kernel: ext4: fix check to prevent initializing reserved inodes

The broken check would detect a newly generated root filesystem as corrupt
under certain circumstances, in some cases actually currupting the it while
trying to handle the error condition.

This is a regression introduced in kernel 4.4.140. The 4.14.y stable series
has already received this fix, while it is still pending for 4.4.y and
4.9.y.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

target/linux/generic/patches-4.4/005-ext4-fix-check-to-prevent-initializing-reserved-inod.patch

@@ -0,0 +1,68 @@
+From 7cb53d379df849d843cfd658f351b79b41b09051 Mon Sep 17 00:00:00 2001
+Message-Id: <7cb53d379df849d843cfd658f351b79b41b09051.1533837011.git.mschiffer@universe-factory.net>
+From: Theodore Ts'o <tytso@mit.edu>
+Date: Sat, 28 Jul 2018 08:12:04 -0400
+Subject: [PATCH] ext4: fix check to prevent initializing reserved inodes
+
+Commit 5012284700775a4e6e3fbe7eac4c543c4874b559 upstream.
+
+Commit 8844618d8aa7: "ext4: only look at the bg_flags field if it is
+valid" will complain if block group zero does not have the
+EXT4_BG_INODE_ZEROED flag set.  Unfortunately, this is not correct,
+since a freshly created file system has this flag cleared.  It gets
+almost immediately after the file system is mounted read-write --- but
+the following somewhat unlikely sequence will end up triggering a
+false positive report of a corrupted file system:
+
+   mkfs.ext4 /dev/vdc
+   mount -o ro /dev/vdc /vdc
+   mount -o remount,rw /dev/vdc
+
+Instead, when initializing the inode table for block group zero, test
+to make sure that itable_unused count is not too large, since that is
+the case that will result in some or all of the reserved inodes
+getting cleared.
+
+This fixes the failures reported by Eric Whiteney when running
+generic/230 and generic/231 in the the nojournal test case.
+
+Fixes: 8844618d8aa7 ("ext4: only look at the bg_flags field if it is valid")
+Reported-by: Eric Whitney <enwlinux@gmail.com>
+Signed-off-by: Theodore Ts'o <tytso@mit.edu>
+---
+ fs/ext4/ialloc.c | 5 ++++-
+ fs/ext4/super.c  | 8 +-------
+ 2 files changed, 5 insertions(+), 8 deletions(-)
+
+--- a/fs/ext4/ialloc.c
++++ b/fs/ext4/ialloc.c
+@@ -1308,7 +1308,10 @@ int ext4_init_inode_table(struct super_b
+ 			    ext4_itable_unused_count(sb, gdp)),
+ 			    sbi->s_inodes_per_block);
+ 
+-	if ((used_blks < 0) || (used_blks > sbi->s_itb_per_group)) {
++	if ((used_blks < 0) || (used_blks > sbi->s_itb_per_group) ||
++	    ((group == 0) && ((EXT4_INODES_PER_GROUP(sb) -
++			       ext4_itable_unused_count(sb, gdp)) <
++			      EXT4_FIRST_INO(sb)))) {
+ 		ext4_error(sb, "Something is wrong with group %u: "
+ 			   "used itable blocks: %d; "
+ 			   "itable unused count: %u",
+--- a/fs/ext4/super.c
++++ b/fs/ext4/super.c
+@@ -2875,14 +2875,8 @@ static ext4_group_t ext4_has_uninit_itab
+ 		if (!gdp)
+ 			continue;
+ 
+-		if (gdp->bg_flags & cpu_to_le16(EXT4_BG_INODE_ZEROED))
+-			continue;
+-		if (group != 0)
++		if (!(gdp->bg_flags & cpu_to_le16(EXT4_BG_INODE_ZEROED)))
+ 			break;
+-		ext4_error(sb, "Inode table for bg 0 marked as "
+-			   "needing zeroing");
+-		if (sb->s_flags & MS_RDONLY)
+-			return ngroups;
+ 	}
+ 
+ 	return group;