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

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Date:	Tue, 21 Jun 2016 09:43:53 +0200
From:	Jan Kara <jack@...e.cz>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Jan Kara <jack@...e.cz>, 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 20-06-16 22:53:26, Dan Carpenter wrote:
> 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.

I see, but match_int() - or more precisely match_number() returns -ERANGE
when the number is > INT_MAX, subsequently we check whether the number is <
0 (Opt_inode_readahead_blks has flag MOPT_GTE0 set) and bail out if yes. So
at the place you are modifying we are sure the number is in [0, INT_MAX].
So the condition (arg > (1 << 30)) is pointless - just defensive
programming in case we decide e.g. to upgrade the type of 'arg' to long - but
not wrong...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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