.. highlight:: none

Debugging kernel and modules via gdb
====================================

The kernel debugger kgdb, hypervisors like QEMU or JTAG-based hardware
interfaces allow to debug the Linux kernel and its modules during runtime
using gdb. Gdb comes with a powerful scripting interface for python. The
kernel provides a collection of helper scripts that can simplify typical
kernel debugging steps. This is a short tutorial about how to enable and use
them. It focuses on QEMU/KVM virtual machines as target, but the examples can
be transferred to the other gdb stubs as well.


Requirements
------------

- gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
  for distributions)


Setup
-----

- Create a virtual Linux machine for QEMU/KVM (see www.linux-kvm.org and
  www.qemu.org for more details). For cross-development,
  http://landley.net/aboriginal/bin keeps a pool of machine images and
  toolchains that can be helpful to start from.

- Build the kernel with CONFIG_GDB_SCRIPTS enabled, but leave
  CONFIG_DEBUG_INFO_REDUCED off. If your architecture supports
  CONFIG_FRAME_POINTER, keep it enabled.

- Install that kernel on the guest.
  Alternatively, QEMU allows to boot the kernel directly using -kernel,
  -append, -initrd command line switches. This is generally only useful if
  you do not depend on modules. See QEMU documentation for more details on
  this mode.

- Enable the gdb stub of QEMU/KVM, either

    - at VM startup time by appending "-s" to the QEMU command line

  or

    - during runtime by issuing "gdbserver" from the QEMU monitor
      console

- cd /path/to/linux-build

- Start gdb: gdb vmlinux

  Note: Some distros may restrict auto-loading of gdb scripts to known safe
  directories. In case gdb reports to refuse loading vmlinux-gdb.py, add::

    add-auto-load-safe-path /path/to/linux-build

  to ~/.gdbinit. See gdb help for more details.

- Attach to the booted guest::

    (gdb) target remote :1234


Examples of using the Linux-provided gdb helpers
------------------------------------------------

- Load module (and main kernel) symbols::

    (gdb) lx-symbols
    loading vmlinux
    scanning for modules in /home/user/linux/build
    loading @0xffffffffa0020000: /home/user/linux/build/net/netfilter/xt_tcpudp.ko
    loading @0xffffffffa0016000: /home/user/linux/build/net/netfilter/xt_pkttype.ko
    loading @0xffffffffa0002000: /home/user/linux/build/net/netfilter/xt_limit.ko
    loading @0xffffffffa00ca000: /home/user/linux/build/net/packet/af_packet.ko
    loading @0xffffffffa003c000: /home/user/linux/build/fs/fuse/fuse.ko
    ...
    loading @0xffffffffa0000000: /home/user/linux/build/drivers/ata/ata_generic.ko

- Set a breakpoint on some not yet loaded module function, e.g.::

    (gdb) b btrfs_init_sysfs
    Function "btrfs_init_sysfs" not defined.
    Make breakpoint pending on future shared library load? (y or [n]) y
    Breakpoint 1 (btrfs_init_sysfs) pending.

- Continue the target::

    (gdb) c

- Load the module on the target and watch the symbols being loaded as well as
  the breakpoint hit::

    loading @0xffffffffa0034000: /home/user/linux/build/lib/libcrc32c.ko
    loading @0xffffffffa0050000: /home/user/linux/build/lib/lzo/lzo_compress.ko
    loading @0xffffffffa006e000: /home/user/linux/build/lib/zlib_deflate/zlib_deflate.ko
    loading @0xffffffffa01b1000: /home/user/linux/build/fs/btrfs/btrfs.ko

    Breakpoint 1, btrfs_init_sysfs () at /home/user/linux/fs/btrfs/sysfs.c:36
    36              btrfs_kset = kset_create_and_add("btrfs", NULL, fs_kobj);

- Dump the log buffer of the target kernel::

    (gdb) lx-dmesg
    [     0.000000] Initializing cgroup subsys cpuset
    [     0.000000] Initializing cgroup subsys cpu
    [     0.000000] Linux version 3.8.0-rc4-dbg+ (...
    [     0.000000] Command line: root=/dev/sda2 resume=/dev/sda1 vga=0x314
    [     0.000000] e820: BIOS-provided physical RAM map:
    [     0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
    [     0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
    ....

- Examine fields of the current task struct::

    (gdb) p $lx_current().pid
    $1 = 4998
    (gdb) p $lx_current().comm
    $2 = "modprobe\000\000\000\000\000\000\000"

- Make use of the per-cpu function for the current or a specified CPU::

    (gdb) p $lx_per_cpu("runqueues").nr_running
    $3 = 1
    (gdb) p $lx_per_cpu("runqueues", 2).nr_running
    $4 = 0

- Dig into hrtimers using the container_of helper::

    (gdb) set $next = $lx_per_cpu("hrtimer_bases").clock_base[0].active.next
    (gdb) p *$container_of($next, "struct hrtimer", "node")
    $5 = {
      node = {
        node = {
          __rb_parent_color = 18446612133355256072,
          rb_right = 0x0 <irq_stack_union>,
          rb_left = 0x0 <irq_stack_union>
        },
        expires = {
          tv64 = 1835268000000
        }
      },
      _softexpires = {
        tv64 = 1835268000000
      },
      function = 0xffffffff81078232 <tick_sched_timer>,
      base = 0xffff88003fd0d6f0,
      state = 1,
      start_pid = 0,
      start_site = 0xffffffff81055c1f <hrtimer_start_range_ns+20>,
      start_comm = "swapper/2\000\000\000\000\000\000"
    }


List of commands and functions
------------------------------

The number of commands and convenience functions may evolve over the time,
this is just a snapshot of the initial version::

 (gdb) apropos lx
 function lx_current -- Return current task
 function lx_module -- Find module by name and return the module variable
 function lx_per_cpu -- Return per-cpu variable
 function lx_task_by_pid -- Find Linux task by PID and return the task_struct variable
 function lx_thread_info -- Calculate Linux thread_info from task variable
 lx-dmesg -- Print Linux kernel log buffer
 lx-lsmod -- List currently loaded modules
 lx-symbols -- (Re-)load symbols of Linux kernel and currently loaded modules

Detailed help can be obtained via "help <command-name>" for commands and "help
function <function-name>" for convenience functions.