Documentation/dev-tools/gdb-kernel-debugging.rst

   1 .. highlight:: none
   2
   3 Debugging kernel and modules via gdb
   4 ====================================
   5
   6 The kernel debugger kgdb, hypervisors like QEMU or JTAG-based hardware
   7 interfaces allow to debug the Linux kernel and its modules during runtime
   8 using gdb. Gdb comes with a powerful scripting interface for python. The
   9 kernel provides a collection of helper scripts that can simplify typical
  10 kernel debugging steps. This is a short tutorial about how to enable and use
  11 them. It focuses on QEMU/KVM virtual machines as target, but the examples can
  12 be transferred to the other gdb stubs as well.
  13
  14
  15 Requirements
  16 ------------
  17
  18 - gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
  19   for distributions)
  20
  21
  22 Setup
  23 -----
  24
  25 - Create a virtual Linux machine for QEMU/KVM (see www.linux-kvm.org and
  26   www.qemu.org for more details). For cross-development,
  27   http://landley.net/aboriginal/bin keeps a pool of machine images and
  28   toolchains that can be helpful to start from.
  29
  30 - Build the kernel with CONFIG_GDB_SCRIPTS enabled, but leave
  31   CONFIG_DEBUG_INFO_REDUCED off. If your architecture supports
  32   CONFIG_FRAME_POINTER, keep it enabled.
  33
  34 - Install that kernel on the guest, turn off KASLR if necessary by adding
  35   "nokaslr" to the kernel command line.
  36   Alternatively, QEMU allows to boot the kernel directly using -kernel,
  37   -append, -initrd command line switches. This is generally only useful if
  38   you do not depend on modules. See QEMU documentation for more details on
  39   this mode. In this case, you should build the kernel with
  40   CONFIG_RANDOMIZE_BASE disabled if the architecture supports KASLR.
  41
  42 - Enable the gdb stub of QEMU/KVM, either
  43
  44     - at VM startup time by appending "-s" to the QEMU command line
  45
  46   or
  47
  48     - during runtime by issuing "gdbserver" from the QEMU monitor
  49       console
  50
  51 - cd /path/to/linux-build
  52
  53 - Start gdb: gdb vmlinux
  54
  55   Note: Some distros may restrict auto-loading of gdb scripts to known safe
  56   directories. In case gdb reports to refuse loading vmlinux-gdb.py, add::
  57
  58     add-auto-load-safe-path /path/to/linux-build
  59
  60   to ~/.gdbinit. See gdb help for more details.
  61
  62 - Attach to the booted guest::
  63
  64     (gdb) target remote :1234
  65
  66
  67 Examples of using the Linux-provided gdb helpers
  68 ------------------------------------------------
  69
  70 - Load module (and main kernel) symbols::
  71
  72     (gdb) lx-symbols
  73     loading vmlinux
  74     scanning for modules in /home/user/linux/build
  75     loading @0xffffffffa0020000: /home/user/linux/build/net/netfilter/xt_tcpudp.ko
  76     loading @0xffffffffa0016000: /home/user/linux/build/net/netfilter/xt_pkttype.ko
  77     loading @0xffffffffa0002000: /home/user/linux/build/net/netfilter/xt_limit.ko
  78     loading @0xffffffffa00ca000: /home/user/linux/build/net/packet/af_packet.ko
  79     loading @0xffffffffa003c000: /home/user/linux/build/fs/fuse/fuse.ko
  80     ...
  81     loading @0xffffffffa0000000: /home/user/linux/build/drivers/ata/ata_generic.ko
  82
  83 - Set a breakpoint on some not yet loaded module function, e.g.::
  84
  85     (gdb) b btrfs_init_sysfs
  86     Function "btrfs_init_sysfs" not defined.
  87     Make breakpoint pending on future shared library load? (y or [n]) y
  88     Breakpoint 1 (btrfs_init_sysfs) pending.
  89
  90 - Continue the target::
  91
  92     (gdb) c
  93
  94 - Load the module on the target and watch the symbols being loaded as well as
  95   the breakpoint hit::
  96
  97     loading @0xffffffffa0034000: /home/user/linux/build/lib/libcrc32c.ko
  98     loading @0xffffffffa0050000: /home/user/linux/build/lib/lzo/lzo_compress.ko
  99     loading @0xffffffffa006e000: /home/user/linux/build/lib/zlib_deflate/zlib_deflate.ko
 100     loading @0xffffffffa01b1000: /home/user/linux/build/fs/btrfs/btrfs.ko
 101
 102     Breakpoint 1, btrfs_init_sysfs () at /home/user/linux/fs/btrfs/sysfs.c:36
 103     36              btrfs_kset = kset_create_and_add("btrfs", NULL, fs_kobj);
 104
 105 - Dump the log buffer of the target kernel::
 106
 107     (gdb) lx-dmesg
 108     [     0.000000] Initializing cgroup subsys cpuset
 109     [     0.000000] Initializing cgroup subsys cpu
 110     [     0.000000] Linux version 3.8.0-rc4-dbg+ (...
 111     [     0.000000] Command line: root=/dev/sda2 resume=/dev/sda1 vga=0x314
 112     [     0.000000] e820: BIOS-provided physical RAM map:
 113     [     0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
 114     [     0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
 115     ....
 116
 117 - Examine fields of the current task struct::
 118
 119     (gdb) p $lx_current().pid
 120     $1 = 4998
 121     (gdb) p $lx_current().comm
 122     $2 = "modprobe\000\000\000\000\000\000\000"
 123
 124 - Make use of the per-cpu function for the current or a specified CPU::
 125
 126     (gdb) p $lx_per_cpu("runqueues").nr_running
 127     $3 = 1
 128     (gdb) p $lx_per_cpu("runqueues", 2).nr_running
 129     $4 = 0
 130
 131 - Dig into hrtimers using the container_of helper::
 132
 133     (gdb) set $next = $lx_per_cpu("hrtimer_bases").clock_base[0].active.next
 134     (gdb) p *$container_of($next, "struct hrtimer", "node")
 135     $5 = {
 136       node = {
 137         node = {
 138           __rb_parent_color = 18446612133355256072,
 139           rb_right = 0x0 <irq_stack_union>,
 140           rb_left = 0x0 <irq_stack_union>
 141         },
 142         expires = {
 143           tv64 = 1835268000000
 144         }
 145       },
 146       _softexpires = {
 147         tv64 = 1835268000000
 148       },
 149       function = 0xffffffff81078232 <tick_sched_timer>,
 150       base = 0xffff88003fd0d6f0,
 151       state = 1,
 152       start_pid = 0,
 153       start_site = 0xffffffff81055c1f <hrtimer_start_range_ns+20>,
 154       start_comm = "swapper/2\000\000\000\000\000\000"
 155     }
 156
 157
 158 List of commands and functions
 159 ------------------------------
 160
 161 The number of commands and convenience functions may evolve over the time,
 162 this is just a snapshot of the initial version::
 163
 164  (gdb) apropos lx
 165  function lx_current -- Return current task
 166  function lx_module -- Find module by name and return the module variable
 167  function lx_per_cpu -- Return per-cpu variable
 168  function lx_task_by_pid -- Find Linux task by PID and return the task_struct variable
 169  function lx_thread_info -- Calculate Linux thread_info from task variable
 170  lx-dmesg -- Print Linux kernel log buffer
 171  lx-lsmod -- List currently loaded modules
 172  lx-symbols -- (Re-)load symbols of Linux kernel and currently loaded modules
 173
 174 Detailed help can be obtained via "help <command-name>" for commands and "help
 175 function <function-name>" for convenience functions.