linux-kernel - RE: [PATCH v3 3/4] drm/mediatek: Add casting before assign

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Date:   Mon, 17 Jul 2023 13:17:34 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jason-JH Lin (林睿祥)' 
        <Jason-JH.Lin@...iatek.com>,
        CK Hu (胡俊光) <ck.hu@...iatek.com>,
        "amergnat@...libre.com" <amergnat@...libre.com>,
        "chunkuang.hu@...nel.org" <chunkuang.hu@...nel.org>,
        "angelogioacchino.delregno@...labora.com" 
        <angelogioacchino.delregno@...labora.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        Singo Chang (張興國) 
        <Singo.Chang@...iatek.com>,
        Johnson Wang (王聖鑫) 
        <Johnson.Wang@...iatek.com>,
        Jason-ch Chen (陳建豪) 
        <Jason-ch.Chen@...iatek.com>,
        Shawn Sung (宋孝謙) 
        <Shawn.Sung@...iatek.com>,
        Nancy Lin (林欣螢) <Nancy.Lin@...iatek.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Project_Global_Chrome_Upstream_Group 
        <Project_Global_Chrome_Upstream_Group@...iatek.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>
Subject: RE: [PATCH v3 3/4] drm/mediatek: Add casting before assign

From: Jason-JH Lin
> Sent: 14 July 2023 07:46
> 
> Hi CK,
> 
> Thanks for the reviews.
> 
> On Fri, 2023-07-14 at 05:45 +0000, CK Hu (胡俊光) wrote:
> > Hi, Jason:
> >
> > On Wed, 2023-06-21 at 18:22 +0800, Jason-JH.Lin wrote:
> > > 1. Add casting before assign to avoid the unintentional integer
> > >    overflow or unintended sign extension.
> > > 2. Add a int varriable for multiplier calculation instead of
> > > calculating
> > >    different types multiplier with dma_addr_t varriable directly.
> >
> > I agree with these modification, but the title does not match the
> > modification.
> >
> > Regards,
> > CK
> 
> I'll change the title and commit msg at the next version below:
> 
> Fix unintentional integer overflow in multiplying different types
> 
> 1. Instead of multiplying 2 variable of different types. Change to
> assign a value of one variable and then multiply the other variable.
> 
> 2. Add a int variable for multiplier calculation instead of calculating
> different types multiplier with dma_addr_t variable directly.

I'm pretty sure the patch makes absolutely no difference.
In C all arithmetic is done with char/short (inc. unsigned)
promoted to int.

So the only likely overflow is if the values exceed 2^31.
Since the temporaries you are using are 'int' this isn't true.

	David

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