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 50CE3C4363D for ; Fri, 25 Sep 2020 15:42:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1D9F221741 for ; Fri, 25 Sep 2020 15:42:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729073AbgIYPms (ORCPT ); Fri, 25 Sep 2020 11:42:48 -0400 Received: from www62.your-server.de ([213.133.104.62]:51560 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729009AbgIYPms (ORCPT ); Fri, 25 Sep 2020 11:42:48 -0400 Received: from sslproxy02.your-server.de ([78.47.166.47]) by www62.your-server.de with esmtpsa (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256) (Exim 4.89_1) (envelope-from ) id 1kLprx-00005L-T9; Fri, 25 Sep 2020 17:42:45 +0200 Received: from [178.196.57.75] (helo=pc-9.home) by sslproxy02.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1kLprx-000JCJ-OL; Fri, 25 Sep 2020 17:42:45 +0200 Subject: Re: [PATCH bpf-next 4/6] bpf, libbpf: add bpf_tail_call_static helper for bpf programs From: Daniel Borkmann To: Andrii Nakryiko Cc: Alexei Starovoitov , john fastabend , Networking , bpf References: <5f3850b2-7346-02d7-50f5-f63355115f35@iogearbox.net> Message-ID: Date: Fri, 25 Sep 2020 17:42:45 +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: <5f3850b2-7346-02d7-50f5-f63355115f35@iogearbox.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit 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 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. 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