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Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.14.0 Subject: Re: [PATCH] USB: cdc-acm: expose serial close_delay and closing_wait in sysfs To: Simon Arlott , Greg Kroah-Hartman Cc: Oliver Neukum , Jiri Slaby , linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org, linux-serial@vger.kernel.org References: <2023082403-masculine-scuttle-f0ad@gregkh> Content-Language: en-US From: Oliver Neukum In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR3P281CA0127.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:94::13) To VI1PR04MB7104.eurprd04.prod.outlook.com (2603:10a6:800:126::9) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VI1PR04MB7104:EE_|GV1PR04MB9119:EE_ X-MS-Office365-Filtering-Correlation-Id: f2cab362-fb79-4ffe-83d7-08dba4fc50e3 X-LD-Processed: f7a17af6-1c5c-4a36-aa8b-f5be247aa4ba,ExtFwd X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:VI1PR04MB7104.eurprd04.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230031)(366004)(376002)(39860400002)(396003)(136003)(346002)(186009)(1800799009)(451199024)(2616005)(5660300002)(4326008)(8676002)(8936002)(36756003)(83380400001)(6666004)(38100700002)(66556008)(66946007)(66476007)(54906003)(316002)(110136005)(478600001)(31686004)(53546011)(41300700001)(6512007)(6506007)(2906002)(86362001)(6486002)(31696002)(45980500001)(43740500002);DIR:OUT;SFP:1101; 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Instead of a > sysfs attribute (that udev has built-in support for writing) it would > require a separate compiled process or other non-trivial dependencies > (e.g. Python) to modify the closing_wait value. There's no shell script > support for read-modify-write of a complex ioctl struct. That, however, is a deficiency of udev. > The ioctl can't be used without opening and closing the tty, which has > side effects. On open() it'll raise DTR/RTS and activate it. For cdc-acm > that will indicate to the device that the serial port has been opened > which will be visible to the software running on the USB device. On > close() it'll be delayed by the close_delay if any process is currently > doing a blocking open() and there's no carrier, then the closing_wait > time if there's been any incomplete transmitted data (by any process). And that is an issue of the generic serial layer. > Those two values require CAP_SYS_ADMIN to modify (which is separately > enforced by many of the tty drivers) so user applications can't change > them even if they're aware of them. That is even more damning. Either something is protected by a capability or it is not. Such a protection must not be circumvented. Regards Oliver