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

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   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.
  35   Alternatively, QEMU allows to boot the kernel directly using -kernel,
  36   -append, -initrd command line switches. This is generally only useful if
  37   you do not depend on modules. See QEMU documentation for more details on
  38   this mode.
  39
  40 - Enable the gdb stub of QEMU/KVM, either
  41
  42     - at VM startup time by appending "-s" to the QEMU command line
  43
  44   or
  45
  46     - during runtime by issuing "gdbserver" from the QEMU monitor
  47       console
  48
  49 - cd /path/to/linux-build
  50
  51 - Start gdb: gdb vmlinux
  52
  53   Note: Some distros may restrict auto-loading of gdb scripts to known safe
  54   directories. In case gdb reports to refuse loading vmlinux-gdb.py, add::
  55
  56     add-auto-load-safe-path /path/to/linux-build
  57
  58   to ~/.gdbinit. See gdb help for more details.
  59
  60 - Attach to the booted guest::
  61
  62     (gdb) target remote :1234
  63
  64
  65 Examples of using the Linux-provided gdb helpers
  66 ------------------------------------------------
  67
  68 - Load module (and main kernel) symbols::
  69
  70     (gdb) lx-symbols
  71     loading vmlinux
  72     scanning for modules in /home/user/linux/build
  73     loading @0xffffffffa0020000: /home/user/linux/build/net/netfilter/xt_tcpudp.ko
  74     loading @0xffffffffa0016000: /home/user/linux/build/net/netfilter/xt_pkttype.ko
  75     loading @0xffffffffa0002000: /home/user/linux/build/net/netfilter/xt_limit.ko
  76     loading @0xffffffffa00ca000: /home/user/linux/build/net/packet/af_packet.ko
  77     loading @0xffffffffa003c000: /home/user/linux/build/fs/fuse/fuse.ko
  78     ...
  79     loading @0xffffffffa0000000: /home/user/linux/build/drivers/ata/ata_generic.ko
  80
  81 - Set a breakpoint on some not yet loaded module function, e.g.::
  82
  83     (gdb) b btrfs_init_sysfs
  84     Function "btrfs_init_sysfs" not defined.
  85     Make breakpoint pending on future shared library load? (y or [n]) y
  86     Breakpoint 1 (btrfs_init_sysfs) pending.
  87
  88 - Continue the target::
  89
  90     (gdb) c
  91
  92 - Load the module on the target and watch the symbols being loaded as well as
  93   the breakpoint hit::
  94
  95     loading @0xffffffffa0034000: /home/user/linux/build/lib/libcrc32c.ko
  96     loading @0xffffffffa0050000: /home/user/linux/build/lib/lzo/lzo_compress.ko
  97     loading @0xffffffffa006e000: /home/user/linux/build/lib/zlib_deflate/zlib_deflate.ko
  98     loading @0xffffffffa01b1000: /home/user/linux/build/fs/btrfs/btrfs.ko
  99
 100     Breakpoint 1, btrfs_init_sysfs () at /home/user/linux/fs/btrfs/sysfs.c:36
 101     36              btrfs_kset = kset_create_and_add("btrfs", NULL, fs_kobj);
 102
 103 - Dump the log buffer of the target kernel::
 104
 105     (gdb) lx-dmesg
 106     [     0.000000] Initializing cgroup subsys cpuset
 107     [     0.000000] Initializing cgroup subsys cpu
 108     [     0.000000] Linux version 3.8.0-rc4-dbg+ (...
 109     [     0.000000] Command line: root=/dev/sda2 resume=/dev/sda1 vga=0x314
 110     [     0.000000] e820: BIOS-provided physical RAM map:
 111     [     0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
 112     [     0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
 113     ....
 114
 115 - Examine fields of the current task struct::
 116
 117     (gdb) p $lx_current().pid
 118     $1 = 4998
 119     (gdb) p $lx_current().comm
 120     $2 = "modprobe\000\000\000\000\000\000\000"
 121
 122 - Make use of the per-cpu function for the current or a specified CPU::
 123
 124     (gdb) p $lx_per_cpu("runqueues").nr_running
 125     $3 = 1
 126     (gdb) p $lx_per_cpu("runqueues", 2).nr_running
 127     $4 = 0
 128
 129 - Dig into hrtimers using the container_of helper::
 130
 131     (gdb) set $next = $lx_per_cpu("hrtimer_bases").clock_base[0].active.next
 132     (gdb) p *$container_of($next, "struct hrtimer", "node")
 133     $5 = {
 134       node = {
 135         node = {
 136           __rb_parent_color = 18446612133355256072,
 137           rb_right = 0x0 <irq_stack_union>,
 138           rb_left = 0x0 <irq_stack_union>
 139         },
 140         expires = {
 141           tv64 = 1835268000000
 142         }
 143       },
 144       _softexpires = {
 145         tv64 = 1835268000000
 146       },
 147       function = 0xffffffff81078232 <tick_sched_timer>,
 148       base = 0xffff88003fd0d6f0,
 149       state = 1,
 150       start_pid = 0,
 151       start_site = 0xffffffff81055c1f <hrtimer_start_range_ns+20>,
 152       start_comm = "swapper/2\000\000\000\000\000\000"
 153     }
 154
 155
 156 List of commands and functions
 157 ------------------------------
 158
 159 The number of commands and convenience functions may evolve over the time,
 160 this is just a snapshot of the initial version::
 161
 162  (gdb) apropos lx
 163  function lx_current -- Return current task
 164  function lx_module -- Find module by name and return the module variable
 165  function lx_per_cpu -- Return per-cpu variable
 166  function lx_task_by_pid -- Find Linux task by PID and return the task_struct variable
 167  function lx_thread_info -- Calculate Linux thread_info from task variable
 168  lx-dmesg -- Print Linux kernel log buffer
 169  lx-lsmod -- List currently loaded modules
 170  lx-symbols -- (Re-)load symbols of Linux kernel and currently loaded modules
 171
 172 Detailed help can be obtained via "help <command-name>" for commands and "help
 173 function <function-name>" for convenience functions.