[PATCH] fs/ntfs3: Avoid UBSAN error on true_sectors_per_clst()

[Shigeru Yoshida]

syzbot reported UBSAN error as below:

[   76.901829][ T6677] ================================================================================
[   76.903908][ T6677] UBSAN: shift-out-of-bounds in fs/ntfs3/super.c:675:13
[   76.905363][ T6677] shift exponent -247 is negative

This patch avoid this error.

Link: https://syzkaller.appspot.com/bug?id=b0299c09a14aababf0f1c862dd4ebc8ab9eb0179
Fixes: a3b774342fa7 (fs/ntfs3: validate BOOT sectors_per_clusters)
Cc: Author: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Reported-by: syzbot+35b87c668935bb55e666@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Shigeru Yoshida <syoshida@xxxxxxxxxx>
---
 fs/ntfs3/super.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ntfs3/super.c b/fs/ntfs3/super.c
index 47012c9bf505..adc4f73722b7 100644
--- a/fs/ntfs3/super.c
+++ b/fs/ntfs3/super.c
@@ -672,7 +672,7 @@ static u32 true_sectors_per_clst(const struct NTFS_BOOT *boot)
 	if (boot->sectors_per_clusters <= 0x80)
 		return boot->sectors_per_clusters;
 	if (boot->sectors_per_clusters >= 0xf4) /* limit shift to 2MB max */
-		return 1U << (0 - boot->sectors_per_clusters);
+		return 1U << -(s8)boot->sectors_per_clusters;
 	return -EINVAL;
 }
 
-- 
2.37.2