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=-17.0 required=3.0 tests=BAYES_00,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT 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 C70FEC43462 for ; Mon, 26 Apr 2021 19:30:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 94ED86135B for ; Mon, 26 Apr 2021 19:30:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240318AbhDZTau convert rfc822-to-8bit (ORCPT ); Mon, 26 Apr 2021 15:30:50 -0400 Received: from mx0b-00082601.pphosted.com ([67.231.153.30]:56156 "EHLO mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S240372AbhDZTar (ORCPT ); Mon, 26 Apr 2021 15:30:47 -0400 Received: from pps.filterd (m0001303.ppops.net [127.0.0.1]) by m0001303.ppops.net (8.16.0.43/8.16.0.43) with SMTP id 13QJRlCd000468 for ; Mon, 26 Apr 2021 12:30:04 -0700 Received: from maileast.thefacebook.com ([163.114.130.16]) by m0001303.ppops.net with ESMTP id 385x39282g-4 (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128 verify=NOT) for ; Mon, 26 Apr 2021 12:30:04 -0700 Received: from intmgw001.05.ash7.facebook.com (2620:10d:c0a8:1b::d) by mail.thefacebook.com (2620:10d:c0a8:83::4) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Mon, 26 Apr 2021 12:30:03 -0700 Received: by devbig012.ftw2.facebook.com (Postfix, from userid 137359) id 255A72ED6122; Mon, 26 Apr 2021 12:29:58 -0700 (PDT) From: Andrii Nakryiko To: , , , CC: , , Lorenz Bauer Subject: [PATCH v2 bpf-next 3/5] selftests/bpf: fix BPF_CORE_READ_BITFIELD() macro Date: Mon, 26 Apr 2021 12:29:47 -0700 Message-ID: <20210426192949.416837-4-andrii@kernel.org> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20210426192949.416837-1-andrii@kernel.org> References: <20210426192949.416837-1-andrii@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8BIT X-FB-Internal: Safe Content-Type: text/plain X-Proofpoint-ORIG-GUID: nM3cjton4JvZsR2WnV9TyTjAFRn2lIjQ X-Proofpoint-GUID: nM3cjton4JvZsR2WnV9TyTjAFRn2lIjQ X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.391,18.0.761 definitions=2021-04-26_09:2021-04-26,2021-04-26 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 adultscore=0 mlxscore=0 phishscore=0 suspectscore=0 bulkscore=0 spamscore=0 priorityscore=1501 lowpriorityscore=0 clxscore=1015 impostorscore=0 mlxlogscore=999 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2104060000 definitions=main-2104260149 X-FB-Internal: deliver Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org Fix BPF_CORE_READ_BITFIELD() macro used for reading CO-RE-relocatable bitfields. Missing breaks in a switch caused 8-byte reads always. This can confuse libbpf because it does strict checks that memory load size corresponds to the original size of the field, which in this case quite often would be wrong. After fixing that, we run into another problem, which quite subtle, so worth documenting here. The issue is in Clang optimization and CO-RE relocation interactions. Without that asm volatile construct (also known as barrier_var()), Clang will re-order BYTE_OFFSET and BYTE_SIZE relocations and will apply BYTE_OFFSET 4 times for each switch case arm. This will result in the same error from libbpf about mismatch of memory load size and original field size. I.e., if we were reading u32, we'd still have *(u8 *), *(u16 *), *(u32 *), and *(u64 *) memory loads, three of which will fail. Using barrier_var() forces Clang to apply BYTE_OFFSET relocation first (and once) to calculate p, after which value of p is used without relocation in each of switch case arms, doing appropiately-sized memory load. Here's the list of relevant relocations and pieces of generated BPF code before and after this patch for test_core_reloc_bitfields_direct selftests. BEFORE ===== #45: core_reloc: insn #160 --> [5] + 0:5: byte_sz --> struct core_reloc_bitfields.u32 #46: core_reloc: insn #167 --> [5] + 0:5: byte_off --> struct core_reloc_bitfields.u32 #47: core_reloc: insn #174 --> [5] + 0:5: byte_off --> struct core_reloc_bitfields.u32 #48: core_reloc: insn #178 --> [5] + 0:5: byte_off --> struct core_reloc_bitfields.u32 #49: core_reloc: insn #182 --> [5] + 0:5: byte_off --> struct core_reloc_bitfields.u32 157: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0 ll 159: 7b 12 20 01 00 00 00 00 *(u64 *)(r2 + 288) = r1 160: b7 02 00 00 04 00 00 00 r2 = 4 ; BYTE_SIZE relocation here ^^^ 161: 66 02 07 00 03 00 00 00 if w2 s> 3 goto +7 162: 16 02 0d 00 01 00 00 00 if w2 == 1 goto +13 163: 16 02 01 00 02 00 00 00 if w2 == 2 goto +1 164: 05 00 12 00 00 00 00 00 goto +18 0000000000000528 : 165: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll 167: 69 11 08 00 00 00 00 00 r1 = *(u16 *)(r1 + 8) ; BYTE_OFFSET relo here w/ WRONG size ^^^^^^^^^^^^^^^^ 168: 05 00 0e 00 00 00 00 00 goto +14 0000000000000548 : 169: 16 02 0a 00 04 00 00 00 if w2 == 4 goto +10 170: 16 02 01 00 08 00 00 00 if w2 == 8 goto +1 171: 05 00 0b 00 00 00 00 00 goto +11 0000000000000560 : 172: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll 174: 79 11 08 00 00 00 00 00 r1 = *(u64 *)(r1 + 8) ; BYTE_OFFSET relo here w/ WRONG size ^^^^^^^^^^^^^^^^ 175: 05 00 07 00 00 00 00 00 goto +7 0000000000000580 : 176: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll 178: 71 11 08 00 00 00 00 00 r1 = *(u8 *)(r1 + 8) ; BYTE_OFFSET relo here w/ WRONG size ^^^^^^^^^^^^^^^^ 179: 05 00 03 00 00 00 00 00 goto +3 00000000000005a0 : 180: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll 182: 61 11 08 00 00 00 00 00 r1 = *(u32 *)(r1 + 8) ; BYTE_OFFSET relo here w/ RIGHT size ^^^^^^^^^^^^^^^^ 00000000000005b8 : 183: 67 01 00 00 20 00 00 00 r1 <<= 32 184: b7 02 00 00 00 00 00 00 r2 = 0 185: 16 02 02 00 00 00 00 00 if w2 == 0 goto +2 186: c7 01 00 00 20 00 00 00 r1 s>>= 32 187: 05 00 01 00 00 00 00 00 goto +1 00000000000005e0 : 188: 77 01 00 00 20 00 00 00 r1 >>= 32 AFTER ===== #30: core_reloc: insn #132 --> [5] + 0:5: byte_off --> struct core_reloc_bitfields.u32 #31: core_reloc: insn #134 --> [5] + 0:5: byte_sz --> struct core_reloc_bitfields.u32 129: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0 ll 131: 7b 12 20 01 00 00 00 00 *(u64 *)(r2 + 288) = r1 132: b7 01 00 00 08 00 00 00 r1 = 8 ; BYTE_OFFSET relo here ^^^ ; no size check for non-memory dereferencing instructions 133: 0f 12 00 00 00 00 00 00 r2 += r1 134: b7 03 00 00 04 00 00 00 r3 = 4 ; BYTE_SIZE relocation here ^^^ 135: 66 03 05 00 03 00 00 00 if w3 s> 3 goto +5 136: 16 03 09 00 01 00 00 00 if w3 == 1 goto +9 137: 16 03 01 00 02 00 00 00 if w3 == 2 goto +1 138: 05 00 0a 00 00 00 00 00 goto +10 0000000000000458 : 139: 69 21 00 00 00 00 00 00 r1 = *(u16 *)(r2 + 0) ; NO CO-RE relocation here ^^^^^^^^^^^^^^^^ 140: 05 00 08 00 00 00 00 00 goto +8 0000000000000468 : 141: 16 03 06 00 04 00 00 00 if w3 == 4 goto +6 142: 16 03 01 00 08 00 00 00 if w3 == 8 goto +1 143: 05 00 05 00 00 00 00 00 goto +5 0000000000000480 : 144: 79 21 00 00 00 00 00 00 r1 = *(u64 *)(r2 + 0) ; NO CO-RE relocation here ^^^^^^^^^^^^^^^^ 145: 05 00 03 00 00 00 00 00 goto +3 0000000000000490 : 146: 71 21 00 00 00 00 00 00 r1 = *(u8 *)(r2 + 0) ; NO CO-RE relocation here ^^^^^^^^^^^^^^^^ 147: 05 00 01 00 00 00 00 00 goto +1 00000000000004a0 : 148: 61 21 00 00 00 00 00 00 r1 = *(u32 *)(r2 + 0) ; NO CO-RE relocation here ^^^^^^^^^^^^^^^^ 00000000000004a8 : 149: 67 01 00 00 20 00 00 00 r1 <<= 32 150: b7 02 00 00 00 00 00 00 r2 = 0 151: 16 02 02 00 00 00 00 00 if w2 == 0 goto +2 152: c7 01 00 00 20 00 00 00 r1 s>>= 32 153: 05 00 01 00 00 00 00 00 goto +1 00000000000004d0 : 154: 77 01 00 00 20 00 00 00 r1 >>= 323 Acked-by: Lorenz Bauer Fixes: ee26dade0e3b ("libbpf: Add support for relocatable bitfields") Signed-off-by: Andrii Nakryiko --- tools/lib/bpf/bpf_core_read.h | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h index 53b3e199fb25..09ebe3db5f2f 100644 --- a/tools/lib/bpf/bpf_core_read.h +++ b/tools/lib/bpf/bpf_core_read.h @@ -88,11 +88,19 @@ enum bpf_enum_value_kind { const void *p = (const void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \ unsigned long long val; \ \ + /* This is a so-called barrier_var() operation that makes specified \ + * variable "a black box" for optimizing compiler. \ + * It forces compiler to perform BYTE_OFFSET relocation on p and use \ + * its calculated value in the switch below, instead of applying \ + * the same relocation 4 times for each individual memory load. \ + */ \ + asm volatile("" : "=r"(p) : "0"(p)); \ + \ switch (__CORE_RELO(s, field, BYTE_SIZE)) { \ - case 1: val = *(const unsigned char *)p; \ - case 2: val = *(const unsigned short *)p; \ - case 4: val = *(const unsigned int *)p; \ - case 8: val = *(const unsigned long long *)p; \ + case 1: val = *(const unsigned char *)p; break; \ + case 2: val = *(const unsigned short *)p; break; \ + case 4: val = *(const unsigned int *)p; break; \ + case 8: val = *(const unsigned long long *)p; break; \ } \ val <<= __CORE_RELO(s, field, LSHIFT_U64); \ if (__CORE_RELO(s, field, SIGNED)) \ -- 2.30.2