On Sun, Jun 22, 2014 at 01:53:52AM +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);
> }
>
> is perfectly valid C, with no warnings in sight. f(1UL << 32) might
> give one, but not this...
PS: I agree that it's worth careful commenting, obviously, but before sending
it to Linus (*with* comments) I want to get a confirmation that this one-liner
actually fixes what Ted is seeing. I have reproduced it here, and that change
makes the breakage go away in my testing, but I'd like to make sure that we are
seeing the same thing. Ted?
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