Re: [PATCH] fs/nilfs2: prevent int overflow in btree binary search

[Sabyrzhan Tasbolatov]

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 AM Ryusuke Konishi
<konishi.ryusuke@xxxxxxxxx> wrote:
>
> On Wed, Apr 3, 2024 at 3:00 AM 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@xxxxxxxxx>
> > ---
> >  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 nilfs_btree_node *node,
> >         index = 0;
> >         s = 0;
> >         while (low <= high) {
> > -               index = (low + high) / 2;
> > +               index = low + (high - low) / 2;
> >                 nkey = nilfs_btree_node_get_key(node, index);
> >                 if (nkey == key) {
> >                         s = 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