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lgadban / debug-linux

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Install build deps. In fedora silverblue, enter a toolsbox first.

toolbox enter

sudo dnf install -y qemu-img qemu-kvm
sudo dnf install -y gcc pkg-config ncurses-devel flex bison elfutils-libelf-devel openssl-devel dwarves kmod

Build VM

# build a debian image with tools
packer build packer.json

We expect linux to be present in a folder called linux. you can get it there for example with git clone https://github.com/torvalds/linux.git

Build linux. if make ask questions, just press enter.

cp config ./linux/.config
make -C linux
# add linux gdb scripts
mkdir -p ~/.config/gdb/
echo "add-auto-load-safe-path $PWD/linux/scripts/gdb/vmlinux-gdb.py" >> ~/.config/gdb/gdbinit

To be able to revert, build a diff qemu image. to revert just run this command again

qemu-img create -F qcow2 -b qemu-images/my-build.qcow2 -f qcow2 img.qcow2

Start the VM. it will not do anything until you connect a debugger to it and hit "continue".

Explanation of args:

  • kernel - the kernel that qemu loads. this means that the kernel in the image is ignored, and the kernel headers will not be present, nor can you get them with "apt-get". If config from this repo is used, kernel headers will be available in /proc
  • serial - output serial consul to stdin/stdout. you can use this to login without ssh
  • hda - the disk to use
  • net - create a network interface so we have internet access
  • m - memory
  • nographic - don't open a window
  • append - Arguments to the kernel. important to have nokaslr there for debugging to work.
  • virtfs - make the current directory available to the guest
  • -s start gdb stub
  • -S essentially waits until gdb connects
  1 qemu-system-x86_64 \
  2         -kernel ./linux/arch/x86/boot/bzImage \
  3         -serial mon:stdio \
  4         -hda custom-deb11.qcow2 \
  5         -net nic \
  6         -net user,hostfwd=tcp::10022-:22 \
  7         -smp 4 \
  8         -m 16G \
  9         -nographic \
 10         -append "root=/dev/sda1 rdinit=/sbin/init console=ttyS0 nokaslr" \
 11         -virtfs local,path=$PWD,mount_tag=host0,security_model=mapped,id=host0 \
 12         -s -S
qemu-kvm \
  -kernel ./linux/arch/x86/boot/bzImage \
  -serial mon:stdio \
  -hda img.qcow2 \
  -net nic -net user,hostfwd=tcp::10022-:22 \
  -m 8192 -nographic \
  -append "root=/dev/sda1 rdinit=/sbin/init console=ttyS0 nokaslr" \
  -virtfs local,path=$PWD,mount_tag=host0,security_model=mapped,id=host0 \
  -s -S

debug with gdb ./linux/vmlinux -ex "target remote localhost:1234". Or with vscode you can cp launch.json ./.vscode/launch.json and hit debug.

Ssh into the VM with:

ssh -p 10022 debian@localhost

Though if you are testing networking, ssh-ing in might add noise.

Sources

https://superuser.com/questions/628169/how-to-share-a-directory-with-the-host-without-networking-in-qemu

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