161 lines
5.9 KiB
Plaintext
161 lines
5.9 KiB
Plaintext
Debugging kernel and modules via gdb
|
|
====================================
|
|
|
|
The kernel debugger kgdb, hypervisors like QEMU or JTAG-based hardware
|
|
interfaces allow to debug the Linux kernel and its modules during runtime
|
|
using gdb. Gdb comes with a powerful scripting interface for python. The
|
|
kernel provides a collection of helper scripts that can simplify typical
|
|
kernel debugging steps. This is a short tutorial about how to enable and use
|
|
them. It focuses on QEMU/KVM virtual machines as target, but the examples can
|
|
be transferred to the other gdb stubs as well.
|
|
|
|
|
|
Requirements
|
|
------------
|
|
|
|
o gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
|
|
for distributions)
|
|
|
|
|
|
Setup
|
|
-----
|
|
|
|
o Create a virtual Linux machine for QEMU/KVM (see www.linux-kvm.org and
|
|
www.qemu.org for more details). For cross-development,
|
|
http://landley.net/aboriginal/bin keeps a pool of machine images and
|
|
toolchains that can be helpful to start from.
|
|
|
|
o Build the kernel with CONFIG_GDB_SCRIPTS enabled, but leave
|
|
CONFIG_DEBUG_INFO_REDUCED off. If your architecture supports
|
|
CONFIG_FRAME_POINTER, keep it enabled.
|
|
|
|
o Install that kernel on the guest.
|
|
|
|
Alternatively, QEMU allows to boot the kernel directly using -kernel,
|
|
-append, -initrd command line switches. This is generally only useful if
|
|
you do not depend on modules. See QEMU documentation for more details on
|
|
this mode.
|
|
|
|
o Enable the gdb stub of QEMU/KVM, either
|
|
- at VM startup time by appending "-s" to the QEMU command line
|
|
or
|
|
- during runtime by issuing "gdbserver" from the QEMU monitor
|
|
console
|
|
|
|
o cd /path/to/linux-build
|
|
|
|
o Start gdb: gdb vmlinux
|
|
|
|
Note: Some distros may restrict auto-loading of gdb scripts to known safe
|
|
directories. In case gdb reports to refuse loading vmlinux-gdb.py, add
|
|
|
|
add-auto-load-safe-path /path/to/linux-build
|
|
|
|
to ~/.gdbinit. See gdb help for more details.
|
|
|
|
o Attach to the booted guest:
|
|
(gdb) target remote :1234
|
|
|
|
|
|
Examples of using the Linux-provided gdb helpers
|
|
------------------------------------------------
|
|
|
|
o Load module (and main kernel) symbols:
|
|
(gdb) lx-symbols
|
|
loading vmlinux
|
|
scanning for modules in /home/user/linux/build
|
|
loading @0xffffffffa0020000: /home/user/linux/build/net/netfilter/xt_tcpudp.ko
|
|
loading @0xffffffffa0016000: /home/user/linux/build/net/netfilter/xt_pkttype.ko
|
|
loading @0xffffffffa0002000: /home/user/linux/build/net/netfilter/xt_limit.ko
|
|
loading @0xffffffffa00ca000: /home/user/linux/build/net/packet/af_packet.ko
|
|
loading @0xffffffffa003c000: /home/user/linux/build/fs/fuse/fuse.ko
|
|
...
|
|
loading @0xffffffffa0000000: /home/user/linux/build/drivers/ata/ata_generic.ko
|
|
|
|
o Set a breakpoint on some not yet loaded module function, e.g.:
|
|
(gdb) b btrfs_init_sysfs
|
|
Function "btrfs_init_sysfs" not defined.
|
|
Make breakpoint pending on future shared library load? (y or [n]) y
|
|
Breakpoint 1 (btrfs_init_sysfs) pending.
|
|
|
|
o Continue the target
|
|
(gdb) c
|
|
|
|
o Load the module on the target and watch the symbols being loaded as well as
|
|
the breakpoint hit:
|
|
loading @0xffffffffa0034000: /home/user/linux/build/lib/libcrc32c.ko
|
|
loading @0xffffffffa0050000: /home/user/linux/build/lib/lzo/lzo_compress.ko
|
|
loading @0xffffffffa006e000: /home/user/linux/build/lib/zlib_deflate/zlib_deflate.ko
|
|
loading @0xffffffffa01b1000: /home/user/linux/build/fs/btrfs/btrfs.ko
|
|
|
|
Breakpoint 1, btrfs_init_sysfs () at /home/user/linux/fs/btrfs/sysfs.c:36
|
|
36 btrfs_kset = kset_create_and_add("btrfs", NULL, fs_kobj);
|
|
|
|
o Dump the log buffer of the target kernel:
|
|
(gdb) lx-dmesg
|
|
[ 0.000000] Initializing cgroup subsys cpuset
|
|
[ 0.000000] Initializing cgroup subsys cpu
|
|
[ 0.000000] Linux version 3.8.0-rc4-dbg+ (...
|
|
[ 0.000000] Command line: root=/dev/sda2 resume=/dev/sda1 vga=0x314
|
|
[ 0.000000] e820: BIOS-provided physical RAM map:
|
|
[ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
|
|
[ 0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
|
|
....
|
|
|
|
o Examine fields of the current task struct:
|
|
(gdb) p $lx_current().pid
|
|
$1 = 4998
|
|
(gdb) p $lx_current().comm
|
|
$2 = "modprobe\000\000\000\000\000\000\000"
|
|
|
|
o Make use of the per-cpu function for the current or a specified CPU:
|
|
(gdb) p $lx_per_cpu("runqueues").nr_running
|
|
$3 = 1
|
|
(gdb) p $lx_per_cpu("runqueues", 2).nr_running
|
|
$4 = 0
|
|
|
|
o Dig into hrtimers using the container_of helper:
|
|
(gdb) set $next = $lx_per_cpu("hrtimer_bases").clock_base[0].active.next
|
|
(gdb) p *$container_of($next, "struct hrtimer", "node")
|
|
$5 = {
|
|
node = {
|
|
node = {
|
|
__rb_parent_color = 18446612133355256072,
|
|
rb_right = 0x0 <irq_stack_union>,
|
|
rb_left = 0x0 <irq_stack_union>
|
|
},
|
|
expires = {
|
|
tv64 = 1835268000000
|
|
}
|
|
},
|
|
_softexpires = {
|
|
tv64 = 1835268000000
|
|
},
|
|
function = 0xffffffff81078232 <tick_sched_timer>,
|
|
base = 0xffff88003fd0d6f0,
|
|
state = 1,
|
|
start_pid = 0,
|
|
start_site = 0xffffffff81055c1f <hrtimer_start_range_ns+20>,
|
|
start_comm = "swapper/2\000\000\000\000\000\000"
|
|
}
|
|
|
|
|
|
List of commands and functions
|
|
------------------------------
|
|
|
|
The number of commands and convenience functions may evolve over the time,
|
|
this is just a snapshot of the initial version:
|
|
|
|
(gdb) apropos lx
|
|
function lx_current -- Return current task
|
|
function lx_module -- Find module by name and return the module variable
|
|
function lx_per_cpu -- Return per-cpu variable
|
|
function lx_task_by_pid -- Find Linux task by PID and return the task_struct variable
|
|
function lx_thread_info -- Calculate Linux thread_info from task variable
|
|
lx-dmesg -- Print Linux kernel log buffer
|
|
lx-lsmod -- List currently loaded modules
|
|
lx-symbols -- (Re-)load symbols of Linux kernel and currently loaded modules
|
|
|
|
Detailed help can be obtained via "help <command-name>" for commands and "help
|
|
function <function-name>" for convenience functions.
|