linux-kernel - Re: 32-bit bug in iovec iterator changes

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Message-ID: <1403397164.2177.40.camel@dabdike.int.hansenpartnership.com>
Date:	Sat, 21 Jun 2014 17:32:44 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Theodore Ts'o <tytso@....edu>, Dave Chinner <david@...morbit.com>,
	Jens Axboe <axboe@...com>, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org
Subject: Re: 32-bit bug in iovec iterator changes

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


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