Re: [PATCH] iomap: avoid avoid truncating 64-bit offset to 32 bits

[Darrick J. Wong]

On Wed, Jan 08, 2025 at 08:45:07PM -0800, Marco Nelissen wrote:
> On Wed, Jan 8, 2025 at 8:38 PM Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
> >
> > On Wed, Jan 08, 2025 at 08:11:50PM -0800, Marco Nelissen wrote:
> > > on 32-bit kernels, iomap_write_delalloc_scan() was inadvertently using a
> > > 32-bit position due to folio_next_index() returning an unsigned long.
> > > This could lead to an infinite loop when writing to an xfs filesystem.
> > >
> > > Signed-off-by: Marco Nelissen <marco.nelissen@xxxxxxxxx>
> > > ---
> > >  fs/iomap/buffered-io.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > > index 54dc27d92781..d303e6c8900c 100644
> > > --- a/fs/iomap/buffered-io.c
> > > +++ b/fs/iomap/buffered-io.c
> > > @@ -1138,7 +1138,7 @@ static void iomap_write_delalloc_scan(struct inode *inode,
> > >                               start_byte, end_byte, iomap, punch);
> > >
> > >               /* move offset to start of next folio in range */
> > > -             start_byte = folio_next_index(folio) << PAGE_SHIFT;
> > > +             start_byte = folio_pos(folio) + folio_size(folio);
> >
> > eeek.  Yeah, I guess that would happen towards the upper end of the 16T
> > range on 32-bit.
> 
> By "16T" do you mean 16 TeraByte? I'm able to reproduce the infinite loop
> with files around 4 GB.

Yes.  On 32-bit, everything between 2^32 and 16T is the upper end. :)

--D

> > I wonder if perhaps pagemap.h should have:
> >
> > static inline loff_t folio_next_pos(struct folio *folio)
> > {
> >         return folio_pos(folio) + folio_size(folio);
> > }
> >
> > But I think this is the only place in the kernel that uses this
> > construction?  So maybe not worth the fuss.
> >
> > Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>
> >
> > --D
> >
> > >               folio_unlock(folio);
> > >               folio_put(folio);
> > >       }
> > > --
> > > 2.39.5
> > >