ext3 compatibility between 2.4 and 2.6 kernels

'Andreas Dilger' adilger at clusterfs.com
Tue Feb 22 17:26:46 UTC 2005

On Feb 22, 2005  09:47 -0700, Howdy Pierce wrote:
> Thanks for your prompt help.  The problem (as you hinted) turned out
> to be SELinux.
> So -- the extended attributes get turned on using SE Linux, and this
> happens regardless of whether you use the -Onouser_xattr flag to
> mount.  

The user_xattr flag only allows/prevents regular users from storing EAs
to the filesystem.

> When I disabled SE Linux and repeated the test, the dumpe2fs from
> before and after the mount/tar/umount was more similar.  Also,
> symbolic links were now readable from a 2.4 kernel, and my embedded
> system boots correctly (and finds init). 

I would guess that /sbin/init is a symlink then?

> I can't say I really understand enough about SE Linux to have an
> opinion as to whether this is a bug or not.  It seems like there
> should be some option, even on an SE Linux system, to write ext2/3
> filesystems in a backward-compatible manner.

Note that EAs (generally, this EA-on-symlink problem being an unfortunate
exception) are compatible with older kernels.  This functionality was 
added after EAs were developed and added to the kernel already.

I agree it might be useful to have an option to prevent any EAs from
being stored on a filesystem, since this also consumes space in the
filesystem that has no value to your embedded device.  I don't think
that mounting the filesystem as ext2 will help as it also has EA support.

As I mentioned previously, it is possible with a tiny patch to the 2.4
kernel to fix ext3_inode_is_fast_symlink() to understand symlinks with EAs.

Cheers, Andreas
Andreas Dilger
http://members.shaw.ca/adilger/             http://members.shaw.ca/golinux/

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