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

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Message-ID: <1403398834.2177.41.camel@dabdike.int.hansenpartnership.com>
Date:	Sat, 21 Jun 2014 18:00:34 -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 01:53 +0100, Al Viro wrote:
> On Sat, Jun 21, 2014 at 05:32:44PM -0700, James Bottomley wrote:
> > > 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.
> 
> In which universe?
> 
> extern void f(unsigned int);
> 
> void g(unsigned long x)
> {
> 	f(x);
> }

In the one where the code is compiled with -Wconversion ... I'm just
surprised, I thought we had this enabled.

James


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