Re: 32-bit bug in iovec iterator changes

Al Viro
2014-06-22T00:54:01Z

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...