[PATCH] ntfs: fix undefined behavior in mft/index record size calculation

The boot sector validation allows clusters_per_mft_record and
clusters_per_index_record to range from 0xE1 (-31) to 0xF7 (-9) when
interpreted as signed values. When these are used as negative shift
counts in expressions like `1 << -clusters_per_mft_record`, values
like 0xE1 cause `1 << 31`, which shifts into the sign bit of a 32-bit
signed integer, resulting in undefined behavior.

Fix by using unsigned shift (1U << ...) instead of signed shift.
This prevents undefined behavior while preserving the full valid
range of negative values (-31 to -9) that may appear in NTFS boot
sectors.

The encoding scheme uses negative values to represent record sizes
smaller than cluster_size: -log2(record_size). Common values include
-10 (1024 bytes) for mft_record_size and -12 (4096 bytes) for
index_record_size.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: [email protected]
Signed-off-by: Hongling Zeng <[email protected]>
---
 fs/ntfs/super.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ntfs/super.c b/fs/ntfs/super.c
index 30481e5d5dd4..a1813093222b 100644
--- a/fs/ntfs/super.c
+++ b/fs/ntfs/super.c
@@ -695,7 +695,7 @@ static bool parse_ntfs_boot_sector(struct ntfs_volume *vol,
 		 * = -log2(mft_record_size) bytes. mft_record_size normaly is
 		 * 1024 bytes, which is encoded as 0xF6 (-10 in decimal).
 		 */
-		vol->mft_record_size = 1 << -clusters_per_mft_record;
+		vol->mft_record_size = 1U << -clusters_per_mft_record;
 	vol->mft_record_size_mask = vol->mft_record_size - 1;
 	vol->mft_record_size_bits = ffs(vol->mft_record_size) - 1;
 	ntfs_debug("vol->mft_record_size = %i (0x%x)", vol->mft_record_size,
@@ -732,7 +732,7 @@ static bool parse_ntfs_boot_sector(struct ntfs_volume *vol,
 		 * index_record_size normaly equals 4096 bytes, which is
 		 * encoded as 0xF4 (-12 in decimal).
 		 */
-		vol->index_record_size = 1 << -clusters_per_index_record;
+		vol->index_record_size = 1U << -clusters_per_index_record;
 	vol->index_record_size_mask = vol->index_record_size - 1;
 	vol->index_record_size_bits = ffs(vol->index_record_size) - 1;
 	ntfs_debug("vol->index_record_size = %i (0x%x)",
-- 
2.25.1