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
