[PATCH 4.5 112/128] btrfs: avoid overflowing f_bfree

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4.5-stable review patch.  If anyone has any objections, please let me know.

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

From: Luis de Bethencourt <luisbg@xxxxxxxxxxxxxxx>

commit 41b34accb265e3a20211a7a8ef3625678f1c6ec7 upstream.

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>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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

--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2073,7 +2073,11 @@ static int btrfs_statfs(struct dentry *d
 
 	/* 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);


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