Re: [PATCH 06/20] staging/lustre: fix comparison between signed and unsigned - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Oleg Drokin <green@linuxhacker.ru>
Cc: Dan Carpenter <dan.carpenter@oracle.com>,
	devel@driverdev.osuosl.org,
	Dmitry Eremin <dmitry.eremin@intel.com>,
	Andreas Dilger <andreas.dilger@intel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 06/20] staging/lustre: fix comparison between signed and unsigned
Date: Mon, 2 Feb 2015 12:51:43 -0800	[thread overview]
Message-ID: <20150202205143.GA6369@kroah.com> (raw)
In-Reply-To: <210DE60C-B152-4DF5-B6FC-06CD47CD36B9@linuxhacker.ru>

On Mon, Feb 02, 2015 at 03:25:58PM -0500, Oleg Drokin wrote:
> Hello!
> 
> On Feb 2, 2015, at 10:44 AM, Greg Kroah-Hartman wrote:
> 
> > On Mon, Feb 02, 2015 at 04:02:31PM +0300, Dan Carpenter wrote:
> >> On Sun, Feb 01, 2015 at 09:52:05PM -0500, green@linuxhacker.ru wrote:
> >>> From: Dmitry Eremin <dmitry.eremin@intel.com>
> >>> 
> >>> Expression if (size != (ssize_t)size) is always false.
> >>> Therefore no bounds check errors detected.
> >> 
> >> The original code actually worked as designed.  The integer overflow
> >> could only happen on 32 bit systems and the test only was true for 32
> >> bit systems.
> 
> Hm, indeed.
> Originally I fell into the trap thinking we are trying to protect against
> negative results here too. But in fact callers all check for the return
> to be negative as an error sign. Not to mention that we cannot overflow
> 64bit integer here as explained by the comment just 2 lines above the
> default patch context.
> 
> >> 
> >>> -	if (size != (ssize_t)size)
> >>> +	if (size > ~((size_t)0)>>1)
> >>> 		return -1;
> >> 
> >> The problem is that the code was unclear.  I think the new code is even
> >> more complicated to look at.
> > I agree, I don't even understand what the new code is doing.
> 
> Sorry, this patch indeed should be dropped.
> 
> > What is this code supposed to be protecting from?  And -1?  That should
> > never be a return value…
> 
> Why is -1 a bad return value if all callsites check for that as an
> indication of error?

Because you should use "real" error values, don't make them up with
random negative numbers that mean nothing.

> (granted there's only one caller at this point in kernel space:
> lustre/llite/dir.c::ll_dir_ioctl()
>                 totalsize = hur_len(hur);
>                 OBD_FREE_PTR(hur);
>                 if (totalsize < 0)
>                         return -E2BIG;
> )

Shouldn't you have returned the error that hur_len() passed you?

thanks,

greg k-h