From add3efdd78b8a0478ce423bb9d4df6bd95e8b335 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Tue, 5 Nov 2019 17:44:07 +0100
Subject: [PATCH] jbd2: Fix possible overflow in jbd2_log_space_left()
Git-commit: add3efdd78b8a0478ce423bb9d4df6bd95e8b335
Patch-mainline: v5.5-rc1
References: bsc#1163860

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>
Acked-by: Jan Kara <jack@suse.cz>

---
 include/linux/jbd2.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index 603fbc4e2f70..10e6049c0ba9 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1582,7 +1582,7 @@ 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->
@@ -1591,7 +1591,7 @@ 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);
 }
 
 /*
-- 
2.16.4