Documentation/gdb-kernel-debugging.txt

   1 Debugging kernel and modules via gdb
   2 ====================================
   3
   4 The kernel debugger kgdb, hypervisors like QEMU or JTAG-based hardware
   5 interfaces allow to debug the Linux kernel and its modules during runtime
   6 using gdb. Gdb comes with a powerful scripting interface for python. The
   7 kernel provides a collection of helper scripts that can simplify typical
   8 kernel debugging steps. This is a short tutorial about how to enable and use
   9 them. It focuses on QEMU/KVM virtual machines as target, but the examples can
  10 be transferred to the other gdb stubs as well.
  11
  12
  13 Requirements
  14 ------------
  15
  16  o gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
  17    for distributions)
  18
  19
  20 Setup
  21 -----
  22
  23  o Create a virtual Linux machine for QEMU/KVM (see www.linux-kvm.org and
  24    www.qemu.org for more details). For cross-development,
  25    http://landley.net/aboriginal/bin keeps a pool of machine images and
  26    toolchains that can be helpful to start from.
  27
  28  o Build the kernel with CONFIG_GDB_SCRIPTS enabled, but leave
  29    CONFIG_DEBUG_INFO_REDUCED off. If your architecture supports
  30    CONFIG_FRAME_POINTER, keep it enabled.
  31
  32  o Install that kernel on the guest.
  33
  34    Alternatively, QEMU allows to boot the kernel directly using -kernel,
  35    -append, -initrd command line switches. This is generally only useful if
  36    you do not depend on modules. See QEMU documentation for more details on
  37    this mode.
  38
  39  o Enable the gdb stub of QEMU/KVM, either
  40     - at VM startup time by appending "-s" to the QEMU command line
  41    or
  42     - during runtime by issuing "gdbserver" from the QEMU monitor
  43       console
  44
  45  o cd /path/to/linux-build
  46
  47  o Start gdb: gdb vmlinux
  48
  49    Note: Some distros may restrict auto-loading of gdb scripts to known safe
  50    directories. In case gdb reports to refuse loading vmlinux-gdb.py, add
  51
  52     add-auto-load-safe-path /path/to/linux-build
  53
  54    to ~/.gdbinit. See gdb help for more details.
  55
  56  o Attach to the booted guest:
  57     (gdb) target remote :1234
  58
  59
  60 Examples of using the Linux-provided gdb helpers
  61 ------------------------------------------------
  62
  63  o Load module (and main kernel) symbols:
  64     (gdb) lx-symbols
  65     loading vmlinux
  66     scanning for modules in /home/user/linux/build
  67     loading @0xffffffffa0020000: /home/user/linux/build/net/netfilter/xt_tcpudp.ko
  68     loading @0xffffffffa0016000: /home/user/linux/build/net/netfilter/xt_pkttype.ko
  69     loading @0xffffffffa0002000: /home/user/linux/build/net/netfilter/xt_limit.ko
  70     loading @0xffffffffa00ca000: /home/user/linux/build/net/packet/af_packet.ko
  71     loading @0xffffffffa003c000: /home/user/linux/build/fs/fuse/fuse.ko
  72     ...
  73     loading @0xffffffffa0000000: /home/user/linux/build/drivers/ata/ata_generic.ko
  74
  75  o Set a breakpoint on some not yet loaded module function, e.g.:
  76     (gdb) b btrfs_init_sysfs
  77     Function "btrfs_init_sysfs" not defined.
  78     Make breakpoint pending on future shared library load? (y or [n]) y
  79     Breakpoint 1 (btrfs_init_sysfs) pending.
  80
  81  o Continue the target
  82     (gdb) c
  83
  84  o Load the module on the target and watch the symbols being loaded as well as
  85    the breakpoint hit:
  86     loading @0xffffffffa0034000: /home/user/linux/build/lib/libcrc32c.ko
  87     loading @0xffffffffa0050000: /home/user/linux/build/lib/lzo/lzo_compress.ko
  88     loading @0xffffffffa006e000: /home/user/linux/build/lib/zlib_deflate/zlib_deflate.ko
  89     loading @0xffffffffa01b1000: /home/user/linux/build/fs/btrfs/btrfs.ko
  90
  91     Breakpoint 1, btrfs_init_sysfs () at /home/user/linux/fs/btrfs/sysfs.c:36
  92     36              btrfs_kset = kset_create_and_add("btrfs", NULL, fs_kobj);
  93
  94  o Dump the log buffer of the target kernel:
  95     (gdb) lx-dmesg
  96     [     0.000000] Initializing cgroup subsys cpuset
  97     [     0.000000] Initializing cgroup subsys cpu
  98     [     0.000000] Linux version 3.8.0-rc4-dbg+ (...
  99     [     0.000000] Command line: root=/dev/sda2 resume=/dev/sda1 vga=0x314
 100     [     0.000000] e820: BIOS-provided physical RAM map:
 101     [     0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
 102     [     0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
 103     ....
 104
 105  o Examine fields of the current task struct:
 106     (gdb) p $lx_current().pid
 107     $1 = 4998
 108     (gdb) p $lx_current().comm
 109     $2 = "modprobe\000\000\000\000\000\000\000"
 110
 111  o Make use of the per-cpu function for the current or a specified CPU:
 112     (gdb) p $lx_per_cpu("runqueues").nr_running
 113     $3 = 1
 114     (gdb) p $lx_per_cpu("runqueues", 2).nr_running
 115     $4 = 0
 116
 117  o Dig into hrtimers using the container_of helper:
 118     (gdb) set $next = $lx_per_cpu("hrtimer_bases").clock_base[0].active.next
 119     (gdb) p *$container_of($next, "struct hrtimer", "node")
 120     $5 = {
 121       node = {
 122         node = {
 123           __rb_parent_color = 18446612133355256072,
 124           rb_right = 0x0 <irq_stack_union>,
 125           rb_left = 0x0 <irq_stack_union>
 126         },
 127         expires = {
 128           tv64 = 1835268000000
 129         }
 130       },
 131       _softexpires = {
 132         tv64 = 1835268000000
 133       },
 134       function = 0xffffffff81078232 <tick_sched_timer>,
 135       base = 0xffff88003fd0d6f0,
 136       state = 1,
 137       start_pid = 0,
 138       start_site = 0xffffffff81055c1f <hrtimer_start_range_ns+20>,
 139       start_comm = "swapper/2\000\000\000\000\000\000"
 140     }
 141
 142
 143 List of commands and functions
 144 ------------------------------
 145
 146 The number of commands and convenience functions may evolve over the time,
 147 this is just a snapshot of the initial version:
 148
 149  (gdb) apropos lx
 150  function lx_current -- Return current task
 151  function lx_module -- Find module by name and return the module variable
 152  function lx_per_cpu -- Return per-cpu variable
 153  function lx_task_by_pid -- Find Linux task by PID and return the task_struct variable
 154  function lx_thread_info -- Calculate Linux thread_info from task variable
 155  lx-dmesg -- Print Linux kernel log buffer
 156  lx-lsmod -- List currently loaded modules
 157  lx-symbols -- (Re-)load symbols of Linux kernel and currently loaded modules
 158
 159 Detailed help can be obtained via "help <command-name>" for commands and "help
 160 function <function-name>" for convenience functions.