Re: [PATCH 1/4] btrfs: fix 64bit division in btrfs_insert_striped_mirrored_raid_extents

Geert Uytterhoeven
2023-09-18T18:31:38Z

Hi David,

On Mon, Sep 18, 2023 at 6:31 PM David Sterba <dsterba@suse.cz> wrote:
> On Mon, Sep 18, 2023 at 03:03:10PM +0000, Johannes Thumshirn wrote:
> > On 18.09.23 16:19, Geert Uytterhoeven wrote:
> > > Hi Johannes,
> > >
> > > On Mon, Sep 18, 2023 at 4:14 PM Johannes Thumshirn
> > > <johannes.thumshirn@wdc.com> wrote:
> > >> Fix modpost error due to 64bit division on 32bit systems in
> > >> btrfs_insert_striped_mirrored_raid_extents.
> > >>
> > >> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> > >> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> > >
> > > Thanks for your patch!
> > >
> > >> --- a/fs/btrfs/raid-stripe-tree.c
> > >> +++ b/fs/btrfs/raid-stripe-tree.c
> > >> @@ -148,10 +148,10 @@ static int btrfs_insert_striped_mirrored_raid_extents(
> > >> {
> > >> struct btrfs_io_context *bioc;
> > >> struct btrfs_io_context *rbioc;
> > >> - const int nstripes = list_count_nodes(&ordered->bioc_list);
> > >> - const int index = btrfs_bg_flags_to_raid_index(map_type);
> > >> - const int substripes = btrfs_raid_array[index].sub_stripes;
> > >> - const int max_stripes = trans->fs_info->fs_devices->rw_devices / substripes;
> > >> + const size_t nstripes = list_count_nodes(&ordered->bioc_list);
> > >> + const enum btrfs_raid_types index = btrfs_bg_flags_to_raid_index(map_type);
> > >> + const u8 substripes = btrfs_raid_array[index].sub_stripes;
> > >> + const int max_stripes = div_u64(trans->fs_info->fs_devices->rw_devices, substripes);
> > >
> > > What if the quotient does not fit in a signed 32-bit value?
> >
> > Then you've bought a lot of HDDs ;-)
> >
> > Jokes aside, yes this is theoretically correct. Dave can you fix
> > max_stripes up to be u64 when applying?
>
> I think we can keep it int, or unsigned int if needed, we can't hit such
> huge values for rw_devices. The 'theoretically' would fit for a machine
> with infinite resources, otherwise the maximum number of devices I'd
> expect is a few thousand.

rw_devices and various other *_devices are u64.
Is there a good reason they are that big?
With the fs fuzzing threads in mind, is any validation done on their values?

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds