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Fri, 18 Jul 2025 23:48:07 +0000 Message-ID: <99baa18c-ae1c-47e1-8bbe-e411570df8f1@intel.com> Date: Fri, 18 Jul 2025 16:48:06 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86/fpu: Fix potential NULL dereference in avx512_status() Content-Language: en-US To: Fushuai Wang CC: , , , , , , , , , , , , , , , References: <89987231-37ce-4d49-a1d7-6e699e8ab0d2@intel.com> <20250718071250.36019-1-wangfushuai@baidu.com> From: Sohil Mehta In-Reply-To: <20250718071250.36019-1-wangfushuai@baidu.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BY3PR10CA0026.namprd10.prod.outlook.com (2603:10b6:a03:255::31) To BYAPR11MB3320.namprd11.prod.outlook.com (2603:10b6:a03:18::25) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BYAPR11MB3320:EE_|SA3PR11MB7625:EE_ X-MS-Office365-Filtering-Correlation-Id: 1567eab8-d080-4e15-fc11-08ddc6558bb9 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|7416014; 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If not, then we can do what you currently have but without the >> CONFIG_X86_DEBUG_FPU restriction. All kernel threads would always print >> the AVX512_elapsed_ms as -1. >> Let's go with this approach. See below. >> However, this would be a user visible change so we should probably get >> more inputs. I tried this experiment on an older kernel without the >> above issue. Among all the active kthreads on my system a handful of >> them show a valid value for AVX512 usage. The rest of them all show -1. >> >> PID: 2594 >> CMD: avahi-daemon: running [SAP.local] >> /proc/2594/arch_status content: >> AVX512_elapsed_ms: 46032 >> >> PID: 2729 >> CMD: sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups >> /proc/2729/arch_status content: >> AVX512_elapsed_ms: 396656 >> Correction: These aren't really Kthreads. There was a slight error in the script that I used. Reporting AVX512 usage doesn't seem very useful for Kthreads. The usage is mainly for userspace schedulers. Let's just report -1 for all Kthreads. How about something like below. This should work with and without CONFIG_X86_DEBUG_FPU. diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c index 9aa9ac8399ae..10c3994295f2 100644 --- a/arch/x86/kernel/fpu/xstate.c +++ b/arch/x86/kernel/fpu/xstate.c @@ -1855,19 +1855,18 @@ long fpu_xstate_prctl(int option, unsigned long arg2) #ifdef CONFIG_PROC_PID_ARCH_STATUS /* * Report the amount of time elapsed in millisecond since last AVX512 - * use in the task. + * use in the task. Report -1 if no AVX512 usage. */ static void avx512_status(struct seq_file *m, struct task_struct *task) { - unsigned long timestamp = READ_ONCE(x86_task_fpu(task)->avx512_timestamp); - long delta; + unsigned long timestamp = 0; + long delta = -1; - if (!timestamp) { - /* - * Report -1 if no AVX512 usage - */ - delta = -1; - } else { + /* Do not report AVX512 usage for kernel threads */ + if (!(task->flags & (PF_KTHREAD | PF_USER_WORKER))) + timestamp = READ_ONCE(x86_task_fpu(task)->avx512_timestamp); + + if (timestamp) { delta = (long)(jiffies - timestamp); /* * Cap to LONG_MAX if time difference > LONG_MAX