xfs: don't over-report free space or inodes in statvfs

[ Upstream commit 4b8d867ca6e2fc6d152f629fdaf027053b81765a ]

Emmanual Florac reports a strange occurrence when project quota limits
are enabled, free space is lower than the remaining quota, and someone
runs statvfs:

  # mkfs.xfs -f /dev/sda
  # mount /dev/sda /mnt -o prjquota
  # xfs_quota  -x -c 'limit -p bhard=2G 55' /mnt
  # mkdir /mnt/dir
  # xfs_io -c 'chproj 55' -c 'chattr +P' -c 'stat -vvvv' /mnt/dir
  # fallocate -l 19g /mnt/a
  # df /mnt /mnt/dir
  Filesystem      Size  Used Avail Use% Mounted on
  /dev/sda         20G   20G  345M  99% /mnt
  /dev/sda        2.0G     0  2.0G   0% /mnt

I think the bug here is that xfs_fill_statvfs_from_dquot unconditionally
assigns to f_bfree without checking that the filesystem has enough free
space to fill the remaining project quota.  However, this is a
longstanding behavior of xfs so it's unclear what to do here.

Cc: <stable@vger.kernel.org> # v2.6.18
Fixes: 932f2c32 ("[XFS] statvfs component of directory/project quota support, code originally by Glen.")
Reported-by: Emmanuel Florac <eflorac@intellique.com>
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/xfs/xfs_qm_bhv.c

@@ -32,21 +32,28 @@ xfs_fill_statvfs_from_dquot(
 	limit = blkres->softlimit ?
 		blkres->softlimit :
 		blkres->hardlimit;
-	if (limit && statp->f_blocks > limit) {
-		statp->f_blocks = limit;
-		statp->f_bfree = statp->f_bavail =
-			(statp->f_blocks > blkres->reserved) ?
-			 (statp->f_blocks - blkres->reserved) : 0;
+	if (limit) {
+		uint64_t	remaining = 0;
+
+		if (limit > blkres->reserved)
+			remaining = limit - blkres->reserved;
+
+		statp->f_blocks = min(statp->f_blocks, limit);
+		statp->f_bfree = min(statp->f_bfree, remaining);
+		statp->f_bavail = min(statp->f_bavail, remaining);
 	}
 
 	limit = dqp->q_ino.softlimit ?
 		dqp->q_ino.softlimit :
 		dqp->q_ino.hardlimit;
-	if (limit && statp->f_files > limit) {
-		statp->f_files = limit;
-		statp->f_ffree =
-			(statp->f_files > dqp->q_ino.reserved) ?
-			 (statp->f_files - dqp->q_ino.reserved) : 0;
+	if (limit) {
+		uint64_t	remaining = 0;
+
+		if (limit > dqp->q_ino.reserved)
+			remaining = limit - dqp->q_ino.reserved;
+
+		statp->f_files = min(statp->f_files, limit);
+		statp->f_ffree = min(statp->f_ffree, remaining);
 	}
 }