linux-ext4 - Re: [PATCH] ext4: avoid arithemetic overflow that can trigger a BUG

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Message-ID: <20180901024936.GA27277@thunk.org>
Date:   Fri, 31 Aug 2018 22:49:36 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] ext4: avoid arithemetic overflow that can trigger a BUG

On Fri, Aug 31, 2018 at 01:31:05PM -0600, Andreas Dilger wrote:
> > 	map.m_lblk = first_block;
> > -	map.m_len = last_block - first_block + 1;
> > +	len = last_block - first_block + 1;
> > +	map.m_len = (len < UINT_MAX) ? len : UINT_MAX;
> 
> Wouldn't "(len < UINT_MAX)" always be true on a 32-bit system, or is there some
> other limitation in that case (e.g. filesystem < 16TB) that prevents it from
> being an issue?  Otherwise, this should use "unsigned long long len".

first_block and last_block are both 32-bit values and defined as
unsigned long.  That's because they are logical block numbers and
should never be more than 2**32.  The fact that last_block had
overflowed was due to i_size being corrupted to being an insanely
large number.

So it's fine that len is an unsigned long, since first_block and
last_block are both unsigned long.

					- Ted