Subject: [PATCH] fs/ntfs3: fix mount failure for sparse runs in run_unpack()

From: Konstantin Komarov <[email-masked]>
Date: Fri, 17 Oct 2025 12:18:47 +0200

Some NTFS volumes failed to mount because sparse data runs were not
handled correctly during runlist unpacking. The code performed arithmetic
on the special SPARSE_LCN64 marker, leading to invalid LCN values and
mount errors.

Add an explicit check for the case described above, marking the run as
sparse without applying arithmetic.

Fixes: 736fc7bf5f68 ("fs: ntfs3: Fix integer overflow in run_unpack()")
Cc: [email-masked]
Signed-off-by: Konstantin Komarov <[email-masked]>
---
 fs/ntfs3/run.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/ntfs3/run.c b/fs/ntfs3/run.c
index 88550085f745..5df55e4adbb1 100644
--- a/fs/ntfs3/run.c
+++ b/fs/ntfs3/run.c
@@ -984,8 +984,12 @@ int run_unpack(struct runs_tree *run, struct ntfs_sb_info *sbi, CLST ino,
 			if (!dlcn)
 				return -EINVAL;
 
-			if (check_add_overflow(prev_lcn, dlcn, &lcn))
+			/* Check special combination: 0 + SPARSE_LCN64. */
+			if (!prev_lcn && dlcn == SPARSE_LCN64) {
+				lcn = SPARSE_LCN64;
+			} else if (check_add_overflow(prev_lcn, dlcn, &lcn)) {
 				return -EINVAL;
+			}
 			prev_lcn = lcn;
 		} else {
 			/* The size of 'dlcn' can't be > 8. */
-- 
2.43.0