The patch titled
Subject: scripts/gdb: add basic documentation
has been added to the -mm tree. Its filename is
scripts-gdb-add-basic-documentation.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/scripts-gdb-add-basic-documentation.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/scripts-gdb-add-basic-documentation.patch
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------------------------------------------------------
From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
Subject: scripts/gdb: add basic documentation
Signed-off-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
Cc: Rob Landley <rob@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
Cc: Ben Widawsky <ben@xxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/gdb-kernel-debugging.txt | 160 +++++++++++++++++++++++
1 file changed, 160 insertions(+)
diff -puN /dev/null Documentation/gdb-kernel-debugging.txt
--- /dev/null
+++ a/Documentation/gdb-kernel-debugging.txt
@@ -0,0 +1,160 @@
+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.
_
Patches currently in -mm which might be from jan.kiszka@xxxxxxxxxxx are
scripts-gdb-add-infrastructure.patch
scripts-gdb-add-cache-for-type-objects.patch
scripts-gdb-add-container_of-helper-and-convenience-function.patch
scripts-gdb-add-module-iteration-class.patch
scripts-gdb-add-lx-symbols-command.patch
module-do-not-inline-do_init_module.patch
scripts-gdb-add-automatic-symbol-reloading-on-module-insertion.patch
scripts-gdb-add-internal-helper-and-convenience-function-to-look-up-a-module.patch
scripts-gdb-add-get_target_endianness-helper.patch
scripts-gdb-add-read_u16-32-64-helpers.patch
scripts-gdb-add-lx-dmesg-command.patch
scripts-gdb-add-task-iteration-class.patch
scripts-gdb-add-helper-and-convenience-function-to-look-up-tasks.patch
scripts-gdb-add-is_target_arch-helper.patch
scripts-gdb-add-internal-helper-and-convenience-function-to-retrieve-thread_info.patch
scripts-gdb-add-get_gdbserver_type-helper.patch
scripts-gdb-add-internal-helper-and-convenience-function-for-per-cpu-lookup.patch
scripts-gdb-add-lx_current-convenience-function.patch
scripts-gdb-add-class-to-iterate-over-cpu-masks.patch
scripts-gdb-add-lx-lsmod-command.patch
scripts-gdb-add-basic-documentation.patch
scripts-gdb-port-to-python3-gdb77.patch
scripts-gdb-ignore-byte-compiled-python-files.patch
scripts-gdb-use-a-generator-instead-of-iterator-for-task-list.patch
scripts-gdb-convert-modulelist-to-generator-function.patch
scripts-gdb-convert-cpulist-to-generator-function.patch
scripts-gdb-define-maintainer.patch
scripts-gdb-disable-pagination-while-printing-from-breakpoint-handler.patch
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