1.. highlight:: none
2
3Debugging kernel and modules via gdb
4====================================
5
6The kernel debugger kgdb, hypervisors like QEMU or JTAG-based hardware
7interfaces allow to debug the Linux kernel and its modules during runtime
8using gdb. Gdb comes with a powerful scripting interface for python. The
9kernel provides a collection of helper scripts that can simplify typical
10kernel debugging steps. This is a short tutorial about how to enable and use
11them. It focuses on QEMU/KVM virtual machines as target, but the examples can
12be transferred to the other gdb stubs as well.
13
14
15Requirements
16------------
17
18- gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
19  for distributions)
20
21
22Setup
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  https://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
67Examples 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(supported by x86 and arm64 only)::
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
158List of commands and functions
159------------------------------
160
161The number of commands and convenience functions may evolve over the time,
162this 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
174Detailed help can be obtained via "help <command-name>" for commands and "help
175function <function-name>" for convenience functions.
176