I followed the steps here and compiled the kernel successfully in usermode: https://btrfs.wiki.kernel.org/index.php/Debugging_Btrfs_with_GDB

But when I start ./linux in various ways it always gives me a very similar error:

pc@linux-94q0:~/linux-4.11-rc4> ./linux root=/mnt
Core dump limits :
    soft - 0
    hard - NONE
Checking that ptrace can change system call numbers...OK
Checking syscall emulation patch for ptrace...OK
Checking advanced syscall emulation patch for ptrace...OK
Checking environment variables for a tempdir...none found
Checking if /dev/shm is on tmpfs...OK
Checking PROT_EXEC mmap in /dev/shm...OK
Adding 33251328 bytes to physical memory to account for exec-shield gap
Linux version 4.11.0-rc4 (pc@linux-94q0) (gcc version 4.8.5 (SUSE Linux) ) #1 Fri Mar 31 12:40:07 CEST 2017
Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 16087
Kernel command line: root=/mnt
PID hash table entries: 256 (order: -1, 2048 bytes)
Dentry cache hash table entries: 8192 (order: 4, 65536 bytes)
Inode-cache hash table entries: 4096 (order: 3, 32768 bytes)
Memory: 26140K/65240K available (3518K kernel code, 770K rwdata, 948K rodata, 114K init, 195K bss, 39100K reserved, 0K cma-reserved)
clocksource: timer: mask: 0xffffffffffffffff max_cycles: 0x1cd42e205, max_idle_ns: 881590404426 ns
Calibrating delay loop... 6966.47 BogoMIPS (lpj=34832384)
pid_max: default: 32768 minimum: 301
Mount-cache hash table entries: 512 (order: 0, 4096 bytes)
Mountpoint-cache hash table entries: 512 (order: 0, 4096 bytes)
Checking that host ptys support output SIGIO...Yes
Checking that host ptys support SIGIO on close...No, enabling workaround
devtmpfs: initialized
Using 2.6 host AIO
clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 19112604462750000 ns
futex hash table entries: 256 (order: 0, 6144 bytes)
xor: measuring software checksum speed
   8regs     : 19749.600 MB/sec
   8regs_prefetch: 17312.000 MB/sec
   32regs    : 18694.400 MB/sec
   32regs_prefetch: 17317.600 MB/sec
xor: using function: 8regs (19749.600 MB/sec)
NET: Registered protocol family 16
raid6: int64x1  gen()  4139 MB/s
raid6: int64x1  xor()  2318 MB/s
raid6: int64x2  gen()  3758 MB/s
raid6: int64x2  xor()  2685 MB/s
raid6: int64x4  gen()  3413 MB/s
raid6: int64x4  xor()  2153 MB/s
raid6: int64x8  gen()  2865 MB/s
raid6: int64x8  xor()  1626 MB/s
raid6: using algorithm int64x1 gen() 4139 MB/s
raid6: .... xor() 2318 MB/s, rmw enabled
raid6: using intx1 recovery algorithm
clocksource: Switched to clocksource timer
VFS: Disk quotas dquot_6.6.0
VFS: Dquot-cache hash table entries: 512 (order 0, 4096 bytes)
NET: Registered protocol family 2
TCP established hash table entries: 512 (order: 0, 4096 bytes)
TCP bind hash table entries: 512 (order: 0, 4096 bytes)
TCP: Hash tables configured (established 512 bind 512)
UDP hash table entries: 256 (order: 1, 8192 bytes)
UDP-Lite hash table entries: 256 (order: 1, 8192 bytes)
NET: Registered protocol family 1
console [stderr0] disabled
mconsole (version 2) initialized on /home/pc/.uml/y33GMV/mconsole
Checking host MADV_REMOVE support...OK
workingset: timestamp_bits=62 max_order=13 bucket_order=0
io scheduler noop registered
io scheduler deadline registered (default)
io scheduler mq-deadline registered
NET: Registered protocol family 17
Initialized stdio console driver
Console initialized on /dev/tty0
console [tty0] enabled
Initializing software serial port version 1
console [mc-1] enabled
Failed to initialize ubd device 0 :Couldn't determine size of device's file
Btrfs loaded, crc32c=crc32c-generic, debug=on
VFS: Cannot open root device "/mnt" or unknown-block(0,0): error -6
Please append a correct "root=" boot option; here are the available partitions:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
CPU: 0 PID: 1 Comm: swapper Not tainted 4.11.0-rc4 #1
 6381bd80 60066344 602a250a 62cab500
 602a250a 600933ba 6381bd90 60297e6f
 6381beb0 60092b41 6381be30 60380ea1
Call Trace:
 [<600933ba>] ? 
 [<60066344>] ? 
 [<602a250a>] ? 
 [<602a250a>] ? 
 [<600933ba>] ? 
 [<60380ea1>] ? 
 [<600929ce>] ? 
 [<600cac33>] ? 
 [<600f01da>] ? 
 [<600933ba>] ? 
 [<600f012c>] ? 
 [<6029e3f9>] ? 
 [<6029e543>] ? 
 [<600eb9d3>] ? 
 [<600933ba>] ? 

Aborted (core dumped)

I have now tried everything I could think of to satisfy the ./linux root= option but nothing seems to work.

  1. I created a root filesystem with https://buildroot.org/, passed that as .gz, .tar, .tar.gz, uncompressed folder
  2. I put the contents of the buildroot.org into the btrfs loop device, then right clicked in the disks utility and created an .img file. Tried starting with that.
  3. Of course I tried all the usual options I could think of, like ./linux root=/mnt, ./linux root=/dev/loop0

I don't know what else to try. Why is this not working? I tried finding out what the -6 error code means, but it seems all the Linux kernel error codes are positive numbers. https://gist.github.com/nullunar/4553641

I really don't know what else to do, I guess I could start reading up for hours and hours about what exactly the udb stuff means, but I was really hoping somebody could just tell me what I need to pass to the command line as my interest right now is only in debugging btrfs, not Linux in general.


From https://www.linux.com/news/how-run-linux-inside-linux-user-mode-linux :

./linux-2.6.19-rc5 ubda=FedoraCore5-x86-root_fs mem=128M
The ubda parameter in this command is giving the UML kernel the name of a file to use to create the virtual machine's /dev/ubda virtual block device, which will be its root filesystem.
| improve this answer | |
  • Thank you, I actually get the feeling that I found that article before but it looked too much like blog spam for me to actually read enough. It's a very good article. – Blub Mar 31 '17 at 15:52
  • The reason why my filesystem created from buildroot.org probably didn't work, was because the format was wrong. It has to be in 'rawdisk' format, which seems to be a file created by dd and then populated with the necessary content and partitions and partition tables via extra tools. – Blub Mar 31 '17 at 15:56
  • What would be nice to know though, is why the root filesystem is read-only. – Blub Mar 31 '17 at 15:58

I have described a working Buildroot setup in great detail at: How to create rootfs for user mode Linux on Fedora 18?

The main catch is that I had to hack up the BusyBox init to add ::sysinit:/sbin/mdev -s to the inittab.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.