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Miller" , Jakub Kicinski , Paolo Abeni , Jamal Hadi Salim , Cong Wang , Jiri Pirko , netdev@vger.kernel.org, eric.dumazet@gmail.com, syzbot , Vinicius Costa Gomes Subject: Re: [PATCH net] net/sched: taprio: fix duration_to_length() Message-ID: <20240524160718.mak4p7jan2t5qfoz@skbuf> References: <20240523134549.160106-1-edumazet@google.com> <20240524153948.57ueybbqeyb33lxj@skbuf> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: VI1PR07CA0307.eurprd07.prod.outlook.com (2603:10a6:800:130::35) To VE1PR04MB7374.eurprd04.prod.outlook.com (2603:10a6:800:1ac::11) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VE1PR04MB7374:EE_|PR3PR04MB7467:EE_ X-MS-Office365-Filtering-Correlation-Id: 3a8414b5-8fc3-43a3-53a6-08dc7c0b9745 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230031|376005|7416005|1800799015|366007; 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> > > > + return div64_u64(duration * PSEC_PER_NSEC, > > > > + atomic64_read(&q->picos_per_byte)); > > > > } > > > > > > There's a netdev_dbg() in taprio_set_picos_per_byte(). Could you turn > > > that on? I'm curious what was the q->picos_per_byte value that triggered > > > the 64-bit division fault. There are a few weird things about > > > q->picos_per_byte's representation and use as an atomic64_t (s64) type. > > > > > > No repro yet. > > > > Anything with 32 low order bits cleared would trigger a divide by 0. > > > > (1ULL << 32) picoseconds is only 4.294 ms > > BTW, just a reminder, div_u64() is a divide by a 32bit value... > > static inline u64 div_u64(u64 dividend, u32 divisor) > ... The thing is that I don't see how q->picos_per_byte could take any sane value of either 0 or a multiple of 2^32. Its formula is "(USEC_PER_SEC * 8) / speed" where "speed" is the link speed: 10, 100, 1000 etc. The special cases of speed=0 and speed=SPEED_UNKNOWN are handled by falling back to SPEED_10 in the picos_per_byte calculation. For q->picos_per_byte to be larger than 2^32, "speed" would have to be smaller than 8000000 / U32_MAX (0.001862645). For q->picos_per_byte to be exactly 0, "speed" would have to be larger than 8000000. But the largest defined speed in include/uapi/linux/ethtool.h is precisely SPEED_800000, leading to an expected q->picos_per_byte of 1.