[PATCH 3.16 054/148] jbd2: Fix possible overflow in jbd2_log_space_left()

[Ben Hutchings]

3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jan Kara <jack@xxxxxxx>

commit add3efdd78b8a0478ce423bb9d4df6bd95e8b335 upstream.

When number of free space in the journal is very low, the arithmetic in
jbd2_log_space_left() could underflow resulting in very high number of
free blocks and thus triggering assertion failure in transaction commit
code complaining there's not enough space in the journal:

J_ASSERT(journal->j_free > 1);

Properly check for the low number of free blocks.

Reviewed-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
Link: https://lore.kernel.org/r/20191105164437.32602-1-jack@xxxxxxx
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 include/linux/jbd2.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1340,7 +1340,7 @@ static inline int jbd2_space_needed(jour
 static inline unsigned long jbd2_log_space_left(journal_t *journal)
 {
 	/* Allow for rounding errors */
-	unsigned long free = journal->j_free - 32;
+	long free = journal->j_free - 32;
 
 	if (journal->j_committing_transaction) {
 		unsigned long committing = atomic_read(&journal->
@@ -1349,7 +1349,7 @@ static inline unsigned long jbd2_log_spa
 		/* Transaction + control blocks */
 		free -= committing + (committing >> JBD2_CONTROL_BLOCKS_SHIFT);
 	}
-	return free;
+	return max_t(long, free, 0);
 }
 
 /*