Re: Question: fentry on kernel func optimized by compiler

[Tao Chen <chen.dylane@linux.dev>]

在 2025/3/28 01:19, Song Liu 写道:
> 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/
> 
>

Hi Song,

Thank you for the materials you provided. I'll take a look at them.

> On Thu, Mar 27, 2025 at 9:03 AM Tao Chen <chen.dylane@linux.dev> 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
>>


-- 
Best Regards
Tao Chen