fsverity PAGE_SIZE constraints

[Jes Sorensen <jes@trained-monkey.org>]

Hi,

I am working on adding fsverity support to RPM and I am hitting a tricky
problem. I am see this with RPM, but it really isn't specific to RPM,
and will apply to any method for distribution signatures.

fsverity is currently hard-wiring the Merkle tree block size to
PAGE_SIZE. This is problematic for a number of reasons, in particular on
architectures that can be configured with different page sizes, such as
ARM, as well as the case where someone generates a shared 'common'
package to be used cross architectures (noarch package in RPM terms).

For a package manager to be able to create a generic package with
signatures, it basically has to build a signature for every supported
page size of the target architecture.

Chris Mason is working on adding fsverity support to btrfs, and I
understand he is supporting 4K as the default Merkle tree block size,
independent of the PAGE_SIZE.

Would it be feasible to make ext4 and other file systems support 4K for
non 4K page sized systems and make that a general recommendation going
forward?

Thoughts?

Thanks,
Jes