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Debugging linux kernel with GDB and Qemu

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Linux debug gdb qemu kernel

Intro.

In this page I’ll briefly introduce how to setup the environment for debugging a linux kernel with GDB and Qemu. The linux kernel runs on Qemu being virtualized.

Process

  1. Build the linux kernel that you want to debug.
    • Check the configuration. Set CONFIG_DEBUG_INFO=y if not configured.
    • You can build the kernel as deb or rpm to install on remote servers. Use commands: make bindep-pkg -j4, make rpm-pkg -j4. (In the case of deb, package alien is required. Install it with apt.)
  2. Install qemu. Refer to the official site.
    • Configure example: ./configure --target-list=x86_64-softmmu --enable-debug
  3. Install VM. (In this example, I installed ubuntu 16.04.)
    • Download ubuntu iso file.
    • Create qemu img named as ‘test.qcow2’: ./qemu-img create -f qcow2 test.qcow2 16G
    • Boot VM with ubuntu iso image: x86_64-softmmu/qemu-system-x86_64 -m 2048 -enable-kvm -drive if=virtio,file=test.qcow2,cache=none -cdrom ubuntu-16.04.5.iso
    • You can use -vnc :1 option when accessing remotly through ssh.
    • After installation, start VM without the --cdrom ~~.iso option.
  4. Replace the default kernel of VM with the version you built.
    • Copy deb files to the VM and sudo dpkg -i *.deb.
  5. Add kernel parameter nokaslr. It is required to make breakpoint work correctly.
    • Append nokaslr to the GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub.
    • Execute update-grub
    • Restart the VM.
  6. Make symbol file from vmlinux kernel image. vmlinux file is located in the kernel source directory (generated after compilation). objcopy --only-keep-debug vmlinux kernel.sym

  7. Run the qemu with the following options. -S stops qemu waiting gdb and -s makes gdb be able to attach through localhost:1234.

    # Example for qemu execution.
./x86_64-softmmu/qemu-system-x86_64 -s -S \
-m 2048 \
-chardev stdio,id=gdb0 \
-drive if=virtio,file=nfstest.qcow2,cache=none \
-device isa-debugcon,iobase=0x402,chardev=gdb0,id=d1 \
-vga virtio \
-enable-kvm \
-vnc :1

    The qemu stops waiting for the debugger to be attached.

  8. Run gdb, load symbol file, and attach to the qemu execution.

    gdb
(gdb) file ./kernel.sym
(gdb) target remote :1234
(gdb) hbreak start_kernel
(gdb) c

    After type c, qemu continues to run and break at the start_kernel breakpoint.

Other issues

  • gdb does not find the source directory.
    • Use dir /src/dir/path command. (Not tested.)

  • Change optimization options of a kernel compilation to eliminate any confusion while tracking the code with gdb.

    • Set -Og options for the files. As below example.

      # Added to mm/Makefile for debugging.
CFLAGS_ksm.o = -Og
CFLAGS_huge_memory.o = -Og
CFLAGS_memory.o = -Og
CFLAGS_migrate.o = -Og
CFLAGS_page_alloc.o = -Og

References

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