Linux kernel live debugging, how it's done and what tools are used?

55
votes

What are the most common and why not uncommon methods and tools used to do live debugging on the Linux kernel? I know that Linus for eg. is against this kind of debugging for the Linux Kernel or it least was and thus nothing much has been done in that sense in those years, but honestly a lot of time has passed since 2000 and i am interested if that mentality has changed regarding the Linux project and what current methods are used to do live debugging on the Linux kernel at the moment(either local or remote)?

References to walkthroughs and tutorials on mentioned techniques and tools are welcome.

11
linuxdebugginglinux-kernelkernel
Also see lwn.net/Articles/280912 (search for kgdb) – Toby Brull
What do you mean by live? The one you are currently running or? Or also one in a VM? – Ciro Santilli 新疆再教育营六四事件法轮功郝海东

11 Answers

29
votes

Another option is to use ICE/JTAG controller, and GDB. This 'hardware' solution is especially used with embedded systems,

but for instance Qemu offers similar features:

  • start qemu with a gdb 'remote' stub which listens on 'localhost:1234' : qemu -s ...,

  • then with GDB you open the kernel file vmlinux compiled with debug information (you can take a look a this mailing list thread where they discuss the unoptimization of the kernel).

  • connect GDB and Qemu: target remote localhost:1234

  • see your live kernel:

    (gdb) where
#0  cpu_v7_do_idle () at arch/arm/mm/proc-v7.S:77
#1  0xc0029728 in arch_idle () atarm/mach-realview/include/mach/system.h:36
#2  default_idle () at arm/kernel/process.c:166
#3  0xc00298a8 in cpu_idle () at arch/arm/kernel/process.c:199
#4  0xc00089c0 in start_kernel () at init/main.c:713

unfortunately, user-space debugging is not possible so far with GDB (no task list information, no MMU reprogramming to see different process contexts, ...), but if you stay in kernel-space, that's quite convenient.

  • info threads will give you the list and states of the different CPUs

EDIT:

You can get more details about the procedure in this PDF:

Debugging Linux systems using GDB and QEMU.

23
votes

While debugging Linux kernel we can utilize several tools, for example, debuggers (KDB, KGDB), dumping while crashed (LKCD), tracing toolkit (LTT, LTTV, LTTng), custom kernel instruments (dprobes, kprobes). In the following section I tried to summarized most of them, hope these will help.

LKCD (Linux Kernel Crash Dump) tool allows the Linux system to write the contents of its memory when a crash occurs. These logs can be further analyzed for the root cause of the crash. Resources regarding LKCD

Oops when kernel detects a problem, it prints an Oops message. Such a message is generated by printk statements in the fault handler (arch/*/kernel/traps.c). A dedicated ring buffer in the kernel being used by the printk statements. Oops contains information like, the CPU where the Oops occurred on, contents of CPU registers, number of Oops, description, stack back trace and others. Resources regarding kernel Oops

Dynamic Probes is one of the popular debugging tool for Linux which developed by IBM. This tool allows the placement of a “probe” at almost any place in the system, in both user and kernel space. The probe consists of some code (written in a specialized, stack-oriented language) that is executed when control hits the given point. Resources regarding Dynamic Probe listed below

Linux Trace Toolkit is a kernel patch and a set of related utilities that allow the tracing of events in the kernel. The trace includes timing information and can create a reasonably complete picture of what happened over a given period of time. Resources of LTT, LTT Viewer and LTT Next Generation

MEMWATCH is an open source memory error detection tool. It works by defining MEMWATCH in gcc statement and by adding a header file to our code. Through this we can track memory leaks and memory corruptions. Resources regarding MEMWATCH

ftrace is a good tracing framework for Linux kernel. ftrace traces internal operations of the kernel. This tool included in the Linux kernel in 2.6.27. With its various tracer plugins, ftrace can be targeted at different static tracepoints, such as scheduling events, interrupts, memory-mapped I/O, CPU power state transitions, and operations related to file systems and virtualization. Also, dynamic tracking of kernel function calls is available, optionally restrictable to a subset of functions by using globs, and with the possibility to generate call graphs and provide stack usage. You can find a good tutorial of ftrace at https://events.linuxfoundation.org/slides/2010/linuxcon_japan/linuxcon_jp2010_rostedt.pdf

ltrace is a debugging utility in Linux, used to display the calls a user space application makes to shared libraries. This tool can be used to trace any dynamic library function call. It intercepts and records the dynamic library calls which are called by the executed process and the signals which are received by that process. It can also intercept and print the system calls executed by the program.

KDB is the in-kernel debugger of the Linux kernel. KDB follows simplistic shell-style interface. We can use it to inspect memory, registers, process lists, dmesg, and even set breakpoints to stop in a certain location. Through KDB we can set breakpoints and execute some basic kernel run control (Although KDB is not source level debugger). Several handy resources regarding KDB

KGDB is intended to be used as a source level debugger for the Linux kernel. It is used along with gdb to debug a Linux kernel. Two machines are required for using kgdb. One of these machines is a development machine and the other is the target machine. The kernel to be debugged runs on the target machine. The expectation is that gdb can be used to "break in" to the kernel to inspect memory, variables and look through call stack information similar to the way an application developer would use gdb to debug an application. It is possible to place breakpoints in kernel code and perform some limited execution stepping. Several handy resources regarding KGDB

21
votes

According to the wiki, kgdb was merged into the kernel in 2.6.26 which is within the last few years. kgdb is a remote debugger, so you activate it in your kernel then you attach gdb to it somehow. I say somehow as there seems to be lots of options - see connecting gdb. Given that kgdb is now in the source tree, I'd say going forward this is what you want to be using.

So it looks like Linus gave in. However, I would emphasize his argument - you should know what you're doing and know the system well. This is kernel land. If anything goes wrong, you don't get segfault, you get anything from some obscure problem later on to the whole system coming down. Here be dragons. Proceed with care, you have been warned.

15
votes

Another good tool for "live" debugging is kprobes / dynamic probes.

This lets you dynamically build little tiny modules which run when certain addresses are executed - sort of like a breakpoint.

The big advantage of them are:

  1. They do not impact the system - i.e. when a location is hit - it just excecutes the code - it doesn't halt the whole kernel.
  2. You don't need two different systems interconnected (target and debug) like with kgdb

It is best for doing things like hitting a breakpoint, and seeing what data values are, or checking if things have been changed/overwritten, etc. If you want to "step through code" - it doesn't do that.

Addition - 2018:

Another very powerful method is a program simply called "perf" which kind of rolls-up many tools (like Dynamic probes) and kind of replaces/depricates others (like oprofile).

In particular, the perf probe command can be used to easily create/add dynamic probes to the system, afterwhich perf record can sample the system and report info (and backtraces) when the probe is hit for reporting via perf report (or perf script). If you have good debug symbols in the kernel you can get great intel out of the system without even taking the kernel down. Do a man perf (in Google or on your system) for more info on this tool or see this great page on it:

http://www.brendangregg.com/perf.html

7
votes

KGDB + QEMU step-by-step

KGDB is a kernel subsystem that allows you to step debug the kernel itself from a host GDB.

My QEMU + Buildroot example is a good way to get a taste of it without real hardware: https://github.com/cirosantilli/linux-kernel-module-cheat/tree/1969cd6f8d30dace81d9848c6bacbb8bad9dacd8#kgdb

Pros and cons vs other methods:

  • advantage vs QEMU:
    • you often don't have software emulation for your device as hardware vendors don't like to release accurate software models for their devices
    • real hardware way faster than QEMU
  • advantage vs JTAG: no need for extra JTAG hardware, easier to setup
  • disadvantages vs QEMU and JTAG: less visibility and more intrusive. KGDB relies on the certain parts of the kernel working to be able to communicate with the host. So e.g. it breaks down in panic, you can't view the boot sequence.

The main steps are:

  1. Compile the kernel with:

    CONFIG_DEBUG_KERNEL=y
CONFIG_DEBUG_INFO=y

CONFIG_CONSOLE_POLL=y
CONFIG_KDB_CONTINUE_CATASTROPHIC=0
CONFIG_KDB_DEFAULT_ENABLE=0x1
CONFIG_KDB_KEYBOARD=y
CONFIG_KGDB=y
CONFIG_KGDB_KDB=y
CONFIG_KGDB_LOW_LEVEL_TRAP=y
CONFIG_KGDB_SERIAL_CONSOLE=y
CONFIG_KGDB_TESTS=y
CONFIG_KGDB_TESTS_ON_BOOT=n
CONFIG_MAGIC_SYSRQ=y
CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE=0x1
CONFIG_SERIAL_KGDB_NMI=n

    Most of those are not mandatory, but this is what I've tested.

  2. Add to your QEMU command:

    -append 'kgdbwait kgdboc=ttyS0,115200' \
-serial tcp::1234,server,nowait

  3. Run GDB with from the root of the Linux kernel source tree with:

    gdb -ex 'file vmlinux' -ex 'target remote localhost:1234'

  4. In GDB:

    (gdb) c

    and the boot should finish.

  5. In QEMU:

    echo g > /proc/sysrq-trigger

    And GDB should break.

  6. Now we are done, you can use GDB as usual:

    b sys_write
c

Tested in Ubuntu 14.04.

KGDB + Raspberry Pi

The exact same setup as above almost worked on a Raspberry Pi 2, Raspbian Jessie 2016-05-27.

You just have to learn to do the QEMU steps on the Pi, which are easily Googlable:

  • add the configuration options and recompile the kernel as explained at https://www.raspberrypi.org/documentation/linux/kernel/building.md There were unfortunately missing options on the default kernel build, notably no debug symbols, so the recompile is needed.

  • edit cmdline.txt of the boot partition and add:

    kgdbwait kgdboc=ttyAMA0,115200

  • connect gdb to the serial with:

    arm-linux-gnueabihf-gdb -ex 'file vmlinux' -ex 'target remote /dev/ttyUSB0'

    If you are not familiar with the serial, check out this: https://www.youtube.com/watch?v=da5Q7xL_OTo All you need is a cheap adapter like this one. Make sure you can get a shell through the serial to ensure that it is working before trying out KGDB.

  • do:

    echo g | sudo tee /proc/sysrq-trigger

    from inside an SSH session, since the serial is already taken by GDB.

With this setup, I was able to put a breakpoint in sys_write, pause program execution, list source and continue.

However, sometimes when I did next in sys_write GDB just hung and printed this error message several times:

Ignoring packet error, continuing...

so I'm not sure if something is wrong with my setup, or if this is expected because of what some background process is doing in the more complex Raspbian image.

I've also been told to try and disable multiprocessing with the Linux boot options, but I haven't tried it yet.

5
votes

QEMU + GDB step-by-step procedure tested on Ubuntu 16.10 host

To get started from scratch quickly I've made a minimal fully automated QEMU + Buildroot example at: https://github.com/cirosantilli/linux-kernel-module-cheat Major steps are covered below.

First get a root filesystem rootfs.cpio.gz. If you need one, consider:

Then on the Linux kernel:

git checkout v4.9
make mrproper
make x86_64_defconfig
cat <<EOF >.config-fragment
CONFIG_DEBUG_INFO=y
CONFIG_DEBUG_KERNEL=y
CONFIG_GDB_SCRIPTS=y
EOF
./scripts/kconfig/merge_config.sh .config .config-fragment
make -j"$(nproc)"
qemu-system-x86_64 -kernel arch/x86/boot/bzImage \
                   -initrd rootfs.cpio.gz -S -s

On another terminal, supposing you want to start debugging from start_kernel:

gdb \
    -ex "add-auto-load-safe-path $(pwd)" \
    -ex "file vmlinux" \
    -ex 'set arch i386:x86-64:intel' \
    -ex 'target remote localhost:1234' \
    -ex 'break start_kernel' \
    -ex 'continue' \
    -ex 'disconnect' \
    -ex 'set arch i386:x86-64' \
    -ex 'target remote localhost:1234'

and we are done!!

For kernel modules see: How to debug Linux kernel modules with QEMU?

For Ubuntu 14.04, GDB 7.7.1, hbreak was needed, break software breakpoints were ignored. Not the case anymore in 16.10. See also: https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/901944

The messy disconnect and what come after it are to work around the error:

Remote 'g' packet reply is too long: 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

Related threads:

See also:

Known limitations:

4
votes

Actually the joke is that Linux has had an in-kernel debugger since 2.2.12, xmon, but only for the powerpc architecture (actually it was ppc back then).

It's not a source level debugger, and it's almost entirely undocumented, but still.

http://lxr.linux.no/linux-old+v2.2.12/arch/ppc/xmon/xmon.c#L119

3
votes

As someone who writes kernel code a lot I have to say I have never used kgdb, and only rarely use kprobes etc.

It is still often the best approach to throw in some strategic printks. In more recent kernels trace_printk is a good way to do that without spamming dmesg.

3
votes

User mode Linux (UML)

https://en.wikipedia.org/wiki/User-mode_Linux

Another virtualization another method that allows step debugging kernel code.

UML is very ingenious: it is implemented as an ARCH, just like x86, but instead of using low level instructions, it implements the ARCH functions with userland system calls.

The result is that you are able to run Linux kernel code as a userland process on a Linux host!

First make a rootfs and run it as shown at: https://unix.stackexchange.com/questions/73203/how-to-create-rootfs-for-user-mode-linux-on-fedora-18/372207#372207

The um defconfig sets CONFIG_DEBUG_INFO=y by default (yup, it is a development thing), so we are fine.

On guest:

i=0
while true; do echo $i; i=$(($i+1)); done

On host in another shell:

ps aux | grep ./linux
gdb -pid "$pid"

In GDB:

break sys_write
continue
continue

And now you are controlling the count from GDB, and can see source as expected.

Pros:

  • fully contained in the Linux kernel mainline tree
  • more lightweight than QEMU's full system emulation

Cons:

  • very invasive, as it changes how the kernel itself is compiled.

    But the higher level APIs outside of ARCH specifics should remain unchanged.

  • arguably not very active: Is user mode linux (UML) project stopped?

See also: https://unix.stackexchange.com/questions/127829/why-would-someone-want-to-run-usermode-linux-uml

1
votes

You guys are wrong, the kgdb still works well for latest kernel, you need to take care of kernel configuration of split image, randomization optimization.

kgdb over serial port is useless because no computer today supports DB9 on a motherboard serial port, USB serial port doesn't support the polling mode.

The new game is kgdboe, following is the log trace:

following is the host machine, vmlinux is from the target machine

root@Thinkpad-T510:~/KGDBOE# gdb vmlinux
Reading symbols from vmlinux...done.
(gdb) target remote udp:192.168.1.22:31337
1077    kernel/debug/debug_core.c: No such file or directory.
(gdb) l oom_kill_process 
828 mm/oom_kill.c: No such file or directory.
(gdb) l oom_kill_process 
828 in mm/oom_kill.c
(gdb) break oom_kill_process
Breakpoint 1 at 0xffffffff8119e0c0: file mm/oom_kill.c, line 833.
(gdb) c
Continuing.
[New Thread 1779]
[New Thread 1782]
[New Thread 1777]
[New Thread 1778]
[New Thread 1780]
[New Thread 1781]
[Switching to Thread 1779]

Thread 388 hit Breakpoint 1, oom_kill_process (oc=0xffffc90000d93ce8, message=0xffffffff82098fbc "Out of memory")
at mm/oom_kill.c:833
833 in mm/oom_kill.c
(gdb) s
834 in mm/oom_kill.c
(gdb)

On peer target machine, following is how to get it crash and to be captured by host machine 

#swapoff -a
#stress -m 4 --vm-bytes=500m
-4
votes

kgdb and gdb are almost useless for debugging the kernel because the code is so optimised it bears no relation to the orioginal source and many varuiables are optimised out. This makes steppijng , hence stepping through the source is impossible, examining variables is impossible and is therefore aolmost pointles.

Actually it is worse than useless, it actually gives you false infoprmation so detached is the code you are ollooking at to the actual running code.

And no, you cant turn off optimisations in the kernel, it doesnt compile.

I have to say, coming from a windows kernel environment, the lack of decent debugger is anoying, given that there is junk code out there to maintain.