Libguestfs - Aug 2023 - debug a corrupted VHDX file with libguestfs

Hi

I am using libguestfs-1.46.2 on Ubuntu 22.04

I already enabled
export LIBGUESTFS_DEBUG=1
export LIBGUESTFS_TRACE=1

I want to mount a vhdx file (which is known to be corrupted) with
root at HenrikUbuntu2204:/mnt/sdj# guestmount -a
c1eef115-00000006-ec888b8d-64888b8d-00155000-4a448456 -m /dev/sda1 --ro
/mnt/sdk/

where I can see the following error message from output
[   46.629055] EXT4-fs (sda1): ext4_check_descriptors: Block bitmap for group
524288 not in group (block 0)!
[   46.629656] EXT4-fs (sda1): group descriptors corrupted!
command: mount returned 32

I want to actually add debug logs into ext4_check_descriptors with printk
To print out the hex values of superblock in the corrupted vhdx file

I tried to recompile Ubuntu kernel and add some printk messages, they don't
show up in dmesg when I run
root at HenrikUbuntu2204:/mnt/sdj# guestmount -a
c1eef115-00000006-ec888b8d-64888b8d-00155000-4a448456 -m /dev/sda1 --ro
/mnt/sdk/

I guess the following messages are from libvirt / KVM ?
[   46.629055] EXT4-fs (sda1): ext4_check_descriptors: Block bitmap for group
524288 not in group (block 0)!
[   46.629656] EXT4-fs (sda1): group descriptors corrupted!

How do I use printk to add logs in the kernel messages from libguestfs?



Regards
Henrik



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On Fri, Aug 04, 2023 at 10:37:50PM +0000, Lin, HongYi - Dell Team
wrote:
> I tried to recompile Ubuntu kernel and add some printk messages, they don?t
> show up in dmesg when I run
> 
> root at HenrikUbuntu2204:/mnt/sdj# guestmount -a
> c1eef115-00000006-ec888b8d-64888b8d-00155000-4a448456 -m /dev/sda1 --ro
/mnt/
> sdk/
>  
> 
> I guess the following messages are from libvirt / KVM ?
> 
> [   46.629055] EXT4-fs (sda1): ext4_check_descriptors: Block bitmap for
group
> 524288 not in group (block 0)!
> 
> [   46.629656] EXT4-fs (sda1): group descriptors corrupted!
These messages are from the kernel.

However you will need to tell libguestfs to use your recompiled
kernel.  This can be done easily by setting some environment
variables.  This is what I do when I want to test a new kernel:

  rm -rf /tmp/kernel/*
  make
  make INSTALL_MOD_PATH=/tmp/kernel modules_install

  # In case the kernel modules are built with debug symbols:
  find /tmp/kernel/ -name '*.ko' -exec strip --strip-debug {} \;

  modules=`echo /tmp/kernel/lib/modules/*/`
  export SUPERMIN_KERNEL=$PWD/arch/x86/boot/bzImage
  export SUPERMIN_MODULES=$modules

  rm -rf /var/tmp/.guestfs-`id -u`/

Then run guestmount in the usual way.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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