From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Carpenter Date: Tue, 21 Jun 2016 13:06:44 +0000 Subject: Re: [patch] ext4: underflow in alignment check Message-Id: <20160621130644.GO32247@mwanda> List-Id: References: <20160616070709.GC23129@mwanda> <20160620160204.GG6882@quack2.suse.cz> <20160620195325.GM32247@mwanda> <20160621074353.GB3750@quack2.suse.cz> In-Reply-To: <20160621074353.GB3750@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Jan Kara Cc: Theodore Ts'o , Andreas Dilger , linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org On Tue, Jun 21, 2016 at 09:43:53AM +0200, Jan Kara wrote: > 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 > > > > > > 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... Ah. Smatch wasn't able to figure out that MOPT_GTE0 was set. Thanks for reviewing this. regards, dan carpenter