From: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
To: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: linux-nilfs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fs/nilfs2: prevent int overflow in btree binary search
Date: Wed, 3 Apr 2024 22:08:09 +0500 [thread overview]
Message-ID: <CACzwLxhgq0rsrEi3Ye7ExrF6KMZEXeX6VeMehmZD5d3HR+eonQ@mail.gmail.com> (raw)
In-Reply-To: <CAKFNMonDO_diEo+_JXKgHMTmOPv5T30TALb-ZvsOprcwSJCtFQ@mail.gmail.com>
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
prev parent reply other threads:[~2024-04-03 17:08 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-02 18:00 [PATCH] fs/nilfs2: prevent int overflow in btree binary search Sabyrzhan Tasbolatov
2024-04-02 20:55 ` Ryusuke Konishi
2024-04-03 17:08 ` Sabyrzhan Tasbolatov [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CACzwLxhgq0rsrEi3Ye7ExrF6KMZEXeX6VeMehmZD5d3HR+eonQ@mail.gmail.com \
--to=snovitoll@gmail.com \
--cc=konishi.ryusuke@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-nilfs@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).