Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <b4f66729-90ab-080a-51ec-bf435ad6199d@meta.com>
Date:   Sat, 20 May 2023 20:58:25 -0700
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0)
 Gecko/20100101 Thunderbird/102.10.1
Subject: Re:
Content-Language: en-US
To:     Ze Gao <zegao2021@gmail.com>, jolsa@kernel.org
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Hao Luo <haoluo@google.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Song Liu <song@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Yonghong Song <yhs@fb.com>, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
        kafai@fb.com, kpsingh@chromium.org, netdev@vger.kernel.org,
        paulmck@kernel.org, songliubraving@fb.com,
        Ze Gao <zegao@tencent.com>
References: <20220515203653.4039075-1-jolsa@kernel.org>
 <20230520094722.5393-1-zegao@tencent.com>
From:   Yonghong Song <yhs@meta.com>
In-Reply-To: <20230520094722.5393-1-zegao@tencent.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?OTdpdk10SENHSTFoS2NHbTRtb2ZwQys5QnVCVUZMS3RHS2ZPaVZoV21uUDV1?=
 =?utf-8?B?cGljUFF1eWUweGIzN0xodlBDdmVianFBWWlmODRaUVYzV2pTNWRVVnBUakFs?=
 =?utf-8?B?TW9qenB6OFQ3alIwVTlQS0xWYmZIWUUwK0VoQkFHMGNxbmpjYTVwbEhmZWRw?=
 =?utf-8?B?UGdvb1oyTmN1aFdOc3J2OVB4UHU4a0x3d1o2dnZRYmVLWm9IbWJNam9XNm1w?=
 =?utf-8?B?RVhJVUpmbXA3QytucGk3dzFObnFVZWVDRmIyNFYzWlV1TzZqMUVoZm5GR0pI?=
 =?utf-8?B?NlFjNGgvUjF4NUFuWS9CWkY1VTNTVytpakh5QjhmL2EvcEYzMjJUNEc5V2tI?=
 =?utf-8?B?ZmNBMnE2Sk1TMG5zM3d1bDBra3plY2FiV0k5a2hyZmZtOHVKWVd6Z3hBN1lC?=
 =?utf-8?B?NC9oR29UNkgrTFZ1Y05GU1pXblZ5aXdTMzIyYzk2UGlnd1RRUEphY04wTDAr?=
 =?utf-8?B?SkNjVTRuSVlINHIzVHAzUWR3QzFORW1RcStJamlXTWJiYS9TWTJFTktjZENN?=
 =?utf-8?B?NThOVEwvWVNPR3dNaHdldXltditrRENKUGU2Si90RTEyWnlESUtrczFXQysx?=
 =?utf-8?B?Ty8yeC9Dd2dadFBuRE9WdU4ycFJtNVJieUNTMTlRcGRBaU9TVWVpUk1LNUZi?=
 =?utf-8?B?ekFUR0ozOWw4ZzdBMTI1bEhiT2xSWGp1WVkzQWVxbUlna1h4c0Fzdi9KTXUw?=
 =?utf-8?B?SU9XR2dKcUthQmpaaUdaS1habURKelMrRVEySEkzQzVqS0tFdXozcnlmYVhL?=
 =?utf-8?B?SUhjWlI2RklGd1NnWHdsYXBkdVdESy9kVDhvWm8wbVdpQmR0UHl3ZDFneks1?=
 =?utf-8?B?bXlUcGNwNmdTTkt0N2pLNnRKM0wyNnJMdjVOUGxocWtMQXpWeDVqb0o3YytM?=
 =?utf-8?B?ZThHWjJKNjBFLzd3dXNYM2hzMGRWRDN3ZDFpNlJ5RFZLNEZnbWZFQklrZ0Y0?=
 =?utf-8?B?SlRNK2pad1gvM3FRSFd6MTN3cnFKVVZBak9leE51TDBzZFd4eVVoU1ArZnA4?=
 =?utf-8?B?NUp0TmtjeGNsMmlNbGRSVGZFVjRmbnRNajJrWXlnT2M1MjNrb2xHazNFbWVk?=
 =?utf-8?B?N21LUGYreE53bks0OG9VR0xWUllENUVBSFhSTjZmYjJJaWRkMFdiSjVsSVV4?=
 =?utf-8?B?SUFkVytYTHJ2NWlheDRRZHZoUm84VHFTeHJvMFZOZWlXMVBFNlBUcGNPcEtn?=
 =?utf-8?B?eW4xQVdMOXRzVi9vUTN4c1VGV2tMczZNay95ZFgvOUJxUGJsYXdncVdEMm9y?=
 =?utf-8?B?Z1lyalpTRFU5TDZuVXF5OFZpZ0x1YXFJQVdNZGJ5V2IvS0g3YlRNa2tHUTF5?=
 =?utf-8?B?VXZvWUNwaTVPSHlxSUZkUVJCMFlLdmZhc0RvT0k0YVJzTjEzLzRzNWdjSERW?=
 =?utf-8?B?Q3BSZk1JZWVqekttYkNRaXU4WGwwSWhsOE1PNlpoSWZtVWc5bXF6OVVSQXVs?=
 =?utf-8?B?Nmk2K0psbEE3OFdjYUFzR3E4ZVg4b1RZMmdHcXozU2YyQnFZeERjVzdZQk9U?=
 =?utf-8?B?RVdhVmd5eFpMNGdCMHZnS3lKK3h0YkVTTzN2NFdOa3M1SDhFU29UM2lvU3lC?=
 =?utf-8?B?bnRuZlNDOWdZWmowRzZLczYrVGhyTUIxVmJWalJjUStCREZldndYNngxRHZF?=
 =?utf-8?B?TnVDYi9oVzJyQ2V0bW9RWGY1MSthWlFEbFJvMm9lZlpDWHBIRjF6VzB0VFpY?=
 =?utf-8?B?WjUzZ3Vhbm9WWDAyNVhKTE1KbzNpUWdpQWcxb3Zlbjh3U3F4akcwdndMS0RY?=
 =?utf-8?B?QWtaWDZzL1JRM3NqRTNScFFqWXlkbmtuQ1Zkaml1Z1ZibE1hSXo0bTIzUXY0?=
 =?utf-8?B?SUdZTmp2U2dVc1NwWTdPbDcrMm95cmtkdUZrcHBhOE05WEJHNkZrMmtKeWhw?=
 =?utf-8?B?OU4vUEFaT1FhYWs4R3dTQjNEdEhSZDlHbUd5OHgzY0hRK0IvV292TU94ZEl2?=
 =?utf-8?B?clV4cmVENE1WWjcvdVJxNGh5dXZtSk5sVE4yZDNyb0lMcTdQanlFZGlFcGNY?=
 =?utf-8?B?dHdMR0hRSElhMmdlZ3BqekZWekVYU0JvL0RVVnF1NVZjOWtaVjBvbEhuOTlv?=
 =?utf-8?B?eFAvZkFkUmlXTmYzd2pSMDRVWUQvLzJaVHdYM1RiYUR4NmtRbk1sSm9oOCth?=
 =?utf-8?B?YVZ3YUpxU2tlVEFPWm5RMXgzVDJxbGdDdGdnemVCZVJHZVBxTG1Qdk5XNmh4?=
 =?utf-8?B?SlE9PQ==?=
X-OriginatorOrg: meta.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 1a0e4438-c3a5-4b86-aa3d-08db59afa416
X-MS-Exchange-CrossTenant-AuthSource: SN6PR1501MB2064.namprd15.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 21 May 2023 03:58:31.1530
 (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: YGw3PU7CgxNYG151VBjH0UfkHHq3hkAjhOFUnzjSfJZMdrEFREQm/xFBUnEYHZ9O
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR15MB3912
X-Proofpoint-ORIG-GUID: SYnOvrOtduPz7Lulv5jIEqEXjk1szRyy
X-Proofpoint-GUID: SYnOvrOtduPz7Lulv5jIEqEXjk1szRyy
X-Proofpoint-Virus-Version: vendor=baseguard
 engine=ICAP:2.0.254,Aquarius:18.0.957,Hydra:6.0.573,FMLib:17.11.170.22
 definitions=2023-05-21_01,2023-05-17_02,2023-02-09_01
X-Spam-Status: No, score=-4.4 required=5.0 tests=BAYES_00,DKIM_SIGNED,
        DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,NICE_REPLY_A,RCVD_IN_DNSWL_LOW,
        RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_PASS,
        T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
        lindbergh.monkeyblade.net
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


On 5/20/23 2:47 AM, Ze Gao wrote:
> 
> Hi Jiri,
> 
> Would you like to consider to add rcu_is_watching check in
> to solve this from the viewpoint of kprobe_multi_link_prog_run
> itself? And accounting of missed runs can be added as well
> to imporve observability.
> 
> Regards,
> Ze
> 
> 
> -----------------
>  From 29fd3cd713e65461325c2703cf5246a6fae5d4fe Mon Sep 17 00:00:00 2001
> From: Ze Gao <zegao@tencent.com>
> Date: Sat, 20 May 2023 17:32:05 +0800
> Subject: [PATCH] bpf: kprobe_multi runs bpf progs only when rcu_is_watching
> 
>  From the perspective of kprobe_multi_link_prog_run, any traceable
> functions can be attached while bpf progs need specical care and
> ought to be under rcu protection. To solve the likely rcu lockdep
> warns once for good, when (future) functions in idle path were
> attached accidentally, we better paying some cost to check at least
> in kernel-side, and return when rcu is not watching, which helps
> to avoid any unpredictable results.

kprobe_multi/fprobe share the same set of attachments with fentry.
Currently, fentry does not filter with !rcu_is_watching, maybe
because this is an extreme corner case. Not sure whether it is
worthwhile or not.

Maybe if you can give a concrete example (e.g., attachment point)
with current code base to show what the issue you encountered and
it will make it easier to judge whether adding !rcu_is_watching()
is necessary or not.

> 
> Signed-off-by: Ze Gao <zegao@tencent.com>
> ---
>   kernel/trace/bpf_trace.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 9a050e36dc6c..3e6ea7274765 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2622,7 +2622,7 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
>   	struct bpf_run_ctx *old_run_ctx;
>   	int err;
>   
> -	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
> +	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1 || !rcu_is_watching())) {
>   		err = 0;
>   		goto out;
>   	}