How to attach a disk image to QEMU containing root filesystem for linux kernel?

Issue

I am able to launch the linux kernel on QEMU using -kernel and that's pretty straight-forward and simple.

But now I am attempting something different. I am simulating the combination of UEFI + GRUB + Kernel.

I have built OVMF and GRUB2 from source and also able to load the kernel image, but having difficulty creating a rootfs disk image and attaching it to qemu so that kernel detects it.

This is how I prepared the rootfs.img for the kernel :

#Create an empty ext2 disk image of size 10M
dd if=/dev/zero of=rootfs.img bs=1M count=10
mke2fs -j rootfs.img

#Populate the disk
mkdir /mnt/rootfs
mount -o loop rootfs.img /mnt/rootfs
rsync -a busybox/_install/ /mnt/rootfs
chown -R root:root /mnt/rootfs
sync
umount /mnt/rootfs

This is how my GRUB config looks like :

menuentry "Linux Kernel Boot" {
        set root=(hd0,msdos1)
        linux (hd0,msdos1)/bzImage-x86_64 root=/dev/sdb1 rootwait console=ttyS0,115200n8 console=tty0
}

This is the command that I use to launch qemu :

qemu-system-x86_64 -m 2G -drive  if=pflash,format=raw,readonly,file=OVMF_CODE.fd -drive if=pflash,format=raw,file=OVMF_VARS.fd -net none -serial stdio -display none -hda fat:disk -drive file=rootfs.img,index=1

The FAT disk that I have attached as hda contains my GRUB and kernel image. I want a separate disk for rootfs and do not want to use this FAT for that purpose.

The setup boots till kernel but kernel is not able to mount the disk sdb1 and hence does not see the init in rootfs and hence I get a kernel panic. Below are the kernel logs :

[    1.030290] NET: Registered protocol family 17
[    1.033540] ata1.01: configured for MWDMA2
[    1.042119] Key type dns_resolver registered
[    1.045150] scsi 0:0:0:0: Direct-Access     ATA      QEMU HARDDISK    2.5+ PQ: 0 ANSI: 5
[    1.049509] sched_clock: Marking stable (1049235510, 0)->(1378961625, -329726115)
[    1.058429] registered taskstats version 1
[    1.060917] Loading compiled-in X.509 certificates
[    1.066242] sd 0:0:0:0: [sda] 1032192 512-byte logical blocks: (528 MB/504 MiB)
[    1.070386] sd 0:0:0:0: Attached scsi generic sg0 type 0
[    1.074945] sd 0:0:0:0: [sda] Write Protect is off
[    1.079585] scsi 0:0:1:0: Direct-Access     ATA      QEMU HARDDISK    2.5+ PQ: 0 ANSI: 5
[    1.082483] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[    1.090820]   Magic number: 0:346:692
[    1.094970] sd 0:0:1:0: [sdb] 20480 512-byte logical blocks: (10.5 MB/10.0 MiB)
[    1.097740] sd 0:0:1:0: [sdb] Write Protect is off
[    1.101097] sd 0:0:1:0: Attached scsi generic sg1 type 0
[    1.108436] console [netcon0] enabled
[    1.110947] netconsole: network logging started
[    1.114830] sd 0:0:1:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[    1.122345] scsi 1:0:0:0: CD-ROM            QEMU     QEMU DVD-ROM     2.5+ PQ: 0 ANSI: 5
[    1.138010]  sda: sda1
[    1.142638] sr 1:0:0:0: [sr0] scsi3-mmc drive: 4x/4x cd/rw xa/form2 tray
[    1.145513] cdrom: Uniform CD-ROM driver Revision: 3.20
[    1.149749] cfg80211: Loading compiled-in X.509 certificates for regulatory database
[    1.164130] sd 0:0:1:0: [sdb] Attached SCSI disk
[    1.169575] sr 1:0:0:0: Attached scsi generic sg2 type 5
[    1.174262] sd 0:0:0:0: [sda] Attached SCSI disk
[    1.190198] cfg80211: Loaded X.509 cert 'sforshee: 00b28ddf47aef9cea7'
[    1.194508] platform regulatory.0: Direct firmware load for regulatory.db failed with error -2
[    1.197606] cfg80211: failed to load regulatory.db
[    1.200837] ALSA device list:
[    1.203626]   No soundcards found.
[    1.531285] input: ImExPS/2 Generic Explorer Mouse as /devices/platform/i8042/serio1/input/input3
[    1.537979] md: Waiting for all devices to be available before autodetect
[    1.541357] md: If you don't use raid, use raid=noautodetect
[    1.548186] md: Autodetecting RAID arrays.
[    1.551815] md: autorun ...
[    1.554889] md: ... autorun DONE.
[    1.559995] VFS: Cannot open root device "sdb1" or unknown-block(8,17): error -6
[    1.563196] Please append a correct "root=" boot option; here are the available partitions:
[    1.566295] 0800          516096 sda
[    1.566326]  driver: sd
[    1.572371]   0801          516064 sda1 be1afdfa-01
[    1.572397]
[    1.577560] 0810           10240 sdb
[    1.577573]  driver: sd
[    1.582573] 0b00         1048575 sr0
[    1.582585]  driver: sr
[    1.587948] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(8,17)
[    1.590831] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.15.3+ #2
[    1.593384] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015
[    1.596327] Call Trace:
[    1.600043]  dump_stack+0x46/0x59
[    1.602665]  panic+0xca/0x218
[    1.604991]  ? printk+0x3e/0x46
[    1.607197]  mount_block_root+0x174/0x223
[    1.609463]  ? set_debug_rodata+0xc/0xc
[    1.611851]  mount_root+0xfb/0x104
[    1.614159]  prepare_namespace+0x130/0x166
[    1.616415]  kernel_init_freeable+0x1c5/0x1d7
[    1.618676]  ? rest_init+0xb0/0xb0
[    1.621326]  kernel_init+0x5/0xf0
[    1.623768]  ret_from_fork+0x35/0x40
[    1.627035] Kernel Offset: 0x24800000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[    1.632240] ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(8,17) 

Can someone please guide the proper way to attach the disk in qemu which can solve this issue, or if I need to change something in the GRUB config?

EDIT

I created the disk image with a partition table this time as per the advice given by @stark.

$ dd if=/dev/zero of=rootfs.img bs=1M count=10

#Using fdisk, created a partition of type Linux (83). This is how it looks now
$ fdisk -l rootfs.img
Disk rootfs.img: 10 MiB, 10485760 bytes, 20480 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xc08ab9b0

Device      Boot Start   End Sectors Size Id Type
rootfs.img1       2048 20479   18432   9M 83 Linux

$ sudo losetup -o 1048576 --sizelimit 10485248 /dev/loop23 rootfs.img
$ sudo mkfs.ext2 /dev/loop23
$ sudo mount /dev/loop23 /mnt/rootfs
$ sudo cp -vrf busybox/_install/* /mnt/rootfs/
$ sudo umount /mnt/rootfs
$ sudo losetup -d /dev/loop23

Now again I launch the kernel with the same command as above, but this time the kernel just hangs :

[    1.593046] EXT4-fs (sdb1): couldn't mount as ext3 due to feature incompatibilities
[    1.599792] EXT4-fs (sdb1): mounting ext2 file system using the ext4 subsystem
[    1.614366] EXT4-fs (sdb1): mounted filesystem without journal. Opts: (null)
[    1.618589] VFS: Mounted root (ext2 filesystem) readonly on device 8:17.
[    1.626643] devtmpfs: mounted
[    1.665158] Freeing unused kernel memory: 1216K
[    1.667934] Write protecting the kernel read-only data: 18432k
[    1.670772] tsc: Refined TSC clocksource calibration: 2207.976 MHz
[    1.673349] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x1fd3a19a709, max_idle_ns: 440795259673 ns
[    1.681441] Freeing unused kernel memory: 2004K
[    1.693107] Freeing unused kernel memory: 1080K
[    1.836603] modprobe (1041) used greatest stack depth: 14072 bytes left
[    1.877586] rcS (1037) used greatest stack depth: 13984 bytes left
[    2.683414] clocksource: Switched to clocksource ts
[  333.025228] kworker/dying (178) used greatest stack depth: 13944 bytes left

No output beyond this. I have created etc/init.d/rcS in the rootfs.img which has below content :

#!/bin/sh
mount -t sysfs none /sys
mount -t configfs none /config
mount -t devtmpfs none /dev

echo "+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++"
echo "+++++++++++++++++++++++++++++++ HELLOO ++++++++++++++++++++++++++++++"
echo "+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++"

EDIT2

I removed console=tty0 from the GRUB config and the kernel booted to shell. But I am not sure why it caused hang in this virtual environment?

I am thinking that since tty0 represent current display and since -display none is set for qemu, this mixup creates some conflict and hence this issue...

But please someone give a correct and concrete conclusion on this issue so that this thread becomes useful to someone in future.

Solution

You created the filesystem on the entire volume (file), not on a partition. If you want it on a partition, then you would have to write a partition table in the file and create the filesystem in the partition.
Edit. Not sure if grub even has a way to boot with no GPT or MBR.

To use this image partitioned, create a 1MB larger file, write the GPT partition table on the first megabyte, use dd to copy your image to the rest.

dd obs=1M seek=1 rootfs.img newroot.img

Answered By - stark