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Date:	Mon, 28 Sep 2015 15:36:26 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	Jan Kara <jack@...e.com>
Cc:	linux-ext4 <linux-ext4@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>,
	"Theodore Ts'o" <tytso@....edu>
Subject: [PATCH] ext2: don't mount filesystems with compat features we know
 only ext4 supports

The ext2 mount code never checks the compat features against the ones
it knows about.  This is correct behavior since compat features are
supposed to be rw-compatible with old drivers; however, for certain
configurations (journalled rootfs) we probably want the ext4 driver
to load, not ext2.

Reported-by: Dave Chinner <david@...morbit.com>
Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
---
 fs/ext2/ext2.h  |    6 +++++-
 fs/ext2/super.c |   13 ++++++++++---
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/fs/ext2/ext2.h b/fs/ext2/ext2.h
index 8d15feb..ce508b1 100644
--- a/fs/ext2/ext2.h
+++ b/fs/ext2/ext2.h
@@ -547,7 +547,11 @@ struct ext2_super_block {
 #define EXT2_FEATURE_INCOMPAT_META_BG		0x0010
 #define EXT2_FEATURE_INCOMPAT_ANY		0xffffffff
 
-#define EXT2_FEATURE_COMPAT_SUPP	EXT2_FEATURE_COMPAT_EXT_ATTR
+#define EXT2_FEATURE_COMPAT_SUPP	(EXT2_FEATURE_COMPAT_DIR_PREALLOC| \
+					 EXT2_FEATURE_COMPAT_IMAGIC_INODES| \
+					 EXT2_FEATURE_COMPAT_EXT_ATTR| \
+					 EXT2_FEATURE_COMPAT_RESIZE_INO| \
+					 EXT2_FEATURE_COMPAT_DIR_INDEX)
 #define EXT2_FEATURE_INCOMPAT_SUPP	(EXT2_FEATURE_INCOMPAT_FILETYPE| \
 					 EXT2_FEATURE_INCOMPAT_META_BG)
 #define EXT2_FEATURE_RO_COMPAT_SUPP	(EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER| \
diff --git a/fs/ext2/super.c b/fs/ext2/super.c
index 900e19c..4efb018 100644
--- a/fs/ext2/super.c
+++ b/fs/ext2/super.c
@@ -896,6 +896,16 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
 	 * previously didn't change the revision level when setting the flags,
 	 * so there is a chance incompat flags are set on a rev 0 filesystem.
 	 */
+#ifdef CONFIG_EXT4_FS
+	/* Journalled FS should mount with ext4 if it's available */
+	features = EXT2_HAS_COMPAT_FEATURE(sb, ~EXT2_FEATURE_COMPAT_SUPP);
+	if (features) {
+		ext2_msg(sb, KERN_ERR,	"error: won't mount because of "
+		       "unsupported optional features (%x); try ext4",
+			le32_to_cpu(features));
+		goto failed_mount;
+	}
+#endif
 	features = EXT2_HAS_INCOMPAT_FEATURE(sb, ~EXT2_FEATURE_INCOMPAT_SUPP);
 	if (features) {
 		ext2_msg(sb, KERN_ERR,	"error: couldn't mount because of "
--
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