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KERNEL.md

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Running a debuggable Linux kernel with QEMU

Setting up the kernel

  1. Make the following directory structure:

    kernel-module
│   ├── build
│   │   ├── busybox-1.32.0
│   │   ├── linux-5.9.1
│   │   └── musl-1.2.1
│   ├── src
│   │   ├── busybox-1.32.0
│   │   ├── linux-5.9.1
│   │   └── musl-1.2.1

  2. Download sources:

  3. Assuming current directory is kernel-module/src/linux-5.9.1, run the following commands:

    $ make O=../../build/linux-5.9.1 x86_64_defconfig
$ make O=../../build/linux-5.9.1 kvmconfig
$ make O=../../build/linux-5.9.1 menuconfig

    A menu will pop up. Find and enable the following options in it:

    • DEBUG_KERNEL (Kernel hacking -> Kernel debugging)
    • DEBUG_INFO (Kernel hacking -> Compile-time checks and compiler options -> Compile the kernel with debug info)
    • CONFIG_GDB_SCRIPTS (Kernel hacking -> Compile-time checks and compiler options -> Compile the kernel with debug info -> Provide GDB scripts for kernel debugging)
    • CONFIG_KGDB (Kernel hacking -> Generic Kernel Debugging Instruments -> KGDB: kernel debugger)
    • CONFIG_KGDB_SERIAL_CONSOLE (Kernel hacking -> Generic Kernel Debugging Instruments -> KGDB: kernel debugger -> KGDB: use kgdb over the serial console)
    • UBSAN (Kernel hacking -> Generic Kernel Debugging Instruments -> Undefined behaviour sanity checker)
    • KASAN (Kernel hacking -> Memory debugging -> KASAN: runtime memory debugger)
    • CONFIG_KALLSYMS (General setup -> Load all symbols for debugging/ksymoops)
    • CONFIG_KALLSYMS_ALL (General setup -> Load all symbols for debugging/ksymoops -> Include all symbols in kallsyms)
    • CONFIG_IKCONFIG (General setup -> Kernel .config support)
    • CONFIG_IKCONFIG_PROC (General setup -> Kernel .config support -> Enable access to .config through /proc/config.gz)

    Press / and find the following options. Make sure that they are enabled. If you see that there are options that need to be enabled as preconditions for them, enable them too.

    • DEBUG_BUGVERBOSE
    • CONFIG_FRAME_POINTER

    Close the menu with saving configuration.

  4. Build the kernel: make O=../../build/linux-5.9.1/ -j4. Parameter j stands for thread count.

  5. BusyBox provides a lot of standard Unix utilities (like ls, pwd, cp, etc.) as a single executable. We need to build it as a static binary. Some linux distributions (e.g. Arch Linux) do not provide static version of libc, that's why wee need to build musl-libc first (musl is a light-weight implementation of libc). Assuming current directory is kernel-module/build/musl-1.2.1, run the following commands:

    $ MUSL_BASE=`pwd`
$ ../../src/musl-1.2.1/configure --prefix=$MUSL_BASE
$ make
$ make install
$
$ cd $MUSL_BASE/bin
$ ln -s `which ar` musl-ar
$ ln -s `which strip` musl-strip
$ cd $MUSL_BASE/include
$ ln -s /usr/include/linux linux
$ ln -s /usr/include/mtd mtd
$ ln -s /usr/include/asm asm
$ ln -s /usr/include/asm-generic asm-generic
$
$ PATH=$MUSL_BASE/bin:$PATH

  6. Assuming current directory is kernel-module/src/busybox-1.32.0, run the following commands:

    $ make O=../../build/busybox-1.32.0 defconfig
$ make O=../../build/busybox-1.32.0 menuconfig

    A menu will pop up. Find and enable STATIC option in Settings -> Build static binary (no shared libs). Also set CROSS_COMPILER_PREFIX to musl- in Settings -> Cross compiler prefix. Close the menu with saving configuration.

  7. Go to kernel-module/build/busybox-1.32.0 and build BusyBox:

    $ make -j4
$ make install

  8. Now we need to create a minimal file system. Create directory kernel-module/build/initramfs and go into it. Run:

    $ mkdir -pv {bin,sbin,etc,proc,sys,usr/{bin,sbin}}
$ cp -rv ../busybox-1.31.1/_install/* .

  9. Create init script:

    $ touch init
$ chmod +x init

    Paste the following code to it:

    #!/bin/sh

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

echo "Welcome to your Linux kernel!"

exec /bin/sh

  10. Build initrams file: find . -print0 | cpio --null -ov --format=newc | gzip -9 > ../initramfs.cpio.gz. It is recommended to create a shell script make-initramfs.sh doing it and place it in the parent directory for future use:

    #!/bin/sh

cd initramfs
find . -print0 | cpio --null -ov --format=newc | gzip -9 > ../initramfs.cpio.gz
cd ..

  11. Go to kernel-module directory and create a shell script start.sh with the following content:

    #!/bin/sh

qemu-system-x86_64 \
    -kernel build/linux-5.9.1/arch/x86_64/boot/bzImage \
    -initrd build/initramfs.cpio.gz \
    -nographic \
    -append "console=ttyS0 nokaslr" \
    -cpu host \
    -enable-kvm \
    -gdb tcp::1234

  12. Run ./start.sh and check that everything works properly. To exit Qemu, press Ctrl+a -> c -> q -> Enter.

Debugging the kernel

  1. Create debug.sh shell script with the following command: gdb build/linux-5.9.1/vmlinux. Run it. If gdb asks you to add some path to .gdbinit, do it.

  2. In gdb, run the following commands:

    (gdb) lx-symbols
(gdb) target remote :1234

    Breakpoints are set like this: b fs/read_write.c:452. To continue running guest kernel, enter c. To break debuggee execution in any place, press Ctrl+c.