linux-kernel - Re: [PATCH][next] afs: fix integer overflow when shifting 1 more than 32 places

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Date:   Wed, 11 Apr 2018 15:17:59 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-afs@...ts.infradead.org, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] afs: fix integer overflow when shifting 1 more than
 32 places

On 11/04/18 15:10, David Howells wrote:
> Colin Ian King <colin.king@...onical.com> wrote:
> 
>>>> -	mask = (1 << nr_slots) - 1;
>>>> +	mask = (1ULL << nr_slots) - 1;
>>>
>>> nr_slots cannot be larger than 9, so what I wrote is actually fine and is
>>> more efficient on a 32-bit machine.
>>
>> ok, sorry about the noise.
> 
> It would be possible to cast the value to u64 before assigning it, I suppose.
> Would that help?  E.g.:
> 
> 	mask = (u64)((1 << nr_slots) - 1);
> 
> It looks a bit odd, though, since the cast is made implicitly anyway.

I'm not sure that actually helps,  1 << nr_slots is still evaluating as
a 32 bit value, so that final cast does not may any difference.  Anyhow,
since nr_slots is less than 32 then keeping it the way it was is fine.

> 
> David
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