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Date: Fri, 28 Mar 2025 00:03:25 +0800
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To: Alexei Starovoitov <ast@kernel.org>, Andrii Nakryiko <andrii@kernel.org>,
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From: Tao Chen <chen.dylane@linux.dev>
Subject: Question: fentry on kernel func optimized by compiler
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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