nilfs2: avoid undefined behavior in nilfs_cnt32_ge macro

According to the C standard 3.4.3p3, the result of signed integer overflow
is undefined.  The macro nilfs_cnt32_ge(), which compares two sequence
numbers, uses signed integer subtraction that can overflow, and therefore
the result of the calculation may differ from what is expected due to
undefined behavior in different environments.

Similar to an earlier change to the jiffies-related comparison macros in
commit 5a581b3 ("jiffies: Avoid undefined behavior from signed
overflow"), avoid this potential issue by changing the definition of the
macro to perform the subtraction as unsigned integers, then cast the
result to a signed integer for comparison.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 9ff0512 ("nilfs2: segment constructor")
Signed-off-by: Ryusuke Konishi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: konis

fs/nilfs2/segment.c

@@ -136,7 +136,7 @@ static void nilfs_dispose_list(struct the_nilfs *, struct list_head *, int);
 
 #define nilfs_cnt32_ge(a, b)   \
 	(typecheck(__u32, a) && typecheck(__u32, b) && \
-	 ((__s32)(a) - (__s32)(b) >= 0))
+	 ((__s32)((a) - (b)) >= 0))
 
 static int nilfs_prepare_segment_lock(struct super_block *sb,
 				      struct nilfs_transaction_info *ti)