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

Jiri Olsa olsajiri at gmail.com
Wed Mar 13 23:27:52 PDT 2024 

On Wed, Mar 13, 2024 at 05:25:35PM -0700, Alexei Starovoitov wrote:
> On Tue, Mar 12, 2024 at 6:53 PM 梦龙董 <dongmenglong.8 at bytedance.com> wrote:
> >
> > On Wed, Mar 13, 2024 at 12:42 AM Alexei Starovoitov
> > <alexei.starovoitov at gmail.com> wrote:
> > >
> > > On Mon, Mar 11, 2024 at 7:42 PM 梦龙董 <dongmenglong.8 at 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?
> 
> I mean what time does it take to attach one program
> to 100 fentry-s ?
> What is the time for 1k and for 10k ?
> 
> The kprobe multi test attaches to pretty much all funcs in
> /sys/kernel/tracing/available_filter_functions
> and it's fast enough to run in test_progs on every commit in bpf CI.
> See get_syms() in prog_tests/kprobe_multi_test.c
> 
> Can this new multi fentry do that?
> and at what speed?
> The answer will decide how applicable this api is going to be.
> Generating different trampolines for every attach point
> is an approach as well. Pls benchmark it too.
> 
> > >
> > > 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.
> 
> I see. The use case makes sense to me.
> Andrii's retsnoop is used to do similar thing before kprobe multi was
> introduced.
> 
> > 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.
> 
> For one trampoline to handle all attach points we might
> need some arch support, but we can start simple.
> Make btf_func_model with MAX_BPF_FUNC_REG_ARGS
> by calling btf_distill_func_proto() with func==NULL.
> And use that to build a trampoline.
> 
> The challenge is how to use minimal number of trampolines
> when bpf_progA is attached for func1, func2, func3
> and bpf_progB is attached to func3, func4, func5.
> We'd still need 3 trampolines:
> for func[12] to call bpf_progA,
> for func3 to call bpf_progA and bpf_progB,
> for func[45] to call bpf_progB.
> 
> Jiri was trying to solve it in the past. His slides from LPC:
> https://lpc.events/event/16/contributions/1350/attachments/1033/1983/plumbers.pdf
> 
> Pls study them and his prior patchsets to avoid stepping on the same rakes.

yep, I refrained from commenting not to take you down the same path
I did, but if you insist.. ;-) 

I managed to forgot almost all of it, but the IIRC the main pain point
was that at some point I had to split existing trampoline which caused
the whole trampolines management and error paths to become a mess

I tried to explain things in [1] changelog and the latest patchset is in [0]

feel free to use/take anything, but I advice strongly against it ;-)
please let me know if I can help

jirka


[0] https://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git/log/?h=bpf/batch
[1] https://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git/commit/?h=bpf/batch&id=52a1d4acdf55df41e99ca2cea51865e6821036ce

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