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=-6.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 77073C433EF for ; Fri, 3 Sep 2021 09:50:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5787860FC4 for ; Fri, 3 Sep 2021 09:50:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348869AbhICJvU (ORCPT ); Fri, 3 Sep 2021 05:51:20 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:55846 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235045AbhICJvO (ORCPT ); Fri, 3 Sep 2021 05:51:14 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1630662614; 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: in-reply-to:in-reply-to:references:references; bh=duzbAIH97p4oVdRMBhioaySsSuzODZFiHTrWeayFJ/A=; b=f4jhOG0UUgyHpuavuqdoEgeyMkyplup3sfLt1n3ryi9x9GJ39gM8AK3aKAuNdTO3a+3lV/ 1B8iFa6pUQic2DBRfux+4rTkUuRDL2U8nJgPRxdhLyn5QZx92gu5cA2sgof9fmcxsFD9R/ CmdWQWbm+QZzs/KCAcLa94eJnS/mJbY= Received: from mail-wm1-f72.google.com (mail-wm1-f72.google.com [209.85.128.72]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-11-LqiDV1rXPt6YvmOLQx1WjA-1; Fri, 03 Sep 2021 05:50:13 -0400 X-MC-Unique: LqiDV1rXPt6YvmOLQx1WjA-1 Received: by mail-wm1-f72.google.com with SMTP id s197-20020a1ca9ce000000b002e72ba822dcso2433635wme.6 for ; Fri, 03 Sep 2021 02:50:13 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=duzbAIH97p4oVdRMBhioaySsSuzODZFiHTrWeayFJ/A=; b=oIMUFpFJe7oR7K03+inJ5J6IXHGfgKrGCqSD2YsfNK5HF0wd41HzZGlD9l45gCfSpw 46g3UBAjQfS/I2jlDak164TKU8iTfhbTzyhI3sl3QHJRTYDKncZCyl8CLlh3mb62xE97 1lHN/Tcs3o9Kt1SWCq3fdzUq9do036HVN86clz0BRceskTy9vBC+X/G1ET+iRGimUPji 79zL4xQ6yQDc5RR6rS1nZW+qe5T0hPd6nYH1lKtPWJ5U5Qgi01o9dKNivAaqBifopHjk 82LoEyrdx7zd0Q6gPW9FmLCX4hd7/2aTl6TWlfE2KOELe59tIcunl3UpM6lK+z/JwVZE OjGg== X-Gm-Message-State: AOAM532hDOjHetTJKK8ye62RL7GX2MvI4IVkTp1r+GciRvBNelcpeiAs EMzcvXkpXsqKWMO5uH3q2OFBUfct71MrB11O1ScgJQYjlsSn0vzMQNLvw6wSl+B71aO1Fq25RI3 VkRUn2rHYKNATuD9J X-Received: by 2002:adf:f747:: with SMTP id z7mr3081543wrp.194.1630662612574; Fri, 03 Sep 2021 02:50:12 -0700 (PDT) X-Google-Smtp-Source: ABdhPJx9ylE45+6PAX7FQM5AJr/mCiW2fz9Pz8cRNkVdIEDEQvG2S68FKjGCgGqgomoOb5JaUeMWUw== X-Received: by 2002:adf:f747:: with SMTP id z7mr3081515wrp.194.1630662612340; Fri, 03 Sep 2021 02:50:12 -0700 (PDT) Received: from krava ([94.113.247.3]) by smtp.gmail.com with ESMTPSA id d5sm4099233wra.38.2021.09.03.02.50.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 03 Sep 2021 02:50:11 -0700 (PDT) Date: Fri, 3 Sep 2021 11:50:09 +0200 From: Jiri Olsa To: Alexei Starovoitov Cc: Andrii Nakryiko , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , "Steven Rostedt (VMware)" , Networking , bpf , Martin KaFai Lau , Song Liu , Yonghong Song , John Fastabend , KP Singh , Daniel Xu , Viktor Malik Subject: Re: [PATCH bpf-next v4 18/27] bpf, x64: Store properly return value for trampoline with multi func programs Message-ID: References: <20210826193922.66204-1-jolsa@kernel.org> <20210826193922.66204-19-jolsa@kernel.org> <20210902215538.a75q7bjcgkpjync4@ast-mbp.dhcp.thefacebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210902215538.a75q7bjcgkpjync4@ast-mbp.dhcp.thefacebook.com> Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Thu, Sep 02, 2021 at 02:55:38PM -0700, Alexei Starovoitov wrote: > On Thu, Sep 02, 2021 at 02:57:11PM +0200, Jiri Olsa wrote: > > > > > > Let's say we have 5 kernel functions: a, b, c, d, e. Say a, b, c all > > > have 1 input args, and d and e have 2. > > > > > > Now let's say we attach just normal fentry program A to function a. > > > Also we attach normal fexit program E to func e. > > > > > > We'll have A attached to a with trampoline T1. We'll also have E > > > attached to e with trampoline T2. Right? > > > > > > And now we try to attach generic fentry (fentry.multi in your > > > terminology) prog X to all 5 of them. If A and E weren't attached, > > > we'd need two generic trampolines, one for a, b, c (because 1 input > > > argument) and another for d,e (because 2 input arguments). But because > > > we already have A and B attached, we'll end up needing 4: > > > > > > T1 (1 arg) for func a calling progs A and X > > > T2 (2 args) for func e calling progs E and X > > > T3 (1 arg) for func b and c calling X > > > T4 (2 args) for func d calling X > > > > so current code would group T3/T4 together, but if we keep > > them separated, then we won't need to use new model and > > cut off some of the code, ok > > We've brainstormed this idea further with Andrii. > (thankfully we could do it in-person now ;) which saved a ton of time) > > It seems the following should work: > 5 kernel functions: a(int), b(long), c(void*), d(int, int), e(long, long). > fentry prog A is attached to 'a'. > fexit prog E is attached to 'e'. > multi-prog X wants to attach to all of them. > It can be achieved with 4 trampolines. > > The trampolines called from funcs 'a' and 'e' can be patched to > call A+X and E+X programs correspondingly. > The multi program X needs to be able to access return values > and arguments of all functions it was attached to. > We can achieve that by always generating a trampoline (both multi and normal) > with extra constant stored in the stack. This constant is the number of > arguments served by this trampoline. > The trampoline 'a' will store nr_args=1. > The tramopline 'e' will store nr_args=2. > We need two multi trampolines. > The multi tramopline X1 that will serve 'b' and 'c' and store nr_args=1 > and multi-tramopline X2 that will serve 'd' and store nr_args=2 > into hidden stack location (like ctx[-2]). > > The multi prog X can look like: > int BPF_PROG(x, __u64 arg1, __u64 arg2, __u64 ret) > in such case it will read correct args and ret when called from 'd' and 'e' > and only correct arg1 when called from 'a', 'b', 'c'. > > To always correctly access arguments and the return value > the program can use two new helpers: bpf_arg(ctx, N) and bpf_ret_value(ctx). > Both will be fully inlined helpers similar to bpf_get_func_ip(). > u64 bpf_arg(ctx, int n) > { > u64 nr_args = ctx[-2]; /* that's the place where _all_ trampoline will store nr_args */ > if (n > nr_args) > return 0; > return ctx[n]; > } > u64 bpf_ret_value(ctx) > { > u64 nr_args = ctx[-2]; > return ctx[nr_args]; > } ok, this is much better then rewiring args access in verifier > > These helpers will be the only recommended way to access args and ret value > in multi progs. > The nice advantage is that normal fentry/fexit progs can use them too. > > We can rearrange ctx[-1] /* func_ip */ and ctx[-2] /* nr_args */ > if it makes things easier. so nr_args will be there all the time, while func_ip is optional at the moment (based on get_func_ip helper presence in program), so we can either switch that: func_ip in ctx[-2] nr_args in ctx[-1] or make func_ip not optional to avoid confusion I think pushing func_ip to ctx-2 is ok > > If multi prog knows that it is attaching to 100 kernel functions > and all of them have 2 arguments it can still do > int BPF_PROG(x, __u64 arg1, __u64 arg2, __u64 ret) > { // access arg1, arg2, ret directly > and it will work correctly. ok, it's user's decision, because at load time we don't know the functions it will be attached to, so verifier can't do anything > > We can make it really strict in the verifier and disallow such > direct access to args from the multi prog and only allow > access via bpf_arg/bpf_ret_value helpers, but I think it's overkill. > Reading garbage values from stack isn't great, but it's not a safety issue. we could also check it in attach time and forbid to attach if there are attach functions with different nr_args and program does not use arg helpers > It means that the verifier will allow something like 16 u64-s args > in multi program. It cannot allow large number, since ctx[1024] > might become a safety issue, while ctx[4] could be a garbage > or a valid value depending on the call site. > > Thoughts? > looks good, thanks for solving this ;-) jirka