Hi Tao, Compiler optimizations can cause issues for tracing kernel functions. Please refer to Yonghong and Alan's presentation [1] for various cases. Thanks, Song [1] https://lpc.events/event/18/contributions/1945/ 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, > 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 >
