Initramfs: Busybox sh makes Kernel panic

[SvenRh]

Hello,

I'm trying to build a small, bloat-free system for network bridging purposes using just Busybox and wpa_supplicant in an initramfs. However, I'm greeted with a kernel panic in the very moment when Busybox should do business.

What have I done so far?

As the target machine conveniently is x86-64, I prepared a freshly downloaded Stage 3 in a chroot on my main workstation, especially setting FEATURES="buildpkg" and running emerge -e @system. Then I emerged gentoo-sources, busybox and wpa_supplicant; wpa_supplicant having all USE flags disabled and busybox having just the ipv6 and - most importantly - static USE flags set. I checked that busybox is indeed static:

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(chroot) r7 / # ldd /bin/busybox
        not a dynamic executable
(chroot) r7 / #

Then I started work on the gentoo-sources with make allnoconfig, gradually adding what I needed and what the target machine needed to actually boot that thing and show something on the screen. (Took me actually a few tries, have never gone the allnoconfig route before.)

I prepared the initramfs in /usr/src/initramfs/ by emerging a baselayout to it, followed by ROOT=/usr/src/initramfs emerge -atv --usepkgonly wpa_supplicant. Then I copied /bin/busybox to /usr/src/initramfs/bin/.

Lastly, I created a minimal /usr/src/initramfs/init file which (for first testing) looks like this:

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(chroot) r7 / # cat /usr/src/initramfs/init
#!/bin/busybox sh

echo "Boot complete. Have fun!"

exec sh
(chroot) r7 / #

I chmoded that file +x, made the kernel (built-in initramfs included) and copied it over to the target machine's EFI boot partition. Then I rebooted that machine and ...

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Run /init as init process
Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000000
CPU: 1 PID: 1 Comm: sh Not tainted 5.4.80-gentoo-r1 #17

Yeah, all I got was Kernel panics when Busybox should have taken over. Given that Busybox is static, I ruled it out as the bully, and fiddled about with the kernel and out of sheer desperation even tried the opposite approach, kernel-wise, make defconfig and disabling only obviously unneeded stuff (mostly a plethora of drivers for various network devices I have never owned). Different approach - no different result: Panic attacks whenever Busybox should do business.

I'm at a loss.

What's wrong here? Where have I missed something?

[NeddySeagoon]

SvenRh,

Well,

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Attempted to kill init! 

means that the kernel loaded and fell off the end of the init process.

I suspect you need to mount /proc or you won't have stdin, stdout and stderr.
That is, busybox won't be able to talk to you.
/dev needs to be mounted too. You need at least /dev/null and /dev/console.

I don't think /sys is required but I'm not sure.

[Goverp]

I'm trying to build a small, bloat-free system for network bridging purposes using just Busybox and wpa_supplicant in an initramfs. However, I'm greeted with a kernel panic in the very moment when Busybox should do business....

SvenRh, Interesting. I was thinking of doing something very similar, without the wifi, to make a stand-alone minimal bootable Busybox as a toolkit for my EFI partition. I know Intel provide an EFI Shell, but AFAIK it doesn't speak bash or anything well-known and hence needs learning afresh, whereas Busybox is pretty easy to use. Keep up your good work; I could then strip out the networking and have just what I wanted (qua probably a wider selection of file systems.)

[NeddySeagoon]

SvenRh,

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#cp /bin/busybox /mnt/gentoo
# chroot /mnt/gentoo/ /busybox sh
/ # ls
busybox
/ # ls /dev
ls: /dev: No such file or directory
/ # ls /proc
ls: /proc: No such file or directory
/ # ls /sys
ls: /sys: No such file or directory
/ # 

that much works

[szatox]

(chroot) r7 / # cat /usr/src/initramfs/init
#!/bin/busybox sh

echo "Boot complete. Have fun!"

exec sh
(chroot) r7 / #

Make sure you have /dev/null and /dev/console nodes created in your initramfs.
AFAIR null is the only hard requirement, you can get all other nodes by mounting /dev (just mount /proc first and /dev second), but in the manual mode console will be needed too.

[SvenRh]

I suspect you need to mount /proc or you won't have stdin, stdout and stderr.
That is, busybox won't be able to talk to you.
/dev needs to be mounted too. You need at least /dev/null and /dev/console.

I don't think /sys is required but I'm not sure.

Make sure you have /dev/null and /dev/console nodes created in your initramfs.
AFAIR null is the only hard requirement, you can get all other nodes by mounting /dev (just mount /proc first and /dev second), but in the manual mode console will be needed too.

Alright, I added the respective mount commands to my /init:

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(chroot) r7 /usr/src/initramfs # cat init
#!/bin/busybox sh

mount -t proc none /proc
mount -t devtmpfs none /dev
mount -t sysfs none /sys

echo "Boot complete. Have fun!"

exec sh
(chroot) r7 /usr/src/initramfs #

I checked my kernel to have the required options set:

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(chroot) r7 /usr/src/linux # grep PROC_FS .config
CONFIG_SCSI_PROC_FS=y
CONFIG_PROC_FS=y
(chroot) r7 /usr/src/linux # grep DEVTMPFS .config
CONFIG_DEVTMPFS=y
CONFIG_DEVTMPFS_MOUNT=y
(chroot) r7 /usr/src/linux # grep SYSFS .config
# CONFIG_SYSFS_DEPRECATED is not set
CONFIG_SYSFS_SYSCALL=y
# CONFIG_DMI_SYSFS is not set
# CONFIG_FW_CFG_SYSFS is not set
# CONFIG_WATCHDOG_SYSFS is not set
CONFIG_EDAC_LEGACY_SYSFS=y
CONFIG_RTC_INTF_SYSFS=y
CONFIG_NVMEM_SYSFS=y
CONFIG_SYSFS=y
(chroot) r7 /usr/src/linux #

For reference here is my complete .config: https://pastebin.com/UCU3G8qp

I removed usr/initramfs_data.cpio from the kernel source tree and re-made the kernel, copied it to the taregt ESP an reboted. However, to no different result.

Here is a "screenshot" of the complete failure: https://imgur.com/a/daS4636

[NeddySeagoon]

SvenRh,

Do those three mount points exist in the initrd?

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mount -t proc none /proc
mount -t devtmpfs none /dev
mount -t sysfs none /sys 

If not, the mounts will fail.

[SvenRh]

Damn, I should adhere to what I quote ...

Make sure you have /dev/null and /dev/console nodes created in your initramfs.
AFAIR null is the only hard requirement, you can get all other nodes by mounting /dev (just mount /proc first and /dev second), but in the manual mode console will be needed too.

Then I would have come across something else missing from my initramfs ...

Do those three mount points exist in the initrd?

Nope, they didn't. I had considered them to be part of sys-apps/baselayout, but apparently they're not. So I mkdir-ed them and copied (--archive) /dev/{null,console} over. Removed initramfs_data.cpio, re-made the kernel, copied it to the target ESP, rebooted. And kaboom: It works!

Thank you very much for all your help!

[szatox]

had considered them to be part of sys-apps/baselayout, but apparently they're not

They are. You just have to enable "build" do-not-USE flag.

Anyway, gratz!