180 lines
6.3 KiB
ReStructuredText
180 lines
6.3 KiB
ReStructuredText
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Debugging kernel and modules via gdb
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====================================
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The kernel debugger kgdb, hypervisors like QEMU or JTAG-based hardware
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interfaces allow to debug the Linux kernel and its modules during runtime
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using gdb. Gdb comes with a powerful scripting interface for python. The
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kernel provides a collection of helper scripts that can simplify typical
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kernel debugging steps. This is a short tutorial about how to enable and use
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them. It focuses on QEMU/KVM virtual machines as target, but the examples can
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be transferred to the other gdb stubs as well.
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Requirements
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------------
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- gdb 7.2+ (recommended: 7.4+) with python support enabled (typically true
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for distributions)
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Setup
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-----
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- Create a virtual Linux machine for QEMU/KVM (see www.linux-kvm.org and
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www.qemu.org for more details). For cross-development,
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https://landley.net/aboriginal/bin keeps a pool of machine images and
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toolchains that can be helpful to start from.
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- Build the kernel with CONFIG_GDB_SCRIPTS enabled, but leave
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CONFIG_DEBUG_INFO_REDUCED off. If your architecture supports
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CONFIG_FRAME_POINTER, keep it enabled.
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- Install that kernel on the guest, turn off KASLR if necessary by adding
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"nokaslr" to the kernel command line.
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Alternatively, QEMU allows to boot the kernel directly using -kernel,
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-append, -initrd command line switches. This is generally only useful if
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you do not depend on modules. See QEMU documentation for more details on
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this mode. In this case, you should build the kernel with
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CONFIG_RANDOMIZE_BASE disabled if the architecture supports KASLR.
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- Build the gdb scripts (required on kernels v5.1 and above)::
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make scripts_gdb
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- Enable the gdb stub of QEMU/KVM, either
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- at VM startup time by appending "-s" to the QEMU command line
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or
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- during runtime by issuing "gdbserver" from the QEMU monitor
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console
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- cd /path/to/linux-build
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- Start gdb: gdb vmlinux
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Note: Some distros may restrict auto-loading of gdb scripts to known safe
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directories. In case gdb reports to refuse loading vmlinux-gdb.py, add::
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add-auto-load-safe-path /path/to/linux-build
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to ~/.gdbinit. See gdb help for more details.
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- Attach to the booted guest::
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(gdb) target remote :1234
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Examples of using the Linux-provided gdb helpers
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------------------------------------------------
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- Load module (and main kernel) symbols::
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(gdb) lx-symbols
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loading vmlinux
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scanning for modules in /home/user/linux/build
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loading @0xffffffffa0020000: /home/user/linux/build/net/netfilter/xt_tcpudp.ko
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loading @0xffffffffa0016000: /home/user/linux/build/net/netfilter/xt_pkttype.ko
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loading @0xffffffffa0002000: /home/user/linux/build/net/netfilter/xt_limit.ko
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loading @0xffffffffa00ca000: /home/user/linux/build/net/packet/af_packet.ko
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loading @0xffffffffa003c000: /home/user/linux/build/fs/fuse/fuse.ko
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...
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loading @0xffffffffa0000000: /home/user/linux/build/drivers/ata/ata_generic.ko
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- Set a breakpoint on some not yet loaded module function, e.g.::
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(gdb) b btrfs_init_sysfs
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Function "btrfs_init_sysfs" not defined.
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Make breakpoint pending on future shared library load? (y or [n]) y
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Breakpoint 1 (btrfs_init_sysfs) pending.
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- Continue the target::
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(gdb) c
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- Load the module on the target and watch the symbols being loaded as well as
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the breakpoint hit::
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loading @0xffffffffa0034000: /home/user/linux/build/lib/libcrc32c.ko
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loading @0xffffffffa0050000: /home/user/linux/build/lib/lzo/lzo_compress.ko
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loading @0xffffffffa006e000: /home/user/linux/build/lib/zlib_deflate/zlib_deflate.ko
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loading @0xffffffffa01b1000: /home/user/linux/build/fs/btrfs/btrfs.ko
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Breakpoint 1, btrfs_init_sysfs () at /home/user/linux/fs/btrfs/sysfs.c:36
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36 btrfs_kset = kset_create_and_add("btrfs", NULL, fs_kobj);
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- Dump the log buffer of the target kernel::
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(gdb) lx-dmesg
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[ 0.000000] Initializing cgroup subsys cpuset
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[ 0.000000] Initializing cgroup subsys cpu
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[ 0.000000] Linux version 3.8.0-rc4-dbg+ (...
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[ 0.000000] Command line: root=/dev/sda2 resume=/dev/sda1 vga=0x314
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[ 0.000000] e820: BIOS-provided physical RAM map:
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[ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
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[ 0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
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....
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- Examine fields of the current task struct(supported by x86 and arm64 only)::
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(gdb) p $lx_current().pid
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$1 = 4998
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(gdb) p $lx_current().comm
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$2 = "modprobe\000\000\000\000\000\000\000"
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- Make use of the per-cpu function for the current or a specified CPU::
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(gdb) p $lx_per_cpu("runqueues").nr_running
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$3 = 1
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(gdb) p $lx_per_cpu("runqueues", 2).nr_running
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$4 = 0
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- Dig into hrtimers using the container_of helper::
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(gdb) set $next = $lx_per_cpu("hrtimer_bases").clock_base[0].active.next
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(gdb) p *$container_of($next, "struct hrtimer", "node")
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$5 = {
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node = {
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node = {
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__rb_parent_color = 18446612133355256072,
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rb_right = 0x0 <irq_stack_union>,
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rb_left = 0x0 <irq_stack_union>
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},
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expires = {
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tv64 = 1835268000000
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}
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},
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_softexpires = {
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tv64 = 1835268000000
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},
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function = 0xffffffff81078232 <tick_sched_timer>,
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base = 0xffff88003fd0d6f0,
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state = 1,
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start_pid = 0,
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start_site = 0xffffffff81055c1f <hrtimer_start_range_ns+20>,
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start_comm = "swapper/2\000\000\000\000\000\000"
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}
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List of commands and functions
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------------------------------
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The number of commands and convenience functions may evolve over the time,
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this is just a snapshot of the initial version::
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(gdb) apropos lx
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function lx_current -- Return current task
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function lx_module -- Find module by name and return the module variable
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function lx_per_cpu -- Return per-cpu variable
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function lx_task_by_pid -- Find Linux task by PID and return the task_struct variable
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function lx_thread_info -- Calculate Linux thread_info from task variable
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lx-dmesg -- Print Linux kernel log buffer
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lx-lsmod -- List currently loaded modules
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lx-symbols -- (Re-)load symbols of Linux kernel and currently loaded modules
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Detailed help can be obtained via "help <command-name>" for commands and "help
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function <function-name>" for convenience functions.
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