btrfs: avoid overflowing f_bfree

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@osg.samsung.com>
Suggested-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
(cherry picked from commit 41b34accb265e3a20211a7a8ef3625678f1c6ec7)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 052b957f909b1a0f66317cbaacc1c9972df31515..2cbd2b37fb5a13ce03f903060a5ee7ad21aeb1af 100644 (file)
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2112,7 +2112,11 @@ 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);