ext4: check mount's handling for very large s_first_meta_bg

On ext4 filesystem, the kernel carshes at mount time when
s_first_meta_bg's value exceeds the largest possible meta_bg
number.  This kernel bug has been fixed in:

3a4b77c ext4: validate s_first_meta_bg at mount time

[eguan: add comments on the first_meta_bg value]

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/ext4/025

@@ -0,0 +1,65 @@
+#! /bin/bash
+# FS QA Test ext4/025
+#
+# Regression test for commit:
+# 3a4b77c ("ext4: validate s_first_meta_bg at mount time").
+#
+#-----------------------------------------------------------------------
+# Copyright (c) 2017 Fujitsu.  All Rights Reserved.
+# Author: Xiao Yang <yangx.jy@cn.fujitsu.com>
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it would be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write the Free Software Foundation,
+# Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+#-----------------------------------------------------------------------
+
+
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+_cleanup()
+{
+	rm -f $tmp.*
+}
+
+# get standard environment and checks
+. ./common/rc
+. ./common/filter
+
+# remove previous $seqres.full before test
+rm -f $seqres.full
+
+# real QA test starts here
+_supported_fs ext4
+_supported_os Linux
+_require_scratch_nocheck
+_require_command "$DEBUGFS_PROG" debugfs
+_require_ext4_mkfs_feature "bigalloc,meta_bg,^resize_inode"
+
+echo "Create ext4 fs and modify first_meta_bg's value"
+_scratch_mkfs "-O bigalloc,meta_bg,^resize_inode" >> $seqres.full 2>&1
+
+# set a big enough first_meta_bg to trigger buffer overrun
+# 842150400 is from original fuzzed ext4 image in bug report
+$DEBUGFS_PROG -w -R "ssv first_meta_bg 842150400" $SCRATCH_DEV >> $seqres.full 2>&1
+
+echo "Try to mount a modified ext4 fs"
+_scratch_mount >> $seqres.full 2>&1 || echo "Fail to mount ext4 fs expectedly"
+
+# success, all done
+status=0
+exit

tests/ext4/025.out

@@ -0,0 +1,4 @@
+QA output created by 025
+Create ext4 fs and modify first_meta_bg's value
+Try to mount a modified ext4 fs
+Fail to mount ext4 fs expectedly

tests/ext4/group

@@ -27,6 +27,7 @@
 022 auto quick attr dangerous
 023 auto quick scrub
 024 auto quick encrypt dangerous
+025 auto quick fuzzers dangerous 
 271 auto rw quick
 301 aio auto ioctl rw stress defrag
 302 aio auto ioctl rw stress defrag