[PATCH 2/2] xfs: avoid f_bfree overflow

[Zheng Bin <zhengbin13@huawei.com>]

If fdblocks < mp->m_alloc_set_aside, statp->f_bfree will overflow.
When we df -h /mnt(xfs mount point), will show this:
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0       13M  -64Z  -32K 100% /mnt

Make sure statp->f_bfree does not underflow.
PS: add fdblocks check in mount.

Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
---
 fs/xfs/xfs_mount.c | 6 ++++++
 fs/xfs/xfs_super.c | 3 ++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index dc41801..a223af4 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -816,6 +816,12 @@ xfs_mountfs(
 	if (error)
 		goto out_log_dealloc;

+	if (sbp->sb_fdblocks < mp->m_alloc_set_aside) {
+		xfs_alert(mp, "Corruption detected. Please run xfs_repair.");
+		error = -EFSCORRUPTED;
+		goto out_log_dealloc;
+	}
+
 	/*
 	 * Get and sanity-check the root inode.
 	 * Save the pointer to it in the mount structure.
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 2094386..9dcf772 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -755,7 +755,8 @@ xfs_fs_statfs(
 	statp->f_blocks = sbp->sb_dblocks - lsize;
 	spin_unlock(&mp->m_sb_lock);

-	statp->f_bfree = fdblocks - mp->m_alloc_set_aside;
+	/* make sure statp->f_bfree does not underflow */
+	statp->f_bfree = max_t(int64_t, fdblocks - mp->m_alloc_set_aside, 0);
 	statp->f_bavail = statp->f_bfree;

 	fakeinos = XFS_FSB_TO_INO(mp, statp->f_bfree);
--
2.7.4