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

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

[Sabyrzhan Tasbolatov]

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 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

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

[Ryusuke Konishi]

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@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 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

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@gmail.com> 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@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 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