Messages in this thread | | | Subject | Re: 32-bit bug in iovec iterator changes | From | James Bottomley <> | Date | Sat, 21 Jun 2014 17:32:44 -0700 |
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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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