Re: [PATCH bpf-next 1/4] bpf: Allow get_func_[arg|arg_cnt] helpers in raw tracepoint programs

[Leon Hwang <leon.hwang@linux.dev>]

On 2025/4/30 20:43, Kafai Wan wrote:
> On Wed, Apr 30, 2025 at 10:46 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
>>
>> On Sat, Apr 26, 2025 at 9:00 AM KaFai Wan <mannkafai@gmail.com> wrote:
>>>

[...]

>>> @@ -2312,7 +2322,7 @@ void __bpf_trace_run(struct bpf_raw_tp_link *link, u64 *args)
>>>  #define REPEAT(X, FN, DL, ...)         REPEAT_##X(FN, DL, __VA_ARGS__)
>>>
>>>  #define SARG(X)                u64 arg##X
>>> -#define COPY(X)                args[X] = arg##X
>>> +#define COPY(X)                args[X + 1] = arg##X
>>>
>>>  #define __DL_COM       (,)
>>>  #define __DL_SEM       (;)
>>> @@ -2323,9 +2333,10 @@ void __bpf_trace_run(struct bpf_raw_tp_link *link, u64 *args)
>>>         void bpf_trace_run##x(struct bpf_raw_tp_link *link,             \
>>>                               REPEAT(x, SARG, __DL_COM, __SEQ_0_11))    \
>>>         {                                                               \
>>> -               u64 args[x];                                            \
>>> +               u64 args[x + 1];                                        \
>>> +               args[0] = x;                                            \
>>>                 REPEAT(x, COPY, __DL_SEM, __SEQ_0_11);                  \
>>> -               __bpf_trace_run(link, args);                            \
>>> +               __bpf_trace_run(link, args + 1);                        \
>>
>> This is neat, but what is this for?
>> The program that attaches to a particular raw_tp knows what it is
>> attaching to and how many arguments are there,
>> so bpf_get_func_arg_cnt() is a 5th wheel.
>>
>> If the reason is "for completeness" then it's not a good reason
>> to penalize performance. Though it's just an extra 8 byte of stack
>> and a single store of a constant.
>>
> If we try to capture all arguments of a specific raw_tp in tracing programs,
> We first obtain the arguments count from the format file in debugfs or BTF
> and pass this count to the BPF program via .bss section or cookie (if
> available).
> 
> If we store the count in ctx and get it via get_func_arg_cnt helper in
> the BPF program，
> a) It's easier and more efficient to get the arguments count in the BPF program.
> b) It could use a single BPF program to capture arguments for multiple raw_tps,
> reduce the number of BPF programs when massive tracing.
> 


bpf_get_func_arg() will be very helpful for bpfsnoop[1] when tracing tp_btf.

In bpfsnoop, it can generate a small snippet of bpf instructions to use
bpf_get_func_arg() for retrieving and filtering arguments. For example,
with the netif_receive_skb tracepoint, bpfsnoop can use
bpf_get_func_arg() to filter the skb argument using pcap-filter(7)[2] or
a custom attribute-based filter. This will allow bpfsnoop to trace
multiple tracepoints using a single bpf program code.

[1] https://github.com/bpfsnoop/bpfsnoop
[2] https://www.tcpdump.org/manpages/pcap-filter.7.html

Thanks,
Leon