linux-kernel - [PATCH v2] ksmbd: vfs: skip lock-range check on equal size to avoid size==0 underflow

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Message-Id: <20251108155712.384021-1-pioooooooooip@gmail.com>
Date: Sun,  9 Nov 2025 00:57:12 +0900
From: Qianchang Zhao <pioooooooooip@...il.com>
To: Namjae Jeon <linkinjeon@...nel.org>,
	Steve French <smfrench@...il.com>
Cc: gregkh@...uxfoundation.org,
	linux-cifs@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	security@...nel.org,
	Zhitong Liu <liuzhitong1993@...il.com>,
	Qianchang Zhao <pioooooooooip@...il.com>,
	stable@...r.kernel.org
Subject: [PATCH v2] ksmbd: vfs: skip lock-range check on equal size to avoid size==0 underflow

When size equals the current i_size (including 0), the code used to call
check_lock_range(filp, i_size, size - 1, WRITE), which computes `size - 1`
and can underflow for size==0. Skip the equal case.

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 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/smb/server/vfs.c b/fs/smb/server/vfs.c
index 891ed2dc2..d068b78a3 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);
 		}
-- 
2.34.1