Re: [PATCH v3 7/7] mm/memfd_luo: fix integer overflow in memfd_luo_preserve_folios

[Pratyush Yadav <pratyush@kernel.org>]

On Thu, Apr 02 2026, Andrew Morton wrote:

> On Thu, 02 Apr 2026 12:06:58 +0000 Pratyush Yadav <pratyush@kernel.org> wrote:
>
>> On Thu, Mar 26 2026, Chenghao Duan wrote:
>> 
>> > In memfd_luo_preserve_folios(), two variables had types that could cause
>> > silent data loss with large files:
>> >
>> > 1. 'size' was declared as 'long', truncating the 64-bit result of
>> >    i_size_read(). On 32-bit systems a 4GB file would be truncated to 0,
>> >    causing the function to return early and discard all data.
>> 
>> As Pasha said, KHO and LUO are not expected to run on 32-bit systems.
>> Plus, since i_size_read() returns loff_t, why use u64 when you can just
>> match the type and just use loff_t (which on 64-bit is long anyway)? I
>> don't get why u64 is any better than long or loff_t.
>> 
>> >
>> > 2. 'max_folios' was declared as 'unsigned int', causing overflow for
>> >    sparse files larger than 4TB. For example, a 16TB+4KB file would
>> >    calculate 0x100000001 folios but truncate to 1 when assigned to
>> >    max_folios, causing memfd_pin_folios() to pin only the first folio.
>> 
>> Using unsigned int was intentional. We pass max_folios to
>> memfd_pin_folios(), which expects an unsigned int. So this change is
>> pointless unless you go and update memfd_pin_folios() too.
>> 
>> I think making memfd_pin_folios() use unsigned long for max_folios makes
>> a lot of sense, so can you please go update that first before making
>> this change? And when you do, please match the type of the argument to
>> the type you use here instead of using u64. This can be a separate,
>> independent patch series.
>
> Thanks.  I'll drop this patch.  The preceding six patches are looking
> well-reviewed and ready to go?

Yes. The first six patches are good to go.

I think the changes in this one can be split off as a separate series
since it will be a bit more involved.

>
> Chenghao, please prepare any update for this patch against the
> preceding six.  Or against tomorrow's mm-unstable or mm-new or
> linux-next.
>

-- 
Regards,
Pratyush Yadav