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

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Message-ID: <20180411155811.2qznymjhi6zlzgbb@mwanda>
Date:   Wed, 11 Apr 2018 18:58:12 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Colin Ian King <colin.king@...onical.com>,
        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 Wed, Apr 11, 2018 at 03:10:16PM +0100, 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.

My feeling is that makes it worse.  It would introduce a secret,
unpublished static checker warning on my build and it doesn't help me as
a reviewer.

Ideally static analyzers should know that nr_slots is 0-9, but right now
that seems pretty tricky to figure out...

regards,
dan carpenter