mkinitramfs fails to detect that kernel module `xxhash_generic` should be
installed to init ramdisk, when the rootfs is on a btrfs filesystem with
checksum algorithm xxhash, and the disk partition table is msdos. This resulted
in the system being unbootable because the init ramdisk lacks kernel xxhash
support:

BTRFS info (device sda2): first mount of filesystem fs-uuid
BTRFS error (device sda2): error allocating xxhash64 hash for checksum
BTRFS error (device sda2): open_ctree failed
mount: mounting /dev/sda2 on /root failed: No such file or directory
Failed to mount /dev/sda2 as root filesystem.

Busybox v1.35.0 (Debian 1:1.35.0-4+b3) built-in shell (ash)
(Omitted)

After I added `xxhash_generic` to `/etc/initramfs-tools/modules` and
regenerated the init ramdisk the system booted without issue.

UEFI-installed system is unaffected (in fact I have been using btrfs rootfs
with xxhash as checksum alg on UEFI-only machines for almost a year so this
came as a big surprise when I found that it doesn't even boot), probably due to
xxhash module being marked as a dependency of another thing?

I would also like to propose that we add a (debug) option to include all
available kernel modules in init ramdisk, which may help debug issues like this
in the future.


For the debugging process I went through see:
https://paste.debian.net/1302284/
if anyone is interested.

Control: reassign -1 src:kmod 30+20221128-1
Control: found -1 33+20240816-2
Control: affects -1 initramfs-tools-core
Control: retitle -1 kmod: modprobe --show-depends may not show all softdeps
[...]

initramfs-tools uses modprobe to find the dependencies of each module it
installs.

The modinfo for btrfs includes:

    softdep:        pre: blake2b-256
    softdep:        pre: sha256
    softdep:        pre: xxhash64
    softdep:        pre: crypto-crc32c

But when we run:

    version=$(uname -r)
    /sbin/modprobe --all --set-version="${version}" --quiet --show-depends btrfs

the results are:

    insmod /lib/modules/6.1.0-31-amd64/kernel/arch/x86/crypto/crc32c-intel.ko
    insmod /lib/modules/6.1.0-31-amd64/kernel/crypto/crc32c_generic.ko
    insmod /lib/modules/6.1.0-31-amd64/kernel/lib/libcrc32c.ko
    insmod /lib/modules/6.1.0-31-amd64/kernel/lib/zstd/zstd_compress.ko
    insmod /lib/modules/6.1.0-31-amd64/kernel/lib/raid6/raid6_pq.ko
    insmod /lib/modules/6.1.0-31-amd64/kernel/crypto/xor.ko
    insmod /lib/modules/6.1.0-31-amd64/kernel/crypto/blake2b_generic.ko
    insmod /lib/modules/6.1.0-31-amd64/kernel/fs/btrfs/btrfs.ko

It seems like only the first softdep is being processed.  (libcrc32c is
a regular dependency so I don't think the crc32c modules are being found
via the softdep on crypto-crc32c.)

Ben.