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=-17.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 85D44C433F5 for ; Wed, 8 Sep 2021 11:46:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 620F3610E9 for ; Wed, 8 Sep 2021 11:46:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1351574AbhIHLr7 (ORCPT ); Wed, 8 Sep 2021 07:47:59 -0400 Received: from www62.your-server.de ([213.133.104.62]:53424 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235453AbhIHLr6 (ORCPT ); Wed, 8 Sep 2021 07:47:58 -0400 Received: from sslproxy02.your-server.de ([78.47.166.47]) by www62.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92.3) (envelope-from ) id 1mNw2N-000G2T-3I; Wed, 08 Sep 2021 13:46:43 +0200 Received: from [85.5.47.65] (helo=linux.home) by sslproxy02.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1mNw2M-000OMW-RR; Wed, 08 Sep 2021 13:46:42 +0200 Subject: Re: [PATCH bpf-next v2 13/13] bpf/tests: Add tail call limit test with external function call To: Johan Almbladh , Ilya Leoshkevich Cc: Alexei Starovoitov , Andrii Nakryiko , Martin KaFai Lau , Song Liu , Yonghong Song , John Fastabend , KP Singh , Networking , bpf References: <20210907222339.4130924-1-johan.almbladh@anyfinetworks.com> <20210907222339.4130924-14-johan.almbladh@anyfinetworks.com> From: Daniel Borkmann Message-ID: Date: Wed, 8 Sep 2021 13:46:42 +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 0.103.2/26288/Wed Sep 8 10:22:21 2021) Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 9/8/21 12:53 PM, Johan Almbladh wrote: > On Wed, Sep 8, 2021 at 12:10 PM Ilya Leoshkevich wrote: >> On Wed, 2021-09-08 at 00:23 +0200, Johan Almbladh wrote: >>> This patch adds a tail call limit test where the program also emits >>> a BPF_CALL to an external function prior to the tail call. Mainly >>> testing that JITed programs preserve its internal register state, for >>> example tail call count, across such external calls. >>> >>> Signed-off-by: Johan Almbladh >>> --- >>> lib/test_bpf.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++--- >>> 1 file changed, 48 insertions(+), 3 deletions(-) >>> >>> diff --git a/lib/test_bpf.c b/lib/test_bpf.c >>> index 7475abfd2186..6e45b4da9841 100644 >>> --- a/lib/test_bpf.c >>> +++ b/lib/test_bpf.c >>> @@ -12259,6 +12259,20 @@ static struct tail_call_test tail_call_tests[] >>> = { >>> }, >>> .result = MAX_TAIL_CALL_CNT + 1, >>> }, >>> + { >>> + "Tail call count preserved across function calls", >>> + .insns = { >>> + BPF_ALU64_IMM(BPF_ADD, R1, 1), >>> + BPF_STX_MEM(BPF_DW, R10, R1, -8), >>> + BPF_CALL_REL(0), >>> + BPF_LDX_MEM(BPF_DW, R1, R10, -8), >>> + BPF_ALU32_REG(BPF_MOV, R0, R1), >>> + TAIL_CALL(0), >>> + BPF_EXIT_INSN(), >>> + }, >>> + .stack_depth = 8, >>> + .result = MAX_TAIL_CALL_CNT + 1, >>> + }, >>> { >>> "Tail call error path, NULL target", >>> .insns = { >> >> There seems to be a problem with BPF_CALL_REL(0) on s390, since it >> assumes that test_bpf_func and __bpf_call_base are within +-2G of >> each other, which is not (yet) the case. > > The idea with this test is to mess up a JITed program's internal state > if it does not properly save/restore those regs. I would like to keep > the test in some form, but I do see the problem here. > > Another option could perhaps be to skip this test at runtime if the > computed offset is outside +-2G. If the offset is greater than that it > does not fit into the 32-bit BPF immediate field, and must therefore > be skipped. This would work for other archs too. Sounds reasonable as a work-around/to move forward. > Yet another solution would be call one or several bpf helpers instead. > As I understand it, they should always be located within this range, > otherwise they would not be callable from a BPF program. The reason I > did not do this was because I found helpers that don't require any > context to be too simple. Ideally one would want to call something > that uses pretty much all available caller-saved CPU registers. I > figured snprintf would be complex/nasty enough for this purpose. Potentially bpf_csum_diff() could also be a candidate, and fairly straight forward to set up from raw asm. >> I can't think of a good fix, so how about something like this? >> >> --- a/lib/test_bpf.c >> +++ b/lib/test_bpf.c >> @@ -12257,6 +12257,7 @@ static struct tail_call_test tail_call_tests[] >> = { >> }, >> .result = MAX_TAIL_CALL_CNT + 1, >> }, >> +#ifndef __s390__ >> { >> "Tail call count preserved across function calls", >> .insns = { >> @@ -12271,6 +12272,7 @@ static struct tail_call_test tail_call_tests[] >> = { >> .stack_depth = 8, >> .result = MAX_TAIL_CALL_CNT + 1, >> }, >> +#endif >> { >> "Tail call error path, NULL target", >> .insns = { >> >> [...] >>