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
  ------------
  
   o gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
     for distributions)
  
  
  Setup
  -----
  
   o 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.
  
   o 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.
  
   o 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.
  
   o 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
  
   o cd /path/to/linux-build
  
   o 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.
  
   o Attach to the booted guest:
      (gdb) target remote :1234
  
  
  Examples of using the Linux-provided gdb helpers
  ------------------------------------------------
  
   o 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
  
   o 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.
  
   o Continue the target
      (gdb) c
  
   o 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);
  
   o 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
      ....
  
   o 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"
  
   o 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
  
   o 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.