Re: [PATCH 1/4] fscrypt: Cap data units at the page size

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Nov 04, 2026 at 10:31:46AM +0800, Baokun Li wrote:
> What I had in mind before was to keep the default as min(bs, PS) and
> allow DU > PS only when explicitly specified, which seemed to give
> better compatibility.  Cross-machine or cross-page-size cases seem
> uncommon, and existing machines have probably already avoided them by
> specifying a 4K or smaller DU.
> 
> Keeping the existing default semantics and returning an error in the
> LBS case for the user to specify seems feasible.  But then
> test_dummy_encryption and the xfstests cases that use the default would
> need to specify the DU explicitly, otherwise the LBS + encrypt test
> coverage would drop a lot.  That is, in the kernel or the tests we
> always have to set a default DU size.
> 
> If you confirm that we should take the route where LBS requires an
> explicit data unit size and the default is rejected, I can adapt the
> kernel and xfstests in v2.  Or should we go back to the more compatible
> DU > page size approach?

It will indeed be annoying to have to explicitly specify the data unit
size, but I don't see a better solution for now, given the complexity of
supporting data unit sizes larger than the page size.  I think making
the on-disk format needing PAGE_SIZE to interpret is a no-go.

I suppose that in theory, how FS_IOC_SET_ENCRYPTION_POLICY handles
log2_data_unit_size=0 could be changed: it could start actually writing
log2_data_unit_size=12 to disk (rather than writing 0 which means the
filesystem block size).  I think that can still lead to problems,
though.  For one FS_IOC_GET_ENCRYPTION_POLICY would return something
different from what was set.

Regarding xfstests, take a look at how common/verity adds $FSV_BLOCKSIZE
to all 'fsverity enable' commands by default, to select some Merkle tree
block size that the filesystem supports.  Something similar could be
done with fscrypt.  Yes, this won't work with v1 encryption policies,
but v1 policies are obsolete.  Most of the xfstests that use v1 policies
should just be updated to use v2 instead.  The btrfs folks were working
on that earlier, since they're not planning to support v1 at all.

- Eric