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Sat, 9 May 2020 06:04:19 +0000 Subject: Re: [PATCH bpf-next v3 13/21] bpf: add bpf_seq_printf and bpf_seq_write helpers To: Alexei Starovoitov , Andrii Nakryiko CC: Andrii Nakryiko , bpf , Martin KaFai Lau , Networking , Daniel Borkmann , Kernel Team References: <20200507053915.1542140-1-yhs@fb.com> <20200507053930.1544090-1-yhs@fb.com> <931b683e-ccb5-f258-f5fb-549b2daf47b3@fb.com> From: Yonghong Song Message-ID: <9993230e-ce46-ba4d-7d5c-e5f8b3b8c078@fb.com> Date: Fri, 8 May 2020 23:04:17 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-ClientProxiedBy: BYAPR08CA0026.namprd08.prod.outlook.com (2603:10b6:a03:100::39) To BYAPR15MB4088.namprd15.prod.outlook.com (2603:10b6:a02:c3::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from macbook-pro-52.local.dhcp.thefacebook.com (2620:10d:c090:400::5:a70f) by BYAPR08CA0026.namprd08.prod.outlook.com (2603:10b6:a03:100::39) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2979.26 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-MS-Exchange-CrossTenant-Network-Message-Id: 82f9af49-25e8-441f-ee0c-08d7f3decfee X-MS-Exchange-CrossTenant-OriginalArrivalTime: 09 May 2020 06:04:19.5194 (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: ca8Nzg68eT+SlcCfYPU7mAzILwMP0+u4LxsLvaCYGZFnsui8rjec02p7WDIoAYKz X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB2984 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.216,18.0.676 definitions=2020-05-09_01:2020-05-08,2020-05-09 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 phishscore=0 lowpriorityscore=0 malwarescore=0 impostorscore=0 mlxlogscore=999 spamscore=0 clxscore=1015 priorityscore=1501 bulkscore=0 adultscore=0 mlxscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2003020000 definitions=main-2005090055 X-FB-Internal: deliver Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On 5/8/20 10:30 PM, Alexei Starovoitov wrote: > On 5/8/20 9:18 PM, Yonghong Song wrote: >> >> >> On 5/8/20 12:44 PM, Andrii Nakryiko wrote: >>> On Wed, May 6, 2020 at 10:40 PM Yonghong Song wrote: >>>> >>>> Two helpers bpf_seq_printf and bpf_seq_write, are added for >>>> writing data to the seq_file buffer. >>>> >>>> bpf_seq_printf supports common format string flag/width/type >>>> fields so at least I can get identical results for >>>> netlink and ipv6_route targets. >>>> >>>> For bpf_seq_printf and bpf_seq_write, return value -EOVERFLOW >>>> specifically indicates a write failure due to overflow, which >>>> means the object will be repeated in the next bpf invocation >>>> if object collection stays the same. Note that if the object >>>> collection is changed, depending how collection traversal is >>>> done, even if the object still in the collection, it may not >>>> be visited. >>>> >>>> bpf_seq_printf may return -EBUSY meaning that internal percpu >>>> buffer for memory copy of strings or other pointees is >>>> not available. Bpf program can return 1 to indicate it >>>> wants the same object to be repeated. Right now, this should not >>>> happen on no-RT kernels since migrate_disable(), which guards >>>> bpf prog call, calls preempt_disable(). >>>> >>>> Signed-off-by: Yonghong Song >>>> --- >>>>   include/uapi/linux/bpf.h       |  32 +++++- >>>>   kernel/trace/bpf_trace.c       | 200 >>>> +++++++++++++++++++++++++++++++++ >>>>   scripts/bpf_helpers_doc.py     |   2 + >>>>   tools/include/uapi/linux/bpf.h |  32 +++++- >>>>   4 files changed, 264 insertions(+), 2 deletions(-) >>>> >>> >>> Was a bit surprised by behavior on failed memory read, I think it's >>> important to emphasize and document this. But otherwise: >>> >>> Acked-by: Andrii Nakryiko >>> >>> [...] >>> >>>> +               if (fmt[i] == 's') { >>>> +                       /* try our best to copy */ >>>> +                       if (memcpy_cnt >= MAX_SEQ_PRINTF_MAX_MEMCPY) { >>>> +                               err = -E2BIG; >>>> +                               goto out; >>>> +                       } >>>> + >>>> +                       bufs->buf[memcpy_cnt][0] = 0; >>>> +                       strncpy_from_unsafe(bufs->buf[memcpy_cnt], >>>> +                                           (void *) (long) >>>> args[fmt_cnt], >>>> +                                           MAX_SEQ_PRINTF_STR_LEN); >>> >>> So the behavior is that we try to read string, but if it fails, we >>> treat it as empty string? That needs to be documented, IMHO. My >>> expectation was that entire printf would fail. >> >> Let me return proper error. Currently, two possible errors may happen: >>    - user provide an invalid address, yes, an error should be returned >>      and we should not do anything >>    - user provide a valid address, but it needs page fault happening >>      to read the content. With current implementation, >>      strncpy_from_unsafe will return fail. Future sleepable >>      bpf program will help for this case, so an error means a >>      real address error. > > It matches what bpf_trace_printk() is doing. > I suggest to defer any improvements to later patches. > Both should be consistent. Sure. We can do that.