linux-kernel - Re: [PATCH bpf-next 1/4] bpf: Allow get_func_[arg|arg

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Message-ID: <f951b81f-1b46-4219-82fd-0839e27ab3f3@linux.dev>
Date: Wed, 30 Apr 2025 23:54:46 +0800
From: Leon Hwang <leon.hwang@...ux.dev>
To: Kafai Wan <mannkafai@...il.com>,
 Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Song Liu <song@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Matt Bobrowski <mattbobrowski@...gle.com>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, Mykola Lysenko <mykolal@...com>,
 Shuah Khan <shuah@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
 bpf <bpf@...r.kernel.org>,
 linux-trace-kernel <linux-trace-kernel@...r.kernel.org>,
 Network Development <netdev@...r.kernel.org>,
 "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/4] bpf: Allow get_func_[arg|arg_cnt] helpers in
 raw tracepoint programs

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