linux-kernel - Re: [PATCH] dmaengine: coh901318: fix integer overflow when shifting more than 32 places

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Message-ID: <1475596893.2340.3.camel@perches.com>
Date:   Tue, 04 Oct 2016 09:01:33 -0700
From:   Joe Perches <joe@...ches.com>
To:     Vinod Koul <vinod.koul@...el.com>,
        Linus Walleij <linus.walleij@...aro.org>
Cc:     Colin King <colin.king@...onical.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, dmaengine@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dmaengine: coh901318: fix integer overflow when
 shifting more than 32 places

On Tue, 2016-10-04 at 21:06 +0530, Vinod Koul wrote:
> On Tue, Oct 04, 2016 at 02:23:51PM +0200, Linus Walleij wrote:
> > On Thu, Sep 29, 2016 at 8:06 PM, Joe Perches <joe@...ches.com> wrote:
> > > On Thu, 2016-09-29 at 18:57 +0100, Colin King wrote:
> > > > Currently U300_DMA_CHANNELS is set to 40, meaning that the shift of 1 can
> > > > be more than 32 places, which leads to a 32 bit integer overflow. Fix this
> > > > by casting 1 to a u64 (the same type as started_channels) before shifting
> > > > it.
> > > trivia:
> > > > diff --git a/drivers/dma/coh901318.c b/drivers/dma/coh901318.c
> > > []
> > > > @@ -1353,7 +1353,7 @@ static ssize_t coh901318_debugfs_read(struct file *file, char __user *buf,
> > > >       tmp += sprintf(tmp, "DMA -- enabled dma channels\n");
> > > > 
> > > >       for (i = 0; i < U300_DMA_CHANNELS; i++)
> > > > -             if (started_channels & (1 << i))
> > > > +             if (started_channels & ((u64)1 << i))
> > > Using
> > >                 if (started_channels & (1ULL << i))
> > > would be more common.
> > Even better (IMO):
> > #include <linux/bitops.h>
> > if (started_channels & BIT(i))
> > Apparently code is there to avoid the bit 31 problem, mea culpa.
> I have already applied this one, so feel free to send this as an update :)

BIT_ULL as it still needs to be u64 not unsigned long.

But if a change is really desired, please use it
consistently in the entire file and not just this
instance.