Re: grub-probe detects ext4 wronly as ext2

["Javier Martín" <lordhabbit@gmail.com>]

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OK, I've addressed all your concerns and here is a new version of the
patch. With it, the delta-size of the compiled ext2.mod against a
completely unpatched one is just 148 bytes.

El vie, 04-07-2008 a las 20:57 +0200, Robert Millan escribió:
> On Fri, Jul 04, 2008 at 04:45:02PM +0200, Javier Martín wrote:
> > 
> > By the way, I'm already using SVN (and thus svn diff) for this patch. Is
> > that right? Was the migration completed already?
> 
> Yep.
Wonderful! I was sick of jumping through hoops with cvs diff.

> I'd suggest making the "RW compatible" etc notes a bit more ellaborate to make
> it clear what they mean (I'm confused myself).
Done, though now I might have over-elaborated

> > +/* The set of back-incompatible features this driver DOES support. Add (OR)
> > + * flags here as the related features are implemented into the driver */
> > +#define EXT2_DRIVER_SUPPORTED_INCOMPAT ( EXT2_FEATURE_INCOMPAT_FILETYPE )
> 
> I suppose we'll want to have EXT4_FEATURE_INCOMPAT_EXTENTS here, now that it's
> been implemented (Bean just sent a patch, which will probably be merged first).
Done too and checked that ext4 filesystems w/o other incompatible
features like 64BIT are now recognized (though I did not check reading
since I haven't yet applied Bean's patch to my tree).

> > +/* The set of back-incompatible features this driver DOES NOT support but are
> > + * ignored for some hackish reason. Flags here should be here _temporarily_!
> > + * Remember that INCOMPAT_* features are so for a reason! */
> > +#define EXT2_DRIVER_IGNORED_INCOMPAT ( EXT3_FEATURE_INCOMPAT_RECOVER )
> 
> Instead of this can we have an explanation of what EXT3_FEATURE_INCOMPAT_RECOVER
> is doing here?  I think the reason was that our code for this feature wasn't
> as mature as it should be, and it appears that not handling it brings better
> results in the short term.
Done - explained why RECOVER is ignored even though it's "incompatible"

> Since we know which one applies, why not tell grub_error about it?  We could
> leave the "not an ext2 filesystem" call unmodified and add another one for
> this particular error.
> 
I may have overstepped a bit, but I've thought it more sensible to
replace all "goto fail;"s for calls to a new macro MOUNT_FAIL taking a
string argument which is saved in the new variable err_msg, and then
jumps to fail which shows _that_ message instead of the old one. Then, I
wrote informative messages for each error condition instead of just "not
an ext2 filesystem".

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Index: fs/ext2.c
===================================================================
--- fs/ext2.c	(revisión: 1691)
+++ fs/ext2.c	(copia de trabajo)
@@ -71,8 +71,53 @@
          ? EXT2_GOOD_OLD_INODE_SIZE \
          : grub_le_to_cpu16 (data->sblock.inode_size))
 
-#define EXT3_FEATURE_COMPAT_HAS_JOURNAL	0x0004
+/* Superblock filesystem feature flags (RW compatible)
+ * A filesystem with any of these enabled can be read and written by a driver
+ * that does not understand them without causing metadata/data corruption */
+#define EXT2_FEATURE_COMPAT_DIR_PREALLOC	0x0001
+#define EXT2_FEATURE_COMPAT_IMAGIC_INODES	0x0002
+#define EXT3_FEATURE_COMPAT_HAS_JOURNAL		0x0004
+#define EXT2_FEATURE_COMPAT_EXT_ATTR		0x0008
+#define EXT2_FEATURE_COMPAT_RESIZE_INODE	0x0010
+#define EXT2_FEATURE_COMPAT_DIR_INDEX		0x0020
+/* Superblock filesystem feature flags (RO compatible)
+ * A filesystem with any of these enabled can be safely read by a driver that
+ * does not understand them, but should not be written to, usually because
+ * additional metadata is required */
+#define EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER	0x0001
+#define EXT2_FEATURE_RO_COMPAT_LARGE_FILE	0x0002
+#define EXT2_FEATURE_RO_COMPAT_BTREE_DIR	0x0004
+#define EXT4_FEATURE_RO_COMPAT_GDT_CSUM		0x0010
+#define EXT4_FEATURE_RO_COMPAT_DIR_NLINK	0x0020
+#define EXT4_FEATURE_RO_COMPAT_EXTRA_ISIZE	0x0040
+/* Superblock filesystem feature flags (back-incompatible)
+ * A filesystem with any of these enabled should not be attempted to be read
+ * by a driver that does not understand them, since they usually indicate
+ * metadata format changes that might confuse the reader. */
+#define EXT2_FEATURE_INCOMPAT_COMPRESSION	0x0001
+#define EXT2_FEATURE_INCOMPAT_FILETYPE		0x0002
+#define EXT3_FEATURE_INCOMPAT_RECOVER		0x0004 /* Needs recovery */
+#define EXT3_FEATURE_INCOMPAT_JOURNAL_DEV	0x0008 /* Volume is journal device */
+#define EXT2_FEATURE_INCOMPAT_META_BG		0x0010
+#define EXT4_FEATURE_INCOMPAT_EXTENTS		0x0040 /* Extents used */
+#define EXT4_FEATURE_INCOMPAT_64BIT		0x0080
+#define EXT4_FEATURE_INCOMPAT_FLEX_BG		0x0200
 
+/* The set of back-incompatible features this driver DOES support. Add (OR)
+ * flags here as the related features are implemented into the driver */
+#define EXT2_DRIVER_SUPPORTED_INCOMPAT ( EXT2_FEATURE_INCOMPAT_FILETYPE \
+                                       | EXT4_FEATURE_INCOMPAT_EXTENTS )
+/* List of rationales for the ignored "incompatible" features:
+ * needs_recovery: Not really back-incompatible - was added as such to forbid
+ *                 ext2 drivers from mounting an ext3 volume with a dirty
+ *                 journal because they will ignore the journal, but the next
+ *                 ext3 driver to mount the volume will find the journal and
+ *                 replay it, potentially corrupting the metadata written by
+ *                 the ext2 drivers
+ */
+#define EXT2_DRIVER_IGNORED_INCOMPAT ( EXT3_FEATURE_INCOMPAT_RECOVER )
+
+
 #define EXT3_JOURNAL_MAGIC_NUMBER	0xc03b3998U
 
 #define EXT3_JOURNAL_DESCRIPTOR_BLOCK	1
@@ -375,10 +420,12 @@
   return 0;
 }
 
+#define EXT2_DRIVER_MOUNT_FAIL(message) { err_msg = (message); goto fail; }
 static struct grub_ext2_data *
 grub_ext2_mount (grub_disk_t disk)
 {
   struct grub_ext2_data *data;
+  const char *err_msg = 0;
 
   data = grub_malloc (sizeof (struct grub_ext2_data));
   if (!data)
@@ -388,12 +435,18 @@
   grub_disk_read (disk, 1 * 2, 0, sizeof (struct grub_ext2_sblock),
                   (char *) &data->sblock);
   if (grub_errno)
-    goto fail;
+    EXT2_DRIVER_MOUNT_FAIL("could not read the superblock")
 
   /* Make sure this is an ext2 filesystem.  */
   if (grub_le_to_cpu16 (data->sblock.magic) != EXT2_MAGIC)
-    goto fail;
+    EXT2_DRIVER_MOUNT_FAIL("not an ext2 filesystem (superblock magic mismatch)")
   
+  /* Check the FS doesn't have feature bits enabled that we don't support. */
+  if (grub_le_to_cpu32 (data->sblock.feature_incompat)
+        & ~(EXT2_DRIVER_SUPPORTED_INCOMPAT | EXT2_DRIVER_IGNORED_INCOMPAT))
+    EXT2_DRIVER_MOUNT_FAIL("filesystem has unsupported incompatible features")
+    
+  
   data->disk = disk;
 
   data->diropen.data = data;
@@ -404,12 +457,14 @@
 
   grub_ext2_read_inode (data, 2, data->inode);
   if (grub_errno)
-    goto fail;
+    EXT2_DRIVER_MOUNT_FAIL("could not read the root directory inode")
   
   return data;
 
  fail:
-  grub_error (GRUB_ERR_BAD_FS, "not an ext2 filesystem");
+  if (!err_msg)
+    err_msg = "DEBUG: mount failed but no error message supplied!";
+  grub_error (GRUB_ERR_BAD_FS, err_msg);
   grub_free (data);
   return 0;
 }

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