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.