1 to all the kernel functions that
I want to trace and whose first arg is skb; attach trace_skb_2 to kernel
functions whose 2nd arg is skb, etc.

Or, I can create only one bpf program and store the index
of skb to the attachment cookie, and attach this program to all
the kernel functions that I want to trace.

This is my use case. With the multi-link, now I only have
1 bpf program, 1 bpf link, 200 trampolines, instead of 200
bpf programs, 200 bpf link and 200 trampolines.

The shared trampoline you mentioned seems to be a
wonderful idea, which can make the 200 trampolines
to one. Let me have a look, we create a trampoline and
record the max args count of all the target functions, let's
mark it as arg_count.

During generating the trampoline, we assume that the
function args count is arg_count. During attaching, we
check the consistency of all the target functions, just like
what we do now.

Am I right?

Thanks!
Menglong Dong