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=-9.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 04D8DC43461 for ; Sat, 5 Sep 2020 00:08:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B75582087C for ; Sat, 5 Sep 2020 00:08:47 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="sQqJG3HY" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728160AbgIEAIr (ORCPT ); Fri, 4 Sep 2020 20:08:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36654 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726208AbgIEAIp (ORCPT ); Fri, 4 Sep 2020 20:08:45 -0400 Received: from mail-yb1-xb41.google.com (mail-yb1-xb41.google.com [IPv6:2607:f8b0:4864:20::b41]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 827AEC061244; Fri, 4 Sep 2020 17:08:44 -0700 (PDT) Received: by mail-yb1-xb41.google.com with SMTP id x2so5501472ybf.12; Fri, 04 Sep 2020 17:08:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=KDX/6zy79NRu72r1RQNQKA3AIJxQMsol9K4um6Utn6A=; b=sQqJG3HY79lMKCfqXY+RAlVqI7KOZ301coe5V6Mr7RW3uEsE38vH/uspH5ci1mkQI1 2qajEpHb9vUcajl54SWtxaBfiq3ngCBKnXunDlXxJtWRV+Xueaq+0QE05DrDPooo+JTk wCIfxBdBy+rVnDFJq/BFtFvWolc0dAqdFov8d6ob7EJNiYUyZnwoInXeBf7Z8RFEGrnn 5CoARl6pX8fvsXAu4u/DmzbyOfCQlwkseVYpvp8NaBuO7w0Nu8DMmBHe09p6wx40Eeyo xps9b43xdB6cpyWE2bRJx0Muui1J+75XtLppeOphP02OKO6pMIbgSYtB2wDW1I5yS8We B/pA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=KDX/6zy79NRu72r1RQNQKA3AIJxQMsol9K4um6Utn6A=; b=mr6lxjqk8yGsn579BNbKNa4oNXzbSPfo22NYSv48WBH3BIyxQCwF47tdqzvFyKj1DU 8b18reXeqVfUhjnUTFp7Hm13nTBVYal7wy8oa+sF02mlrZi+XTNJu0nBwEnUioHHZH9W JLx+khz0pyD2H5NUN7sN0i8s/rHSoed8+LBizWeJ/z/thfzT1Wwywf8SP7SerfOuV6Rd tjnU6EuM0fswhEtcyUzeEeipWOhbQa56Uu6db+DawlJmW7pxvHzDUhZvu3ubtNWrSa23 lI2LkSY9trOBaPMl53ULzGjW3INWeIHs9kaqPtfWQWp5MNymnmRHNgujHIt8XbEt2aI1 fwLg== X-Gm-Message-State: AOAM532rUWIRWZTMPViLJ0ArTVuY+vuEcpOPrxx5D4Vpq7ddg7a1MaYr BSfxJqHKnvlanEbGPjcQ1yf2CN2quw+0Y/WzNvI= X-Google-Smtp-Source: ABdhPJz32ewHgnVTDiLUJ69WjBc/pos+Edn+jVIyHzUFpon1EhI40m64R33yBa6JgJ7GC531OForxTATcI+HwfePz9U= X-Received: by 2002:a25:ef43:: with SMTP id w3mr11668910ybm.230.1599264523663; Fri, 04 Sep 2020 17:08:43 -0700 (PDT) MIME-Version: 1.0 References: <20200904194900.3031319-1-yhs@fb.com> <20200904194900.3031377-1-yhs@fb.com> In-Reply-To: From: Andrii Nakryiko Date: Fri, 4 Sep 2020 17:08:32 -0700 Message-ID: Subject: Re: [PATCH bpf-next 1/2] bpf: permit map_ptr arithmetic with opcode add and offset 0 To: Yonghong Song Cc: bpf , Networking , Alexei Starovoitov , Daniel Borkmann , Kernel Team Content-Type: text/plain; charset="UTF-8" Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Fri, Sep 4, 2020 at 4:20 PM Yonghong Song wrote: > > > > On 9/4/20 1:30 PM, Andrii Nakryiko wrote: > > On Fri, Sep 4, 2020 at 12:49 PM Yonghong Song wrote: > >> > >> Commit 41c48f3a98231 ("bpf: Support access > >> to bpf map fields") added support to access map fields > >> with CORE support. For example, > >> > >> struct bpf_map { > >> __u32 max_entries; > >> } __attribute__((preserve_access_index)); > >> > >> struct bpf_array { > >> struct bpf_map map; > >> __u32 elem_size; > >> } __attribute__((preserve_access_index)); > >> > >> struct { > >> __uint(type, BPF_MAP_TYPE_ARRAY); > >> __uint(max_entries, 4); > >> __type(key, __u32); > >> __type(value, __u32); > >> } m_array SEC(".maps"); > >> > >> SEC("cgroup_skb/egress") > >> int cg_skb(void *ctx) > >> { > >> struct bpf_array *array = (struct bpf_array *)&m_array; > >> > >> /* .. array->map.max_entries .. */ > >> } > >> > >> In kernel, bpf_htab has similar structure, > >> > >> struct bpf_htab { > >> struct bpf_map map; > >> ... > >> } > >> > >> In the above cg_skb(), to access array->map.max_entries, with CORE, the clang will > >> generate two builtin's. > >> base = &m_array; > >> /* access array.map */ > >> map_addr = __builtin_preserve_struct_access_info(base, 0, 0); > >> /* access array.map.max_entries */ > >> max_entries_addr = __builtin_preserve_struct_access_info(map_addr, 0, 0); > >> max_entries = *max_entries_addr; > >> > >> In the current llvm, if two builtin's are in the same function or > >> in the same function after inlining, the compiler is smart enough to chain > >> them together and generates like below: > >> base = &m_array; > >> max_entries = *(base + reloc_offset); /* reloc_offset = 0 in this case */ > >> and we are fine. > >> > >> But if we force no inlining for one of functions in test_map_ptr() selftest, e.g., > >> check_default(), the above two __builtin_preserve_* will be in two different > >> functions. In this case, we will have code like: > >> func check_hash(): > >> reloc_offset_map = 0; > >> base = &m_array; > >> map_base = base + reloc_offset_map; > >> check_default(map_base, ...) > >> func check_default(map_base, ...): > >> max_entries = *(map_base + reloc_offset_max_entries); > >> > >> In kernel, map_ptr (CONST_PTR_TO_MAP) does not allow any arithmetic. > >> The above "map_base = base + reloc_offset_map" will trigger a verifier failure. > >> ; VERIFY(check_default(&hash->map, map)); > >> 0: (18) r7 = 0xffffb4fe8018a004 > >> 2: (b4) w1 = 110 > >> 3: (63) *(u32 *)(r7 +0) = r1 > >> R1_w=invP110 R7_w=map_value(id=0,off=4,ks=4,vs=8,imm=0) R10=fp0 > >> ; VERIFY_TYPE(BPF_MAP_TYPE_HASH, check_hash); > >> 4: (18) r1 = 0xffffb4fe8018a000 > >> 6: (b4) w2 = 1 > >> 7: (63) *(u32 *)(r1 +0) = r2 > >> R1_w=map_value(id=0,off=0,ks=4,vs=8,imm=0) R2_w=invP1 R7_w=map_value(id=0,off=4,ks=4,vs=8,imm=0) R10=fp0 > >> 8: (b7) r2 = 0 > >> 9: (18) r8 = 0xffff90bcb500c000 > >> 11: (18) r1 = 0xffff90bcb500c000 > >> 13: (0f) r1 += r2 > >> R1 pointer arithmetic on map_ptr prohibited > >> > >> To fix the issue, let us permit map_ptr + 0 arithmetic which will > >> result in exactly the same map_ptr. > >> > >> Signed-off-by: Yonghong Song > >> --- > >> kernel/bpf/verifier.c | 3 +++ > >> 1 file changed, 3 insertions(+) > >> > >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > >> index b4e9c56b8b32..92aa985e99df 100644 > >> --- a/kernel/bpf/verifier.c > >> +++ b/kernel/bpf/verifier.c > >> @@ -5317,6 +5317,9 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env, > >> dst, reg_type_str[ptr_reg->type]); > >> return -EACCES; > >> case CONST_PTR_TO_MAP: > >> + if (known && smin_val == 0 && opcode == BPF_ADD) > > > > does smin_val imply that var_off is strictly zero? if that's the case, > > can you please leave a comment stating this clearly, it's hard to tell > > if that's enough of a check. > > It should be, if register state is maintained properly, the following > function (or its functionality) should have been called. > > static void __update_reg64_bounds(struct bpf_reg_state *reg) > { > /* min signed is max(sign bit) | min(other bits) */ > reg->smin_value = max_t(s64, reg->smin_value, > reg->var_off.value | (reg->var_off.mask > & S64_MIN)); > /* max signed is min(sign bit) | max(other bits) */ > reg->smax_value = min_t(s64, reg->smax_value, > reg->var_off.value | (reg->var_off.mask > & S64_MAX)); > reg->umin_value = max(reg->umin_value, reg->var_off.value); > reg->umax_value = min(reg->umax_value, > reg->var_off.value | reg->var_off.mask); > } > > for scalar constant, reg->var_off.mask should be 0. so we will have > reg->smin_value = reg->smax_value = (s64)reg->var_off.value. > > The smin_val is also used below, e.g., BPF_ADD, for a known value. > That is why I am using smin_val here. > > Will add a comment and submit v2. it would be way-way more obvious (and reliable in the long run, probably) if you just used (known && reg->var_off.value == 0). or just tnum_equals_const(reg->var_off, 0)? > > > > >> + break; > >> + /* fall-through */ > >> case PTR_TO_PACKET_END: > >> case PTR_TO_SOCKET: > >> case PTR_TO_SOCKET_OR_NULL: > >> -- > >> 2.24.1 > >>