On Thu, Mar 27, 2025 at 9:03 AM Tao Chen <chen.dylane@xxxxxxxxx> wrote: > > Hi, > > I recently encountered a problem when using fentry to trace kernel > functions optimized by compiler, the specific situation is as follows: > https://github.com/bpftrace/bpftrace/issues/3940 > > Simply put, some functions have been optimized by the compiler. The > original function names are found through BTF, but the optimized > functions are the ones that exist in kallsyms_lookup_name. Therefore, > the two do not match. > > func_proto = btf_type_by_id(desc_btf, func->type); > if (!func_proto || !btf_type_is_func_proto(func_proto)) { > verbose(env, "kernel function btf_id %u does not have a > valid func_proto\n", > func_id); > return -EINVAL; > } > > func_name = btf_name_by_offset(desc_btf, func->name_off); > addr = kallsyms_lookup_name(func_name); > if (!addr) { > verbose(env, "cannot find address for kernel function > %s\n", > func_name); > return -EINVAL; > } > > I have made a simple statistics and there are approximately more than > 2,000 functions in Ubuntu 24.04. > > dylane@2404:~$ cat /proc/kallsyms | grep isra | wc -l > 2324 > > So can we add a judgment from libbpf. If it is an optimized function, No, we cannot. It's a different function at that point and libbpf isn't going to be in the business of guessing on behalf of the user whether it's ok to do or not. But the user can use multi-kprobe with `prefix*` naming, if they encountered (or are anticipating) this situation and think it's fine for them. As for fentry/fexit, you need to have the correct BTF ID associated with that function anyways, so I'm not sure that currently you can attach fentry/fexit to such compiler-optimized functions at all (pahole won't produce BTF for such functions, right?). > pass the suffix of the optimized function from the user space to the > kernel, and then perform a function name concatenation, like: > > func_name = btf_name_by_offset(desc_btf, func->name_off); > if (optimize) { > func_name = func_name + ".isra.0" > } > addr = kallsyms_lookup_name(func_name); > > -- > Best Regards > Tao Chen > >
