Loop-aes and a changed block device size

[Jim MacBaine <jmacbaine@xxxxxxxxx>]

Hello list, Hello Jari,

I'm using loop-AES on my laptop or a few years.  All filesystems are
loop-AES-encrypted on lvm volumes. Today I had to enlarge my /home
partition. As you certainly know, lvm volumes can grow online and
meanwhile at least reiserfs and ext3 have the ability to resize a
filesystem on-line. But unfortunately the loop device layer which
resides between those two does not grow with the underlying block
device.

I was bored tonight, and had a look at the loop-aes source to find out
whether it would be possible to adjust a loop device to the new size
of its its underlying block device.  I think I have found a way, but I
am not competent enough to see whether I break something with it.  It
works for me so far that I can create a file-backed loop device,
create a reiserfs on it and mount it, append a few megabytes to the
container file, call my new LOOP_RESET_SIZE ioctl and resize the file
system with resize_reiserfs, while the fs is mounted.

Maybe you can have a quick look on it. Is it really so simple or am I
missing some important point here?  Would it be worth the effort to
include a this functionality into losetup? I haven't written C for
many years and have never read or written any kernel code before
today, so please don't expect too much.  I would really appreciate any
comment.

Regards,
Jim

--- ../linux/modules/loop-AES-v3.0d/loop.c-2.6.patched	2005-06-11 17:10:40.000000000 +0200
+++ loop-aes-RESIZE/loop.c-2.6.patched	2005-09-16 23:51:52.000000000 +0200
@@ -117,6 +117,10 @@
 # define LOOP_MULTI_KEY_SETUP_V3 0x4C4E
 #endif
 
+#ifndef LOOP_RESET_SIZE 
+# define LOOP_RESET_SIZE 0x4C07
+#endif
+
 #if defined(CONFIG_COMPAT) && !defined(HAVE_COMPAT_IOCTL)
 # include <linux/ioctl32.h>
 # define IOCTL32_COMPATIBLE_PTR ((void*)0)
@@ -634,6 +638,14 @@
 	return err;
 }
 
+static int loop_reset_size(struct loop_device *lo, struct block_device *bdev)
+{
+	if (figure_loop_size(lo, bdev)) {
+		return -EFBIG;
+	}
+	return 0;
+}
+
 static inline int lo_do_transfer(struct loop_device *lo, int cmd, char *rbuf,
 				 char *lbuf, int size, sector_t rblock)
 {
@@ -1747,6 +1759,9 @@
 	case LOOP_GET_STATUS64:
 		err = loop_get_status64(lo, (struct loop_info64 *) arg);
 		break;
+	case LOOP_RESET_SIZE:
+		err = loop_reset_size(lo, bdev);
+		break;
 	default:
 		err = lo->ioctl ? lo->ioctl(lo, cmd, arg) : -EINVAL;
 	}

#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <stdio.h>

#define LOOP_RESET_SIZE 0x4C07

int main(int argc, char **argv) 
{
	int fd, result;

	fd = open(argv[1], O_RDONLY);
	result = ioctl(fd, LOOP_RESET_SIZE);
	
	printf("result: %d\n", result);
}