From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,NICE_REPLY_A,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E1369C4727E for ; Fri, 25 Sep 2020 20:36:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A21A82086A for ; Fri, 25 Sep 2020 20:36:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728764AbgIYUdP (ORCPT ); Fri, 25 Sep 2020 16:33:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52372 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727183AbgIYUXf (ORCPT ); Fri, 25 Sep 2020 16:23:35 -0400 Received: from www62.your-server.de (www62.your-server.de [IPv6:2a01:4f8:d0a:276a::2]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DAD49C0613BD; Fri, 25 Sep 2020 12:57:30 -0700 (PDT) Received: from sslproxy05.your-server.de ([78.46.172.2]) by www62.your-server.de with esmtpsa (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256) (Exim 4.89_1) (envelope-from ) id 1kLtlc-000278-AU; Fri, 25 Sep 2020 21:52:28 +0200 Received: from [178.196.57.75] (helo=pc-9.home) by sslproxy05.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1kLtlc-000NwZ-4t; Fri, 25 Sep 2020 21:52:28 +0200 Subject: Re: [PATCH bpf-next 4/6] bpf, libbpf: add bpf_tail_call_static helper for bpf programs To: Andrii Nakryiko Cc: Alexei Starovoitov , john fastabend , Networking , bpf References: <5f3850b2-7346-02d7-50f5-f63355115f35@iogearbox.net> <52cd972d-c183-5d14-b790-4d3a66b8fda2@iogearbox.net> From: Daniel Borkmann Message-ID: Date: Fri, 25 Sep 2020 21:52:27 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Authenticated-Sender: daniel@iogearbox.net X-Virus-Scanned: Clear (ClamAV 0.102.4/25938/Fri Sep 25 15:54:20 2020) Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On 9/25/20 6:50 PM, Andrii Nakryiko wrote: > On Fri, Sep 25, 2020 at 8:52 AM Daniel Borkmann wrote: >> On 9/25/20 5:42 PM, Daniel Borkmann wrote: >>> On 9/25/20 12:17 AM, Daniel Borkmann wrote: >>>> On 9/24/20 10:53 PM, Andrii Nakryiko wrote: >>>>> On Thu, Sep 24, 2020 at 11:22 AM Daniel Borkmann wrote: >>>>>> >>>>>> Port of tail_call_static() helper function from Cilium's BPF code base [0] >>>>>> to libbpf, so others can easily consume it as well. We've been using this >>>>>> in production code for some time now. The main idea is that we guarantee >>>>>> that the kernel's BPF infrastructure and JIT (here: x86_64) can patch the >>>>>> JITed BPF insns with direct jumps instead of having to fall back to using >>>>>> expensive retpolines. By using inline asm, we guarantee that the compiler >>>>>> won't merge the call from different paths with potentially different >>>>>> content of r2/r3. >>>>>> >>>>>> We're also using __throw_build_bug() macro in different places as a neat >>>>>> trick to trigger compilation errors when compiler does not remove code at >>>>>> compilation time. This works for the BPF backend as it does not implement >>>>>> the __builtin_trap(). >>>>>> >>>>>> [0] https://github.com/cilium/cilium/commit/f5537c26020d5297b70936c6b7d03a1e412a1035 >>>>>> >>>>>> Signed-off-by: Daniel Borkmann >>>>>> --- >>>>>> tools/lib/bpf/bpf_helpers.h | 32 ++++++++++++++++++++++++++++++++ >>>>>> 1 file changed, 32 insertions(+) >>>>>> >>>>>> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h >>>>>> index 1106777df00b..18b75a4c82e6 100644 >>>>>> --- a/tools/lib/bpf/bpf_helpers.h >>>>>> +++ b/tools/lib/bpf/bpf_helpers.h >>>>>> @@ -53,6 +53,38 @@ >>>>>> }) >>>>>> #endif >>>>>> >>>>>> +/* >>>>>> + * Misc useful helper macros >>>>>> + */ >>>>>> +#ifndef __throw_build_bug >>>>>> +# define __throw_build_bug() __builtin_trap() >>>>>> +#endif >>>>> >>>>> this will become part of libbpf stable API, do we want/need to expose >>>>> it? If we want to expose it, then we should probably provide a better >>>>> description. >>>>> >>>>> But also curious, how is it better than _Static_assert() (see >>>>> test_cls_redirect.c), which also allows to provide a better error >>>>> message? >>>> >>>> Need to get back to you whether that has same semantics. We use the __throw_build_bug() >>>> also in __bpf_memzero() and friends [0] as a way to trigger a hard build bug if we hit >>>> a default switch-case [0], so we detect unsupported sizes which are not covered by the >>>> implementation yet. If _Static_assert (0, "foo") does the trick, we could also use that; >>>> will check with our code base. >>> >>> So _Static_assert() won't work here, for example consider: >>> >>> # cat f1.c >>> int main(void) >>> { >>> if (0) >>> _Static_assert(0, "foo"); >>> return 0; >>> } >>> # clang -target bpf -Wall -O2 -c f1.c -o f1.o >>> f1.c:4:3: error: expected expression >>> _Static_assert(0, "foo"); >>> ^ >>> 1 error generated. >> >> .. aaand it looks like I need some more coffee. ;-) But result is the same after all: >> >> # clang -target bpf -Wall -O2 -c f1.c -o f1.o >> f1.c:4:3: error: static_assert failed "foo" >> _Static_assert(0, "foo"); >> ^ ~ >> 1 error generated. >> >> # cat f1.c >> int main(void) >> { >> if (0) { >> _Static_assert(0, "foo"); >> } >> return 0; >> } > > You need still more :-P. For you use case it will look like this: > > $ cat test-bla.c > int bar(int x) { > _Static_assert(!__builtin_constant_p(x), "not a constant!"); > return x; > } > > int foo() { > bar(123); > return 0; > } > $ clang -target bpf -O2 -c test-bla.c -o test-bla.o > $ echo $? > 0 Right, but that won't work for example for the use case to detect switch cases which fall into default case as mentioned with the mem* optimizations earlier in this thread. > But in general to ensure unreachable code it's probably useful anyway > to have this. How about calling it __bpf_build_error() or maybe even > __bpf_unreachable()? I think the __bpf_unreachable() sounds best to me, will use that. >>> In order for it to work as required form the use-case, the _Static_assert() must not trigger >>> here given the path is unreachable and will be optimized away. I'll add a comment to the >>> __throw_build_bug() helper. Given libbpf we should probably also prefix with bpf_. If you see >>> a better name that would fit, pls let me know. >>> >>>> [0] https://github.com/cilium/cilium/blob/master/bpf/include/bpf/builtins.h >>> Thanks, >>> Daniel >>