Writing to read-only devices with aufs2

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© Raoul Fesquet, Fotolia

© Raoul Fesquet, Fotolia

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Add temporary write capability to a read-only device with the stacked filesystem aufs.

Aufs [1] is a stacked filesystem similar to UnionFS [2]. One very common use for aufs is adding "temporary writing" capability to a filesystem residing on read-only media. I use aufs in Knoppix to join read-only data from the compressed Knoppix file (usually located on a CD or DVD) with a read-write filesystem on ramdisk or a USB flash drive. A stacked filesystem creates a stack of existing directories that are "transparent" to the user: Each access to a file is tried for each directory of the stack until it succeeds or the end of the stack is reached.

Even if you aren't building your own Live system, you still might have a practical use for aufs as a means of adding virtual read-write access to files stored on a read-only device. In this article, I describe how to add temporary write capability to a read-only device using aufs.

Gitting Aufs2

Aufs2 comes in two forms: a complete Git-based kernel with aufs2 included and a "standalone" version that contains only the changes that have to be patched into the kernel source.

Because checking out the complete aufs2-patched kernel is probably not the most common scenario and because compiling the ready-to-build version is no real challenge, I will describe the procedure for adding the standalone version of aufs2 to the vanilla kernel source, which you can always get from the Linux Kernel Archives [3].

The following examples are based on kernel version 2.6.29, which resides in a subdirectory of the current working directory and is called linux-2.6.29.

If you have never used git before, it might be missing from your development system. On Debian-based systems, the command for installing it is

aptitude install git-core

For other package management systems, consult your vendor documentation.

The command to check out aufs2 as standalone patch for kernel 2.6.29 is:

git clone http://git.c3sl.ufpr.br/pub/scm/aufs/aufs2-standalone.git aufs2-standalone.git

After this command, aufs2-standalone.git will be a local copy of the aufs2-standalone Git repository.

The command

git checkout origin/aufs2-29

checks out the aufs2 version for kernel 2.6.29 (note the 29 at the end of the command parameters) and defines it as the current working version. You might see a Git note or warning about origin/aufs2-29 not being a local branch, which you can safely ignore at this point.

Listing 1 shows the current working directory aufs2-standalone.git (ls -l). The directory fs includes the aufs2 source, and the include directory holds the necessary kernel include files for aufs2. Both must be copied into the kernel source directory.

Listing 1

aufs2-standalone.git (ls -l )

01 -rw-r--r-- 1 knopper users  9222 15. Apr 00:51 aufs2-standalone.patch
02 -rw-r--r-- 1 knopper users 40296 15. Apr 00:51 ChangeLog
03 -rw-r--r-- 1 knopper users   661 15. Apr 00:47 config.mk
04 -rw-r--r-- 1 knopper users 17990 15. Apr 00:47 COPYING
05 drwxr-xr-x 4 knopper users   104 15. Apr 00:51 Documentation
06 -rw-r--r-- 1 knopper users  1481 15. Apr 00:51 ecryptfs.patch
07 drwxr-xr-x 3 knopper users    72 15. Apr 00:51 fs
08 drwxr-xr-x 3 knopper users    72 15. Apr 00:51 include
09 -rw-r--r-- 1 knopper users   617 15. Apr 00:47 Makefile

Adding Aufs2 Source Files

If your kernel source is located one directory above in linux-2.6.29, enter:

cd ../linux-2.6.29
cp -a ../aufs2-standalone.git/{fs,include,Documentation} .
cp ../aufs2-standalone.git/config.mk fs/aufs/

The aufs2 README describes aufs2 installation differently, but I find that the method of copying all additional aufs2 source files directly to the kernel source tree is a convenient way for building ready-to-use Linux kernel image packages in Debian containing aufs2 without the need to create a modules/aufs subdirectory with a corresponding external module infrastructure.

Patching the Kernel

In the aufs2-standalone.git directory (Listing 1), you might have noticed a file called aufs2-standalone.patch. This file contains changes that have to be made to various filesystem-related places of the original kernel source. The kernel's Makefile and configuration system also require that some changes be made to them so that aufs2 will be visible as a new filesystem in make menuconfig. From the current directory, linux-2.6.29, use patch as follows,

patch -p1 < ../aufs2-standalone.git/aufs2-standalone.patch

which produces the output shown in Listing 2.

If you see no lines to indicate that an error has resulted from the patch, then everything is now set up and ready to compile.

Listing 2

Patching Files

01 patching file fs/Kconfig
02 patching file fs/Makefile
03 patching file fs/namei.c
04 patching file fs/namespace.c
05 patching file fs/open.c
06 patching file fs/splice.c
07 patching file fs/super.c
08 patching file include/linux/Kbuild
09 patching file include/linux/lockdep.h
10 patching file include/linux/namei.h
11 patching file include/linux/splice.h
12 patching file security/device_cgroup.c
13 patching file security/security.c

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