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From: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
Date: Wed, 3 Apr 2024 22:08:09 +0500
Message-ID: <CACzwLxhgq0rsrEi3Ye7ExrF6KMZEXeX6VeMehmZD5d3HR+eonQ@mail.gmail.com>
Subject: Re: [PATCH] fs/nilfs2: prevent int overflow in btree binary search
To: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: linux-nilfs@vger.kernel.org, linux-kernel@vger.kernel.org
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Hi Ryusuke,

> nilfs_btree_node_get_nchildren() returns a value read from a 16-bit
> wide field, so it will never exceed U16_MAX.

You're right, "high" indeed never exceeds INT_MAX as it's limited to 16-bit
in 32-bit integer. Sorry for the confusion, It landed via my grepping tool.

         Sabyrzhan Tasbolatov

On Wed, Apr 3, 2024 at 1:55=E2=80=AFAM Ryusuke Konishi
<konishi.ryusuke@gmail.com> wrote:
>
> On Wed, Apr 3, 2024 at 3:00=E2=80=AFAM Sabyrzhan Tasbolatov wrote:
> >
> > Should prevent int overflow if low + high > INT_MAX in big btree with
> > nchildren in nilfs_btree_node_lookup() binary search.
> >
> > Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
> > ---
> >  fs/nilfs2/btree.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/nilfs2/btree.c b/fs/nilfs2/btree.c
> > index 65659fa03..39ee4fe11 100644
> > --- a/fs/nilfs2/btree.c
> > +++ b/fs/nilfs2/btree.c
> > @@ -300,7 +300,7 @@ static int nilfs_btree_node_lookup(const struct nil=
fs_btree_node *node,
> >         index =3D 0;
> >         s =3D 0;
> >         while (low <=3D high) {
> > -               index =3D (low + high) / 2;
> > +               index =3D low + (high - low) / 2;
> >                 nkey =3D nilfs_btree_node_get_key(node, index);
> >                 if (nkey =3D=3D key) {
> >                         s =3D 0;
> > --
> > 2.34.1
> >
>
> Hi Sabyrzhan,
>
> Thank you for your interesting patch.
>
> In this function, the value of the variable "high" is initialized with
> "nilfs_btree_node_get_nchildren() - 1", and "low" is initialized with
> 0.
>
> nilfs_btree_node_get_nchildren() returns a value read from a 16-bit
> wide field, so it will never exceed U16_MAX.
>
> These index calculations narrow the range between "low" and "high", so
> as long as INT_MAX is 32-bit or more, it seems that the calculation of
> this intermediate value will not overflow.
>
> So while it's a good overflow avoidance technique, it doesn't seem to
> happen in practice.
>
> Am I missing something?
>
> Regards,
> Ryusuke Konishi