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

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Message-ID: <20140622005352.GS18016@ZenIV.linux.org.uk>
Date:	Sun, 22 Jun 2014 01:53:52 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
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 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);
}

is perfectly valid C, with no warnings in sight.  f(1UL << 32) might
give one, but not this...
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