Re: [RFC PATCH bpf-next 00/12] bpf: Introduce static-defined tracing probe for BPF

[Leon Hwang]

On 2026/6/29 19:32, Xu Kuohai wrote:
> On 6/29/2026 6:55 PM, Leon Hwang wrote:
>> On 29/6/26 15:51, Xu Kuohai wrote:
>>>>
>>>> With the elimination, the callsite of 'subprog' has been eliminated.
>>>>
>>>> However, as you mentioned above, the argument preparation insns are
>>>> kept.
>>>>
>>>
>>> I see, the CALL insntruction is removed, not patched to NOP.
>>>
>>> So the call site is either a call to subprog, or totally removed. The
>>> fentry
>>> tracing only works when the CALL instruction and the subprog are not
>>> eliminated.
>>> This is exactly what SDT tries to solve.
>>>
>> So, why not add a macro for the stub following the subprog+fentry idea?
>>
>> #define DEFINE_TRACING_STUB(__stub, ...)     \
>> static __noinline void __stub(...)        \
>> {                        \
>>     ...                    \
>> }
>>
>> DEFINE_TRACING_STUB(my_trace, int, len, int, ret);
>>
>> SEC("xdp")
>> int xdp_prog(struct xdp_md *ctx)
>> {
>>     ...
>>     my_trace(len, ret);
>>     ...
>> }
>>
>> Then, add a libbpf API bpf_program__remove_stub(prog, my_stub) to drop
>> the stub by the way like my demo.
>>
> 
> Since the stub prog is dropped in libbpf, where is the fentry prog
> attached?
> 

No. By default, the stub prog won't be dropped by libbpf. The
bpf_program__remove_stub() API is provided so that users can
intentionally remove the stub prog.

Maybe bpf_program__remove_subprog() would be a better name for the API,
because it is to remove a subprog.

Thanks,
Leon