[PATCH v2 2/2] btrfs: avoid overflowing f_bfree

[Luis de Bethencourt]

Since mixed block groups accounting isn't byte-accurate and f_bree is an
unsigned integer, it could overflow. Avoid this.

Signed-off-by: Luis de Bethencourt <luisbg@xxxxxxxxxxxxxxx>
Suggested-by: David Sterba <dsterba@xxxxxxxx>
---
Hi,

Thanks to Filipe Manana for spotting a mistake in the first version of this
patch.

Luis

 fs/btrfs/super.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index bdca79c..fe03efb 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2100,7 +2100,11 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 
 	/* Account global block reserve as used, it's in logical size already */
 	spin_lock(&block_rsv->lock);
-	buf->f_bfree -= block_rsv->size >> bits;
+	/* Mixed block groups accounting is not byte-accurate, avoid overflow */
+	if (buf->f_bfree >= block_rsv->size >> bits)
+		buf->f_bfree -= block_rsv->size >> bits;
+	else
+		buf->f_bfree = 0;
 	spin_unlock(&block_rsv->lock);
 
 	buf->f_bavail = div_u64(total_free_data, factor);
-- 
2.5.3