linux-ext4 - [PATCH 3/8] ext2: only permit ro mounts with compat features we know we don't support

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Date:	Mon, 12 Oct 2015 14:54:37 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	tytso@....edu, darrick.wong@...cle.com
Cc:	linux-ext4@...r.kernel.org
Subject: [PATCH 3/8] ext2: only permit ro mounts with compat features we
 know we don't support

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) the admin is unlikely to want
no-journal mode.  Since we changed the default probe order to put ext4
first, we can make ext2.ko only allow readonly mounts if has_journal is
found.

(Remember, this only affects mounting ext3 filesystems on ext2.ko.)

Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
---
 fs/ext2/ext2.h  |    6 +++++-
 fs/ext2/super.c |   19 +++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)


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..40c312f 100644
--- a/fs/ext2/super.c
+++ b/fs/ext2/super.c
@@ -896,6 +896,14 @@ 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 */
+	if (EXT2_HAS_COMPAT_FEATURE(sb, EXT3_FEATURE_COMPAT_HAS_JOURNAL)) {
+		ext2_msg(sb, KERN_ERR,	"warning: journal detected; cowardly "
+			"refusing to mount read-write.  Try ext4.");
+		sb->s_flags |= MS_RDONLY;
+	}
+#endif
 	features = EXT2_HAS_INCOMPAT_FEATURE(sb, ~EXT2_FEATURE_INCOMPAT_SUPP);
 	if (features) {
 		ext2_msg(sb, KERN_ERR,	"error: couldn't mount because of "
@@ -1336,6 +1344,17 @@ static int ext2_remount (struct super_block * sb, int * flags, char * data)
 			err = -EROFS;
 			goto restore_opts;
 		}
+#ifdef CONFIG_EXT4_FS
+		/* Journalled FS should mount with ext4 if it's available */
+		if (EXT2_HAS_COMPAT_FEATURE(sb,
+					    EXT3_FEATURE_COMPAT_HAS_JOURNAL)) {
+			ext2_msg(sb, KERN_ERR,	"warning: journal detected; "
+				"cowardly refusing to remount read-write.  "
+				"Try ext4.");
+			err = -EROFS;
+			goto restore_opts;
+		}
+#endif
 		/*
 		 * Mounting a RDONLY partition read-write, so reread and
 		 * store the current valid flag.  (It may have been changed

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