jbd2: Fix possible overflow in jbd2_log_space_left()

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.

CC: stable@vger.kernel.org
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20191105164437.32602-1-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index 65407f6..0010820 100644 (file)
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1440,7 +1440,7 @@ static inline int jbd2_space_needed(journal_t *journal)
 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->
@@ -1449,7 +1449,7 @@ static inline unsigned long jbd2_log_space_left(journal_t *journal)
                /* Transaction + control blocks */
                free -= committing + (committing >> JBD2_CONTROL_BLOCKS_SHIFT);
        }
-       return free;
+       return max_t(long, free, 0);
 }
 
 /*