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Fri, 9 Jul 2021 16:24:22 +0000 Subject: Re: [PATCH v4 04/10] iio: afe: rescale: reduce risk of integer overflow To: Liam Beguin , jic23@kernel.org, lars@metafoo.de, pmeerw@pmeerw.net Cc: linux-kernel@vger.kernel.org, linux-iio@vger.kernel.org, devicetree@vger.kernel.org, robh+dt@kernel.org References: <20210706160942.3181474-1-liambeguin@gmail.com> <20210706160942.3181474-5-liambeguin@gmail.com> From: Peter Rosin Organization: Axentia Technologies AB Message-ID: <13409f37-ecd4-5afb-e1ca-59f1f1f805b8@axentia.se> Date: Fri, 9 Jul 2021 18:24:20 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 In-Reply-To: <20210706160942.3181474-5-liambeguin@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: sv-SE Content-Transfer-Encoding: 7bit X-ClientProxiedBy: AM6P191CA0038.EURP191.PROD.OUTLOOK.COM (2603:10a6:209:7f::15) To DB8PR02MB5482.eurprd02.prod.outlook.com (2603:10a6:10:eb::29) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.13.3] (85.229.94.233) by AM6P191CA0038.EURP191.PROD.OUTLOOK.COM (2603:10a6:209:7f::15) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4308.20 via Frontend Transport; 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> - unsigned long long tmp; > + s64 tmp, tmp2; > + u32 factor; > int ret; > > switch (mask) { > @@ -67,8 +68,13 @@ static int rescale_read_raw(struct iio_dev *indio_dev, > } > switch (ret) { > case IIO_VAL_FRACTIONAL: > - *val *= rescale->numerator; > - *val2 *= rescale->denominator; > + tmp = (s64)*val * rescale->numerator; > + tmp2 = (s64)*val2 * rescale->denominator; > + factor = gcd(tmp, tmp2); Hi! gcd() isn't exactly free. I do not think it is suitable to call it for each and every value. So, if you really need it, then it should only be used when there is an actual overflow (or if there is a high risk if that's somehow easier). Cheers, Peter > + do_div(tmp, factor); > + *val = tmp; > + do_div(tmp2, factor); > + *val2 = tmp2; > return ret; > case IIO_VAL_INT: > *val *= rescale->numerator; >