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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- Build the gdb scripts (required on kernels v5.1 and above)::
43
44 make scripts_gdb
45
46- Enable the gdb stub of QEMU/KVM, either
47
48 - at VM startup time by appending "-s" to the QEMU command line
49
50 or
51
52 - during runtime by issuing "gdbserver" from the QEMU monitor
53 console
54
55- cd /path/to/linux-build
56
57- Start gdb: gdb vmlinux
58
59 Note: Some distros may restrict auto-loading of gdb scripts to known safe
60 directories. In case gdb reports to refuse loading vmlinux-gdb.py, add::
61
62 add-auto-load-safe-path /path/to/linux-build
63
64 to ~/.gdbinit. See gdb help for more details.
65
66- Attach to the booted guest::
67
68 (gdb) target remote :1234
69
70
71Examples of using the Linux-provided gdb helpers
72------------------------------------------------
73
74- Load module (and main kernel) symbols::
75
76 (gdb) lx-symbols
77 loading vmlinux
78 scanning for modules in /home/user/linux/build
79 loading @0xffffffffa0020000: /home/user/linux/build/net/netfilter/xt_tcpudp.ko
80 loading @0xffffffffa0016000: /home/user/linux/build/net/netfilter/xt_pkttype.ko
81 loading @0xffffffffa0002000: /home/user/linux/build/net/netfilter/xt_limit.ko
82 loading @0xffffffffa00ca000: /home/user/linux/build/net/packet/af_packet.ko
83 loading @0xffffffffa003c000: /home/user/linux/build/fs/fuse/fuse.ko
84 ...
85 loading @0xffffffffa0000000: /home/user/linux/build/drivers/ata/ata_generic.ko
86
87- Set a breakpoint on some not yet loaded module function, e.g.::
88
89 (gdb) b btrfs_init_sysfs
90 Function "btrfs_init_sysfs" not defined.
91 Make breakpoint pending on future shared library load? (y or [n]) y
92 Breakpoint 1 (btrfs_init_sysfs) pending.
93
94- Continue the target::
95
96 (gdb) c
97
98- Load the module on the target and watch the symbols being loaded as well as
99 the breakpoint hit::
100
101 loading @0xffffffffa0034000: /home/user/linux/build/lib/libcrc32c.ko
102 loading @0xffffffffa0050000: /home/user/linux/build/lib/lzo/lzo_compress.ko
103 loading @0xffffffffa006e000: /home/user/linux/build/lib/zlib_deflate/zlib_deflate.ko
104 loading @0xffffffffa01b1000: /home/user/linux/build/fs/btrfs/btrfs.ko
105
106 Breakpoint 1, btrfs_init_sysfs () at /home/user/linux/fs/btrfs/sysfs.c:36
107 36 btrfs_kset = kset_create_and_add("btrfs", NULL, fs_kobj);
108
109- Dump the log buffer of the target kernel::
110
111 (gdb) lx-dmesg
112 [ 0.000000] Initializing cgroup subsys cpuset
113 [ 0.000000] Initializing cgroup subsys cpu
114 [ 0.000000] Linux version 3.8.0-rc4-dbg+ (...
115 [ 0.000000] Command line: root=/dev/sda2 resume=/dev/sda1 vga=0x314
116 [ 0.000000] e820: BIOS-provided physical RAM map:
117 [ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
118 [ 0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
119 ....
120
121- Examine fields of the current task struct(supported by x86 and arm64 only)::
122
123 (gdb) p $lx_current().pid
124 $1 = 4998
125 (gdb) p $lx_current().comm
126 $2 = "modprobe\000\000\000\000\000\000\000"
127
128- Make use of the per-cpu function for the current or a specified CPU::
129
130 (gdb) p $lx_per_cpu("runqueues").nr_running
131 $3 = 1
132 (gdb) p $lx_per_cpu("runqueues", 2).nr_running
133 $4 = 0
134
135- Dig into hrtimers using the container_of helper::
136
137 (gdb) set $next = $lx_per_cpu("hrtimer_bases").clock_base[0].active.next
138 (gdb) p *$container_of($next, "struct hrtimer", "node")
139 $5 = {
140 node = {
141 node = {
142 __rb_parent_color = 18446612133355256072,
143 rb_right = 0x0 <irq_stack_union>,
144 rb_left = 0x0 <irq_stack_union>
145 },
146 expires = {
147 tv64 = 1835268000000
148 }
149 },
150 _softexpires = {
151 tv64 = 1835268000000
152 },
153 function = 0xffffffff81078232 <tick_sched_timer>,
154 base = 0xffff88003fd0d6f0,
155 state = 1,
156 start_pid = 0,
157 start_site = 0xffffffff81055c1f <hrtimer_start_range_ns+20>,
158 start_comm = "swapper/2\000\000\000\000\000\000"
159 }
160
161
162List of commands and functions
163------------------------------
164
165The number of commands and convenience functions may evolve over the time,
166this is just a snapshot of the initial version::
167
168 (gdb) apropos lx
169 function lx_current -- Return current task
170 function lx_module -- Find module by name and return the module variable
171 function lx_per_cpu -- Return per-cpu variable
172 function lx_task_by_pid -- Find Linux task by PID and return the task_struct variable
173 function lx_thread_info -- Calculate Linux thread_info from task variable
174 lx-dmesg -- Print Linux kernel log buffer
175 lx-lsmod -- List currently loaded modules
176 lx-symbols -- (Re-)load symbols of Linux kernel and currently loaded modules
177
178Detailed help can be obtained via "help <command-name>" for commands and "help
179function <function-name>" for convenience functions.
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 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
65Examples 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
156List of commands and functions
157------------------------------
158
159The number of commands and convenience functions may evolve over the time,
160this 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
172Detailed help can be obtained via "help <command-name>" for commands and "help
173function <function-name>" for convenience functions.