How to autoprobe block devices in initramfs?

Question

I am building a custom Alpine image based on isolinux. Basically, I am squashing rootfs, and mounting it as overlayfs.

Bootloader does its job fine, kernel loads, but I am stuck at initramfs. Let say I have the following:

#!/bin/sh
export PATH=/sbin:/usr/sbin:/bin:/usr/bin
/bin/busybox --install -s

rescue_shell() {
    echo "Something went wrong. Dropping you to a shell."
    #/bin/busybox --install -s
    /bin/sh || exec /bin/busybox sh
}
mount -t sysfs sysfs /sys
mount -t proc  proc /proc
mkdir  -p /dev/pts

mount -t devtmpfs -o exec,nosuid,mode=0755,size=2M devtmpfs /dev 2>/dev/null \
        || mount -t tmpfs -o exec,nosuid,mode=0755,size=2M tmpfs /dev

[ -c /dev/ptmx ] || mknod -m 666 /dev/ptmx c 5 2
[ -d /dev/pts ] || mkdir -m 755 /dev/pts
mount -t devpts -o gid=5,mode=0620,noexec,nosuid devpts /dev/pts
# shared memory area (later system will need it)
[ -d /dev/shm ] || mkdir /dev/shm
mount -t tmpfs -o nodev,nosuid,noexec shm /dev/shm

/bin/sh
# other code left for simplicity

So once I enters /bin/sh, I don't have any modules loaded, especially I am meaning for block devices, /dev/sda, /dev/sr0, which I need to mount and then extract squashed image, and mount overlay.

Listing /proc/partitions gives me only ram[0-15] devices, which make sense since after boot it's loaded into RAM.

So, my question would be, is there any way that devices gets probed based on available hardware? I have tried with mdev as well, but still can not get my block devices. Proper mdev.conf is there, and tests are performed in VirtualBox. Thank you.

score 1 · Accepted Answer · answered Sep 06 '19 at 14:44

You could try your luck with modalias as exposed through the sysfs interface.

See for example https://patchwork.openembedded.org/patch/148854/ which suggests:

echo "/sbin/mdev" > /proc/sys/kernel/hotplug
mdev -s
find /sys/ -name modalias -print0 | xargs -0 sort -u -z | xargs -0 modprobe -abq

Note that I haven't tested this myself. Also this doesn't seem to be using BusyBox modprobe, which probably does not support -ab. Still, it might be worth checking out what your /sys looks like in early initramfs.

How to autoprobe block devices in initramfs?

1 Answers1