linux-kernel - Re: [patch] ext4: underflow in alignment check

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Message-ID: <20160620195325.GM32247@mwanda>
Date:	Mon, 20 Jun 2016 22:53:26 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Jan Kara <jack@...e.cz>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [patch] ext4: underflow in alignment check

On Mon, Jun 20, 2016 at 06:02:04PM +0200, Jan Kara wrote:
> On Thu 16-06-16 10:07:09, Dan Carpenter wrote:
> > My static checker complains that this can underflow if arg is negative
> > which is true.
> > 
> > Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> 
> How come? (1 << 30) fits even into 32-bit signed type. So where's the
> problem?

Bad changelog...  I was talking about a different issue.  I was casting
it to unsigned to take advantage of type promototion.  Assume we have:

int arg = 1 << 31;

(arg > (1 << 30)) // <-- this is false
(arg > (1U << 30)) // <-- this is true so there is no underflow.

regards,
dan carpenter