Message-ID: <1403397164.2177.40.camel@dabdike.int.hansenpartnership.com>
Subject: Re: 32-bit bug in iovec iterator changes
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: "Theodore Ts'o" <tytso@mit.edu>, Dave Chinner <david@fromorbit.com>,
        Jens Axboe <axboe@fb.com>, linux-kernel@vger.kernel.org,
        linux-scsi@vger.kernel.org
Date: Sat, 21 Jun 2014 17:32:44 -0700
In-Reply-To: <20140622002618.GR18016@ZenIV.linux.org.uk>
References: <20140619153550.GA12836@thunk.org> <53A308DE.7080000@fb.com>
	 <20140619160801.GB4907@thunk.org> <20140619162144.GC4907@thunk.org>
	 <20140619223820.GN4453@dastard> <20140621035144.GA8526@thunk.org>
	 <20140621055306.GP18016@ZenIV.linux.org.uk>
	 <20140621230922.GA13188@thunk.org>
	 <20140621234913.GQ18016@ZenIV.linux.org.uk>
	 <1403395400.2592.4.camel@jarvis.lan>
	 <20140622002618.GR18016@ZenIV.linux.org.uk>
Content-Type: text/plain; charset="ISO-8859-15"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org

On Sun, 2014-06-22 at 01:26 +0100, Al Viro wrote:
> On Sat, Jun 21, 2014 at 05:03:20PM -0700, James Bottomley wrote:
> 
> > > Anyway, does the following alone fix the problem you are seeing?
> > > 
> > > diff --git a/include/linux/uio.h b/include/linux/uio.h
> > > index ddfdb53..dbb02d4 100644
> > > --- a/include/linux/uio.h
> > > +++ b/include/linux/uio.h
> > > @@ -94,7 +94,7 @@ static inline size_t iov_iter_count(struct iov_iter *i)
> > >  	return i->count;
> > >  }
> > >  
> > > -static inline void iov_iter_truncate(struct iov_iter *i, size_t count)
> > > +static inline void iov_iter_truncate(struct iov_iter *i, u64 count)
> > >  {
> > >  	if (i->count > count)
> > >  		i->count = count;
> > 
> > Al, how can that work?  i->count is size_t, which is 32 bit, so we're
> > going to get truncation errors.
> 
> No, we are not.  Look:
> 	* comparison promotes both operands to u64 here, so its result is
> accurate, no matter how large count is.  They are compared as natural
> numbers.

True ... figured this out 10 seconds after sending the email.

> 	* assignment converts count to size_t, which *would* truncate for
> values that are greater than the maximal value representable by size_t.
> But in that case it's by definition greater than i->count, so we do not
> reach that assignment at all.

OK, so what I still don't get is why isn't the compiler warning when we
truncate a u64 to a u32?  We should get that warning in your new code,
and we should have got that warning in fs/block_dev.c where it would
have pinpointed the actual problem.

James


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/