1Debugging kernel and modules via gdb
  2====================================
  3
  4The kernel debugger kgdb, hypervisors like QEMU or JTAG-based hardware
  5interfaces allow to debug the Linux kernel and its modules during runtime
  6using gdb. Gdb comes with a powerful scripting interface for python. The
  7kernel provides a collection of helper scripts that can simplify typical
  8kernel debugging steps. This is a short tutorial about how to enable and use
  9them. It focuses on QEMU/KVM virtual machines as target, but the examples can
 10be transferred to the other gdb stubs as well.
 11
 12
 13Requirements
 14------------
 15
 16 o gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
 17   for distributions)
 18
 19
 20Setup
 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
 60Examples 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
143List of commands and functions
144------------------------------
145
146The number of commands and convenience functions may evolve over the time,
147this 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
159Detailed help can be obtained via "help <command-name>" for commands and "help
160function <function-name>" for convenience functions.