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

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Date:   Tue, 4 Oct 2016 14:23:51 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Colin King <colin.king@...onical.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Vinod Koul <vinod.koul@...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 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.

Yours,
Linus Walleij