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Sat, 28 Nov 2020 05:25:56 +0000 Subject: Re: [PATCH v2 bpf-next 08/13] bpf: Add instructions for atomic_[cmp]xchg To: Brendan Jackman , CC: Alexei Starovoitov , Daniel Borkmann , KP Singh , Florent Revest , , Jann Horn References: <20201127175738.1085417-1-jackmanb@google.com> <20201127175738.1085417-9-jackmanb@google.com> From: Yonghong Song Message-ID: Date: Fri, 27 Nov 2020 21:25:53 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 In-Reply-To: <20201127175738.1085417-9-jackmanb@google.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [2620:10d:c090:400::5:fca2] X-ClientProxiedBy: CO1PR15CA0100.namprd15.prod.outlook.com (2603:10b6:101:21::20) To BYAPR15MB4088.namprd15.prod.outlook.com (2603:10b6:a02:c3::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [IPv6:2620:10d:c085:21c8::102b] (2620:10d:c090:400::5:fca2) by CO1PR15CA0100.namprd15.prod.outlook.com (2603:10b6:101:21::20) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3611.20 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: =?utf-8?B?MlRJWS9qSWtsZjk2QUZkUStFMXhRMERRQjBmdk5xcXpDQnR1cXM4dzd1cGFv?= =?utf-8?B?eTFCNVh1UTA3aUloaFVITG1Uc3M2WGM1bG1Eb2JLZVIzWCs5V2NMUmpWaFND?= =?utf-8?B?NlBtQVN1elVrRUVpbjJaclFxdGgvYUIrMFhWc0Iya2hCYUUrMjVpUzMwNSs2?= =?utf-8?B?ZVVHMy9oTDcrN2JWdGJDUTlZekNPYkJaWnJOei96L2s1Q0M3Wnd6UThMbHRX?= =?utf-8?B?eURvY0d0ZGh4UDBhd283NzgyWTg4Rm54NzJpdmwzSURyVjhwNFRCYUozdGc3?= =?utf-8?B?d2MycHR4c1FVRmJaRHNVdm9EYTYvdENxRDRlZlkyd1RxRkMxci9hQ1Y1b1Z3?= =?utf-8?B?ZWx5c1Y0QmFGaWNka0dmWEsrdFI4K1ZWcS9BdktER2lRVXIwd0UyZ0hjYXp1?= =?utf-8?B?aitYQ0d6SG1vdEE4d0JrMjZuZ0tWWCtDNGE2N0ExRzdLU3Z5WE16Ynp3eUZk?= =?utf-8?B?RlVoVUY4QUU0TXdFRUtxOGxWQld0VkhFWG5EV3p1amJnZU10SjJSNk9YalJY?= =?utf-8?B?S2llenRiU3lFRlh0dDV1blFvOG9WWVh4Ny9Kb0UzbEV2WVdwYlhnb0JoTG0w?= =?utf-8?B?YUJYUUhLbnpjZ1hZOTE0enUxaVZydlFHK282bjBwTW90cllnVGljcSt5YVNN?= =?utf-8?B?aXlqU0ZFd3BDMnBNRU1tdHNCYmcwMHoyTHhGWjdRRFBNNzByT2ljZE5iUVhY?= =?utf-8?B?Kzc1Wk4wWVFWelRBWHV4Tmg0N3BzVXM1Ym80V2cxMHdOU1FBWFNpdDlvK05F?= =?utf-8?B?Z0I1U1lUblkyNTJITzRKcEM0anJMd3gvT1p4MHBvVS9wcDdVZWx5RmJ5TTZp?= =?utf-8?B?UGZzM0k1UGYwWlplMmNnUGFUYVJuVzdzajJvS2dTRWMyNW1pK0tnMFBNQktx?= =?utf-8?B?cXJQUXdJMDRFTUlHdkwrMWVLRi9zc05ZQ2g2SkJ5bzdUV1VLbW1WMzRQWWdM?= =?utf-8?B?T2JWNkM0RzBCMzdWcElPV05DQlZlVGc4Y1ZXa0hMeHNZOGRiTlh1VHRzT1Uy?= =?utf-8?B?RjAzVjdTSjJra2J4d0FYbi9kdHN1NEdoV1Uwbk5wYWJBSGdZUk1DMEdCRm9T?= =?utf-8?B?YTBGYXBnRUZvcVpYWW1ISmtMY3RlUzFsbmlicVJmNWR0OGEzK1gzRDZLRUVC?= =?utf-8?B?bjk3S3lodE9rQ3YrNkhJcXFtR1Vxekg1cmpTVndqbElZQmh3a0JTYlkwRk5T?= =?utf-8?B?aDdVaTJTSDMvdnkydUdPK0Uxd1BEV1lmVE90TFRvS3RvbXpCR0NtUFF4a0pu?= =?utf-8?B?Y3lrL1E4R0sxNkhpc2dzVTRCUXFMdWJwL2V5YXNNeCtYM1RhaE1FWklSRVFG?= =?utf-8?B?Umh0MjdjTVN2dW9ualo0VTBDY0s4ZFFCUnYzOXdNMVhnRVNDQVlFa21sdlpU?= =?utf-8?B?Q1duVU1GSDlpeFE9PQ==?= X-MS-Exchange-CrossTenant-Network-Message-Id: 7e2a6507-7d0b-4009-49a6-08d8935e14f9 X-MS-Exchange-CrossTenant-AuthSource: BYAPR15MB4088.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 28 Nov 2020 05:25:56.3684 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: VSd70gsacnRvQSq8wORZ9F4qhfEQ50UmuxGqxMgUb2AV7kfKsN++s7ny9z+Le1dD X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB2581 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.312,18.0.737 definitions=2020-11-28_02:2020-11-26,2020-11-28 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 lowpriorityscore=0 spamscore=0 mlxlogscore=999 impostorscore=0 priorityscore=1501 adultscore=0 phishscore=0 bulkscore=0 clxscore=1015 mlxscore=0 malwarescore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2011280038 X-FB-Internal: deliver Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/27/20 9:57 AM, Brendan Jackman wrote: > This adds two atomic opcodes, both of which include the BPF_FETCH > flag. XCHG without the BPF_FETCh flag would naturally encode BPF_FETCH > atomic_set. This is not supported because it would be of limited > value to userspace (it doesn't imply any barriers). CMPXCHG without > BPF_FETCH woulud be an atomic compare-and-write. We don't have such > an operation in the kernel so it isn't provided to BPF either. > > There are two significant design decisions made for the CMPXCHG > instruction: > > - To solve the issue that this operation fundamentally has 3 > operands, but we only have two register fields. Therefore the > operand we compare against (the kernel's API calls it 'old') is > hard-coded to be R0. x86 has similar design (and A64 doesn't > have this problem). > > A potential alternative might be to encode the other operand's > register number in the immediate field. > > - The kernel's atomic_cmpxchg returns the old value, while the C11 > userspace APIs return a boolean indicating the comparison > result. Which should BPF do? A64 returns the old value. x86 returns > the old value in the hard-coded register (and also sets a > flag). That means return-old-value is easier to JIT. > > Signed-off-by: Brendan Jackman > --- > arch/x86/net/bpf_jit_comp.c | 8 ++++++++ > include/linux/filter.h | 20 ++++++++++++++++++++ > include/uapi/linux/bpf.h | 4 +++- > kernel/bpf/core.c | 20 ++++++++++++++++++++ > kernel/bpf/disasm.c | 15 +++++++++++++++ > kernel/bpf/verifier.c | 19 +++++++++++++++++-- > tools/include/linux/filter.h | 20 ++++++++++++++++++++ > tools/include/uapi/linux/bpf.h | 4 +++- > 8 files changed, 106 insertions(+), 4 deletions(-) > [...] > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index cd4c03b25573..c8311cc114ec 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -3601,10 +3601,13 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn > static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_insn *insn) > { > int err; > + int load_reg; > > switch (insn->imm) { > case BPF_ADD: > case BPF_ADD | BPF_FETCH: > + case BPF_XCHG: > + case BPF_CMPXCHG: > break; > default: > verbose(env, "BPF_ATOMIC uses invalid atomic opcode %02x\n", insn->imm); > @@ -3626,6 +3629,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i > if (err) > return err; > > + if (insn->imm == BPF_CMPXCHG) { > + /* check src3 operand */ better comment about what src3 means here? > + err = check_reg_arg(env, BPF_REG_0, SRC_OP); > + if (err) > + return err; > + } > + > if (is_pointer_value(env, insn->src_reg)) { > verbose(env, "R%d leaks addr into mem\n", insn->src_reg); > return -EACCES; > @@ -3656,8 +3666,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i > if (!(insn->imm & BPF_FETCH)) > return 0; > > - /* check and record load of old value into src reg */ > - err = check_reg_arg(env, insn->src_reg, DST_OP); > + if (insn->imm == BPF_CMPXCHG) > + load_reg = BPF_REG_0; > + else > + load_reg = insn->src_reg; > + > + /* check and record load of old value */ > + err = check_reg_arg(env, load_reg, DST_OP); > if (err) > return err; > [...]