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+.. highlight:: none
+
+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
+------------
+
+- gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
+ for distributions)
+
+
+Setup
+-----
+
+- Create a virtual Linux machine for QEMU/KVM (see www.linux-kvm.org and
+ www.qemu.org for more details). For cross-development,
+ https://landley.net/aboriginal/bin keeps a pool of machine images and
+ toolchains that can be helpful to start from.
+
+- 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.
+
+- Install that kernel on the guest, turn off KASLR if necessary by adding
+ "nokaslr" to the kernel command line.
+ 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. In this case, you should build the kernel with
+ CONFIG_RANDOMIZE_BASE disabled if the architecture supports KASLR.
+
+- Build the gdb scripts (required on kernels v5.1 and above)::
+
+ make scripts_gdb
+
+- 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
+
+- cd /path/to/linux-build
+
+- 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.
+
+- Attach to the booted guest::
+
+ (gdb) target remote :1234
+
+
+Examples of using the Linux-provided gdb helpers
+------------------------------------------------
+
+- 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
+
+- 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.
+
+- Continue the target::
+
+ (gdb) c
+
+- 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);
+
+- 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
+ ....
+
+- Examine fields of the current task struct(supported by x86 and arm64 only)::
+
+ (gdb) p $lx_current().pid
+ $1 = 4998
+ (gdb) p $lx_current().comm
+ $2 = "modprobe\000\000\000\000\000\000\000"
+
+- 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
+
+- 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.