Re: [patch] ext4: underflow in alignment check

From	Jan Kara <jack@suse.cz>
Newsgroups	linux.kernel
Subject	Re: [patch] ext4: underflow in alignment check
Date	2016-06-21 09:50 +0200
Message-ID	<rMoBz-2OV-7@gated-at.bofh.it> (permalink)
References	<rKzB7-5e9-9@gated-at.bofh.it> <rMa5z-24j-19@gated-at.bofh.it> <rMdwt-44L-27@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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@oracle.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@suse.com>
SUSE Labs, CR

Thread

[patch] ext4: underflow in alignment check Dan Carpenter <dan.carpenter@oracle.com> - 2016-06-16 09:10 +0200
  Re: [patch] ext4: underflow in alignment check Jan Kara <jack@suse.cz> - 2016-06-20 18:20 +0200
    Re: [patch] ext4: underflow in alignment check Dan Carpenter <dan.carpenter@oracle.com> - 2016-06-20 22:00 +0200
      Re: [patch] ext4: underflow in alignment check Jan Kara <jack@suse.cz> - 2016-06-21 09:50 +0200
        Re: [patch] ext4: underflow in alignment check Dan Carpenter <dan.carpenter@oracle.com> - 2016-06-21 15:10 +0200

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