linux-kernel - Re: [PATCH 1/4] btrfs: fix 64bit division in btrfs_insert_striped_mirrored_raid

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Message-ID: <CAMuHMdV_rxSsyURvy_57JX2W1ias0_fuMTE6MpNs7qWaCqibkQ@mail.gmail.com>
Date:   Mon, 18 Sep 2023 20:31:19 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     dsterba@...e.cz
Cc:     Johannes Thumshirn <Johannes.Thumshirn@....com>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, Qu Wenru <wqu@...e.com>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] btrfs: fix 64bit division in btrfs_insert_striped_mirrored_raid_extents

Hi David,

On Mon, Sep 18, 2023 at 6:31 PM David Sterba <dsterba@...e.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@....com> wrote:
> > >> Fix modpost error due to 64bit division on 32bit systems in
> > >> btrfs_insert_striped_mirrored_raid_extents.
> > >>
> > >> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> > >> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@....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@...ux-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