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

[Pasha Tatashin]

On Thu, Mar 26, 2026 at 4:48 AM Chenghao Duan <duanchenghao@xxxxxxxxxx> 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,

This is not an issue, KHO only supports 64-bit systems, but using the
correct type is a good idea anyway.

>    causing the function to return early and discard all data.
>
> 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.
>
> Fix by changing both variables to 'u64' to match the types returned
> by i_size_read() and the folio count calculations.

Strictly speaking, i_size_read() returns loff_t which is 'long long',
so s64 not u64, but anyways u64 works here.

Reviewed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>