linux-kernel - Re: [PATCH] ksmbd: vfs: fix truncate lock-range check for shrink/grow and avoid size==0 underflow

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Message-ID: <CAKYAXd-R8NGDzQ-GTM67QbCxwJTCMGNhxKBo1a0sm0XBDqftLw@mail.gmail.com>
Date: Sat, 8 Nov 2025 23:46:41 +0900
From: Namjae Jeon <linkinjeon@...nel.org>
To: Qianchang Zhao <pioooooooooip@...il.com>
Cc: Steve French <smfrench@...il.com>, linux-cifs@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Zhitong Liu <liuzhitong1993@...il.com>
Subject: Re: [PATCH] ksmbd: vfs: fix truncate lock-range check for shrink/grow
 and avoid size==0 underflow

On Sat, Nov 8, 2025 at 9:36 PM Qianchang Zhao <pioooooooooip@...il.com> wrote:
>
> ksmbd_vfs_truncate() uses check_lock_range() with arguments that are
> incorrect for shrink, and can underflow when size==0:
>
> - For shrink, the code passed [inode->i_size, size-1], which is reversed.
> - When size==0, "size-1" underflows to -1, so the range becomes
>   [old_size, -1], effectively skipping the intended [0, old_size-1].
>
> Fix by:
> - Rejecting negative size with -EINVAL.
> - For shrink (size < old): check [size, old-1].
> - For grow   (size > old): check [old, size-1].
> - Skip lock check when size == old.
> - Keep the return value on conflict as -EAGAIN (no noisy pr_err()).
>
> This avoids the size==0 underflow and uses the correct range order,
> preserving byte-range lock semantics.
>
> Reported-by: Qianchang Zhao <pioooooooooip@...il.com>
> Reported-by: Zhitong Liu <liuzhitong1993@...il.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Qianchang Zhao <pioooooooooip@...il.com>
> ---
>  fs/smb/server/vfs.c | 28 +++++++++++++++++++---------
>  1 file changed, 19 insertions(+), 9 deletions(-)
>
> diff --git a/fs/smb/server/vfs.c b/fs/smb/server/vfs.c
> index 891ed2dc2..e7843ec9b 100644
> --- a/fs/smb/server/vfs.c
> +++ b/fs/smb/server/vfs.c
> @@ -825,17 +825,27 @@ int ksmbd_vfs_truncate(struct ksmbd_work *work,
>         if (!work->tcon->posix_extensions) {
>                 struct inode *inode = file_inode(filp);
>
> -               if (size < inode->i_size) {
> -                       err = check_lock_range(filp, size,
> -                                              inode->i_size - 1, WRITE);
> -               } else {
> -                       err = check_lock_range(filp, inode->i_size,
> -                                              size - 1, WRITE);
> +               loff_t old = i_size_read(inode);
> +               loff_t start = 0, end = -1;
> +               bool need_check = false;
> +
> +               if (size < 0)
> +                       return -EINVAL;
There is no case where size variable is negative.

> +
> +               if (size < old) {
> +                       start = size;
> +                       end   = old - 1;
> +                       need_check = true;
> +               } else if (size > old) {
> +                       start = old;
> +                       end   = size - 1;
> +                       need_check = true;
>                 }
>
> -               if (err) {
> -                       pr_err("failed due to lock\n");
> -                       return -EAGAIN;
> +               if (need_check) {
> +                       err = check_lock_range(filp, start, end, WRITE);
> +                       if (err)
> +                               return -EAGAIN;
>                 }
>         }
Can't you just change it like this?

diff --git a/fs/smb/server/vfs.c b/fs/smb/server/vfs.c
index 891ed2dc2b73..f96f27c60301 100644
--- a/fs/smb/server/vfs.c
+++ b/fs/smb/server/vfs.c
@@ -828,7 +828,7 @@ int ksmbd_vfs_truncate(struct ksmbd_work *work,
                if (size < inode->i_size) {
                        err = check_lock_range(filp, size,
                                               inode->i_size - 1, WRITE);
-               } else {
+               } else if (size > inode->i_size) {
                        err = check_lock_range(filp, inode->i_size,
                                               size - 1, WRITE);
                }

Thanks.
> --
> 2.34.1
>