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

Andrew Morton
2026-04-02T17:58:50Z

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?

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