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diff --git a/Documentation/dev-tools/gdb-kernel-debugging.rst b/Documentation/dev-tools/gdb-kernel-debugging.rst new file mode 100644 index 000000000..895285c03 --- /dev/null +++ b/Documentation/dev-tools/gdb-kernel-debugging.rst @@ -0,0 +1,179 @@ +.. 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. |