On Mon, Oct 24, 2016 at 08:59:26AM +0200, Richard Weinberger wrote: > > So, on ext4 (f2fs too?) root has to enable to feature first before users > can use it? > IOW for ext4 the encryption flag means "file encryption is allowed", for UBIFS it > means "at least one file got encrypted on this fs". Historically there have been one or two feature flags which ext4 has enabled automatically, behind the system administrator's back. We've since decided that's a bad thing to do, and so it's a file system level policy not to do this. An example of why this can cause problems is if a system administrator boots a rescue CD that happens to use a newer kernel, and that newer kernel (plus potentially an updated userspace on said rescue CD) silently the feature flag to enable some feature (say, the first time we have files larger than 2GB, for example), and then when the system administrator books back into their RHEL or SLES kernel, they can no longer mount the file system. So ext4 has a policy now that we won't do that, since it's unfriendly to system administrators. That's a general rule, and not one which is specific to encryption. That way you can create a file system, and for the most part, modulo some really old, ancient flags which hopefully all Linux systems understand (I don't *think* there are any RHEL 2.x systems out there still :-), if a feature flag is not enabled, the kernel will return an error if you try to use some feature which is not enabled, instead of silently enabling it behind the user and/or sysadmin's back. I believe xfs may have a similar policy, and for similar reasons. (Actually, I'm not sure if xfs supports enabling features after the file system has been created at all; certainly Red Hat has a distro-level policy of not supporting features added post-mkfs, although I don't know how much that is enforced at the open source coding level. At one point there were some grumbles that Red Hat might want to disable some of tune2fs's features of enabling features post-mkfs time, but I think they've settled for just laughing at users who show up with bug reports after they converted a file system from ext3 to ext4. :-) > I think I'll do what ext4 does to have common policies. If you want to, feel free; but if there is good reason for Ubifs to want to be more flexible, I think there is room for differences at this level. The specific reason why we do this probably doesn't matter so much on embedded systems --- although I will note that many/most handset manufacturers have a policy of not supporting any new features that require new *kernel* fatures after the device has been released, because they don't want to deal with potential upgrade problems and device instabilities issues; users tend to get unhappy when their phones get bricked after an upgrade. Cheers, - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
