From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-177.mta1.migadu.com (out-177.mta1.migadu.com [95.215.58.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3A04C1E9B06 for ; Tue, 15 Apr 2025 12:10:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.177 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744719023; cv=none; b=MbsAuQmfE4TanbqfhfW9hngHEDDwWLYe/XdYFkYCZex2E8ldMlTRKvUTNS7ujJqkNpakT/7eNJVT/Eos/9WJh0pH0WkJLQUTOov/cignSzXCQnkKLuoTwI6A+V0eTKTgriZSCD5lrqjRaG1tjE3AxCqXsBWpxVfvIATiLmLjcZY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744719023; c=relaxed/simple; bh=TS/NkXPc+RGkVoSBxSzwUeEBb0kJbsvf5fQX3rUmvMc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=PtPhZjpJsXibXNHKWE+DjE7t+jP3pSd/VltYbr67CFTeKKxZax90q+db9Nn0ApV3qxIr/XRZh4xlldeZhkgiKn6tNxdkcsHtNiGirClPxD3aBbcVd+r7k/bEB2iL2T1d/H7eFg8SRaEA796SPN5LMTvzYW40wZXpmMOpyobiFvc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=ej6UeVBp; arc=none smtp.client-ip=95.215.58.177 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="ej6UeVBp" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1744719018; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=hMeS6Prrho83qVgpHmCFKE+Gf2ByHSCiRH7UJDMJW8k=; b=ej6UeVBp2CxDsdAKz6vfFxnfJgp3dU1CMPqcAHuNUBzqiCx9wULL8ZxHdrMmIyR+5ZbMO8 mshiopDHUFWA+0Bg/4T+TWO6cxU3oANwUP6IG0wVKAAjTRGCEzdyC5KJPxLAOkYB56OywH 6mkr5rIQgBdGXbLEdwiwO1oL/t62oZU= Date: Tue, 15 Apr 2025 20:10:09 +0800 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: Question: fentry on kernel func optimized by compiler To: Alan Maguire , Andrii Nakryiko Cc: Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Jiri Olsa , Song Liu , Yonghong Song , bpf References: <7e46c811-e85b-4001-8fac-b16aa0e9815f@linux.dev> <3c6f539b-b498-4587-b0dc-5fdeba717600@oracle.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Tao Chen In-Reply-To: <3c6f539b-b498-4587-b0dc-5fdeba717600@oracle.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 在 2025/3/31 18:13, Alan Maguire 写道: > On 28/03/2025 17:21, Andrii Nakryiko wrote: >> On Thu, Mar 27, 2025 at 9:03 AM Tao Chen 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?). >> > > Yep, BTF will not be there for all cases, but ever since we've had the > "optimized_func" BTF feature, we've have encoded BTF for suffixed > functions as long as their parameters are not optimized away and as long > as we don't have multiple inconsistent representations associated with a > function (say two differing function signatures for the same name). > Optimization away of parameters happens quite frequently, but not always > for .isra.0 functions so they are potentially sometimes safe for fentry. > > The complication here is that - by design - the function name in BTF > will be the prefix; i.e. "foo" not "foo.isra.0". So how we match up the > BTF with the right suffixed function is an issue; a single function > prefix can have ".isra.0" and ".cold.0" suffixes associated for example. > The latter isn't really a function entry point (in the C code at least); > it's just a split of the function into common path and less common path > for better code locality for the more commonly-executed code. > > Yonghong and I talked about this a bit last year in Plumbers, but it did > occur to me that there are conditions where we could match up the prefix > from BTF with a guaranteed fentry point for the function using info we > have today. > > /sys/kernel/tracing/available_filter_functions_addr has similar info to > /proc/kallysyms but as far as I understand it we are also guaranteed > that the associated addresses correspond to real function entry points. > So because the BTF representation currently ensures consistency _and_ > available function parameters, I think we could use > available_filter_functions_addr to carry out the match and provide the > right function address for the BTF representation. > Hi, Alan Sorry for not replying in time. As you said，it seems much simpler when use the func addr from available_filter_functions_addr. It seems to be a bit similar to the way of passing function addresses in kprobe_multi. @Andrii @Jiri what do you think? > In the future, the hope is we can handle inconsistent representations > too in BTF, but the above represents a possible approach we could > implement today I think, though I may be missing something. Thanks! > > Alan -- Best Regards Tao Chen