On Mon, Oct 05, 2020 at 11:18:34PM -0400, Theodore Y. Ts'o wrote: > What Josh is proposing I'm pretty sure would also break "e2fsck -E > unshare_blocks", so that's another reason not to accept this as a > valid format change. The kernel already accepted this as a valid mountable filesystem format, without a single error or warning of any kind, and has done so stably for years. > As far as I'm concerned, contrib/e2fsdroid is the canonical definition > of how to create valid file systems with shared_blocks. I'm not trying to create a problem here; I'm trying to address a whole family of problems. I was generally under the impression that mounting existing root filesystems fell under the scope of the kernel<->userspace or kernel<->existing-system boundary, as defined by what the kernel accepts and existing userspace has used successfully, and that upgrading the kernel should work with existing userspace and systems. If there's some other rule that applies for filesystems, I'm not aware of that. (I'm also not trying to suggest that every random corner case of what the kernel *could* accept needs to be the format definition, but rather, cases that correspond to existing userspace.) It wouldn't be *impossible* to work around this, this time; it may be possible to adapt the existing userspace to work on the new and old kernels. My concern is, if a filesystem format accepted by previous kernels can be rejected by future kernels, what stops a future kernel from further changing the format definition or its strictness (co-evolving with one specific userspace) and causing further regressions? I don't *want* to rely on what apparently turned out to be an undocumented bug in the kernel's validator. That's why I was trying to fix the issue in what seemed like the right way, by detecting the situation and turning off the validator. That seemed like it would fully address the issue. If it would help, I could also supply a tiny filesystem image for regression testing. I'm trying to figure out what solution you'd like to see here, as long as it isn't "any userspace that isn't e2fsdroid can be broken at will". I'd be willing to work to adapt the userspace bits I have to work around the regression, but I'd like to get this on the radar so this doesn't happen again. - Josh Triplett
