xfs: ensure f_ffree returned by statfs() is non-negative

Because of delayed updates to sb_icount field in the super block, it
is possible to allocate over maxicount number of inodes.  This
causes the arithmetic to calculate a negative number of free inodes
in user commands like df or stat -f.

Since maxicount is a somewhat arbitrary number, a slight over
allocation is not critical but user commands should be displayed as
0 or greater and never go negative.  To do this the value in the
stats buffer f_ffree is capped to never go negative.

[ Modified to use max_t as per Christoph's comment. ]

Signed-off-by: Stu Brodsky <sbrodsky@sgi.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/fs/xfs/linux-2.6/xfs_super.c b/fs/xfs/linux-2.6/xfs_super.c
index 15c35b62ff14..c6b24e7c308a 100644
--- a/fs/xfs/linux-2.6/xfs_super.c
+++ b/fs/xfs/linux-2.6/xfs_super.c
@@ -1226,6 +1226,7 @@ xfs_fs_statfs(
         struct xfs_inode        *ip = XFS_I(dentry->d_inode);
         __uint64_t              fakeinos, id;
         xfs_extlen_t            lsize;
+        __int64_t               ffree;
 
         statp->f_type = XFS_SB_MAGIC;
         statp->f_namelen = MAXNAMELEN - 1;
@@ -1249,7 +1250,11 @@ xfs_fs_statfs(
                 statp->f_files = min_t(typeof(statp->f_files),
                                         statp->f_files,
                                         mp->m_maxicount);
-        statp->f_ffree = statp->f_files - (sbp->sb_icount - sbp->sb_ifree);
+
+        /* make sure statp->f_ffree does not underflow */
+        ffree = statp->f_files - (sbp->sb_icount - sbp->sb_ifree);
+        statp->f_ffree = max_t(__int64_t, ffree, 0);
+
         spin_unlock(&mp->m_sb_lock);
 
         if ((ip->i_d.di_flags & XFS_DIFLAG_PROJINHERIT) ||