Re: [PATCH v4] ext4: only allow test_dummy_encryption when supported

[Gabriel Krisman Bertazi <krisman@collabora.com>]

Eric Biggers <ebiggers@kernel.org> writes:

> From: Eric Biggers <ebiggers@google.com>
>
> Make the test_dummy_encryption mount option require that the encrypt
> feature flag be already enabled on the filesystem, rather than
> automatically enabling it.  Practically, this means that "-O encrypt"
> will need to be included in MKFS_OPTIONS when running xfstests with the
> test_dummy_encryption mount option.  (ext4/053 also needs an update.)
>
> Moreover, as long as the preconditions for test_dummy_encryption are
> being tightened anyway, take the opportunity to start rejecting it when
> !CONFIG_FS_ENCRYPTION rather than ignoring it.
>
> The motivation for requiring the encrypt feature flag is that:
>
> - Having the filesystem auto-enable feature flags is problematic, as it
>   bypasses the usual sanity checks.  The specific issue which came up
>   recently is that in kernel versions where ext4 supports casefold but
>   not encrypt+casefold (v5.1 through v5.10), the kernel will happily add
>   the encrypt flag to a filesystem that has the casefold flag, making it
>   unmountable -- but only for subsequent mounts, not the initial one.
>   This confused the casefold support detection in xfstests, causing
>   generic/556 to fail rather than be skipped.
>
> - The xfstests-bld test runners (kvm-xfstests et al.) already use the
>   required mkfs flag, so they will not be affected by this change.  Only
>   users of test_dummy_encryption alone will be affected.  But, this
>   option has always been for testing only, so it should be fine to
>   require that the few users of this option update their test scripts.
>
> - f2fs already requires it (for its equivalent feature flag).
>
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Makes sense to me and code looks good.  Please add:

Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.com>

-- 
Gabriel Krisman Bertazi