ext4: check for ext[23] file system features when mounting as ext[23]

Provide better emulation for ext[23] mode by enforcing that the file
system does not have any unsupported file system features as defined
by ext[23] when emulating the ext[23] file system driver when
CONFIG_EXT4_USE_FOR_EXT23 is defined.

This causes the file system type information in /proc/mounts to be
correct for the automatically mounted root file system.  This also
means that "mount -t ext2 /dev/sda /mnt" will fail if /dev/sda
contains an ext3 or ext4 file system, just as one would expect if the
original ext2 file system driver were in use.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 65 insertions, 9 deletions

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 8553dfb310af..cb22783a4377 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -75,11 +75,27 @@ static void ext4_write_super(struct super_block *sb);
 static int ext4_freeze(struct super_block *sb);
 static struct dentry *ext4_mount(struct file_system_type *fs_type, int flags,
                        const char *dev_name, void *data);
+static inline int ext2_feature_set_ok(struct super_block *sb);
+static inline int ext3_feature_set_ok(struct super_block *sb);
 static int ext4_feature_set_ok(struct super_block *sb, int readonly);
 static void ext4_destroy_lazyinit_thread(void);
 static void ext4_unregister_li_request(struct super_block *sb);
 static void ext4_clear_request_list(void);
 
+#if !defined(CONFIG_EXT2_FS) && !defined(CONFIG_EXT2_FS_MODULE) && defined(CONFIG_EXT4_USE_FOR_EXT23)
+static struct file_system_type ext2_fs_type = {
+        .owner          = THIS_MODULE,
+        .name           = "ext2",
+        .mount          = ext4_mount,
+        .kill_sb        = kill_block_super,
+        .fs_flags       = FS_REQUIRES_DEV,
+};
+#define IS_EXT2_SB(sb) ((sb)->s_bdev->bd_holder == &ext2_fs_type)
+#else
+#define IS_EXT2_SB(sb) (0)
+#endif
+
+
 #if !defined(CONFIG_EXT3_FS) && !defined(CONFIG_EXT3_FS_MODULE) && defined(CONFIG_EXT4_USE_FOR_EXT23)
 static struct file_system_type ext3_fs_type = {
         .owner          = THIS_MODULE,
@@ -3187,6 +3203,28 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
                        "feature flags set on rev 0 fs, "
                        "running e2fsck is recommended");
 
+        if (IS_EXT2_SB(sb)) {
+                if (ext2_feature_set_ok(sb))
+                        ext4_msg(sb, KERN_INFO, "mounting ext2 file system "
+                                 "using the ext4 subsystem");
+                else {
+                        ext4_msg(sb, KERN_ERR, "couldn't mount as ext2 due "
+                                 "to feature incompatibilities");
+                        goto failed_mount;
+                }
+        }
+
+        if (IS_EXT3_SB(sb)) {
+                if (ext3_feature_set_ok(sb))
+                        ext4_msg(sb, KERN_INFO, "mounting ext3 file system "
+                                 "using the ext4 subsystem");
+                else {
+                        ext4_msg(sb, KERN_ERR, "couldn't mount as ext3 due "
+                                 "to feature incompatibilities");
+                        goto failed_mount;
+                }
+        }
+
         /*
          * Check feature flags regardless of the revision level, since we
          * previously didn't change the revision level when setting the flags,
@@ -4772,14 +4810,6 @@ static struct dentry *ext4_mount(struct file_system_type *fs_type, int flags,
 }
 
 #if !defined(CONFIG_EXT2_FS) && !defined(CONFIG_EXT2_FS_MODULE) && defined(CONFIG_EXT4_USE_FOR_EXT23)
-static struct file_system_type ext2_fs_type = {
-        .owner          = THIS_MODULE,
-        .name           = "ext2",
-        .mount          = ext4_mount,
-        .kill_sb        = kill_block_super,
-        .fs_flags       = FS_REQUIRES_DEV,
-};
-
 static inline void register_as_ext2(void)
 {
         int err = register_filesystem(&ext2_fs_type);
@@ -4792,10 +4822,22 @@ static inline void unregister_as_ext2(void)
 {
         unregister_filesystem(&ext2_fs_type);
 }
+
+static inline int ext2_feature_set_ok(struct super_block *sb)
+{
+        if (EXT4_HAS_INCOMPAT_FEATURE(sb, ~EXT2_FEATURE_INCOMPAT_SUPP))
+                return 0;
+        if (sb->s_flags & MS_RDONLY)
+                return 1;
+        if (EXT4_HAS_RO_COMPAT_FEATURE(sb, ~EXT2_FEATURE_RO_COMPAT_SUPP))
+                return 0;
+        return 1;
+}
 MODULE_ALIAS("ext2");
 #else
 static inline void register_as_ext2(void) { }
 static inline void unregister_as_ext2(void) { }
+static inline int ext2_feature_set_ok(struct super_block *sb) { return 0; }
 #endif
 
 #if !defined(CONFIG_EXT3_FS) && !defined(CONFIG_EXT3_FS_MODULE) && defined(CONFIG_EXT4_USE_FOR_EXT23)
@@ -4811,10 +4853,24 @@ static inline void unregister_as_ext3(void)
 {
         unregister_filesystem(&ext3_fs_type);
 }
+
+static inline int ext3_feature_set_ok(struct super_block *sb)
+{
+        if (EXT4_HAS_INCOMPAT_FEATURE(sb, ~EXT3_FEATURE_INCOMPAT_SUPP))
+                return 0;
+        if (!EXT4_HAS_COMPAT_FEATURE(sb, EXT4_FEATURE_COMPAT_HAS_JOURNAL))
+                return 0;
+        if (sb->s_flags & MS_RDONLY)
+                return 1;
+        if (EXT4_HAS_RO_COMPAT_FEATURE(sb, ~EXT3_FEATURE_RO_COMPAT_SUPP))
+                return 0;
+        return 1;
+}
 MODULE_ALIAS("ext3");
 #else
 static inline void register_as_ext3(void) { }
 static inline void unregister_as_ext3(void) { }
+static inline int ext3_feature_set_ok(struct super_block *sb) { return 0; }
 #endif
 
 static struct file_system_type ext4_fs_type = {
@@ -4898,8 +4954,8 @@ static int __init ext4_init_fs(void)
         err = init_inodecache();
         if (err)
                 goto out1;
-        register_as_ext2();
         register_as_ext3();
+        register_as_ext2();
         err = register_filesystem(&ext4_fs_type);
         if (err)
                 goto out;