2:27 p.m.
On Mon, Aug 24, 2020 at 09:35:59PM -0600, Andreas Dilger wrote:
On Aug 24, 2020, at 9:26 PM, Matthew Wilcox
<willy(a)infradead.org> wrote:
>
> On Tue, Aug 25, 2020 at 10:27:35AM +1000, Dave Chinner wrote:
>>> do {
>>> - unsigned offset, bytes;
>>> -
>>> - offset = offset_in_page(pos);
>>> - bytes = min_t(loff_t, PAGE_SIZE - offset, count);
>>> + loff_t bytes;
>>>
>>> if (IS_DAX(inode))
>>> - status = dax_iomap_zero(pos, offset, bytes, iomap);
>>> + bytes = dax_iomap_zero(pos, length, iomap);
>>
>> Hmmm. everything is loff_t here, but the callers are defining length
>> as u64, not loff_t. Is there a potential sign conversion problem
>> here? (sure 64 bit is way beyond anything we'll pass here, but...)
>
> I've gone back and forth on the correct type for 'length' a few times.
> size_t is too small (not for zeroing, but for seek()). An unsigned type
> seems right -- a length can't be negative, and we don't want to give
> the impression that it can. But the return value from these functions
> definitely needs to be signed so we can represent an error. So a u64
> length with an loff_t return type feels like the best solution. And
> the upper layers have to promise not to pass in a length that's more
> than 2^63-1.
The problem with allowing a u64 as the length is that it leads to the
possibility of an argument value that cannot be returned. Checking
length < 0 is not worse than checking length > 0x7ffffffffffffff,
and has the benefit of consistency with the other argument types and
signs...
I think the problem here is that we have no guaranteed 64 bit size
type. when that was the case with off_t, we created loff_t to always
represent a 64 bit offset value. However, we never created one for
the count/size that is passed alongside loff_t in many places - it
was said that "syscalls are limited to 32 bit sizes" and
"size_t is 64 bit on 64 bit platforms" and so on and so we still
don't have a clean way to pass 64 bit sizes through the IO path.
We've been living with this shitty situation for a long time now, so
perhaps it's time for us to define lsize_t for 64 bit lengths and
start using that everywhere that needs a 64 bit clean path
through the code, regardless of whether the arch is 32 or 64 bit...
Thoughts?
-Dave.
--
Dave Chinner
david(a)fromorbit.com