Run one command, get a QEMU Buildroot BusyBox virtual machine built from source with several minimal Linux kernel 4.9 module development example tutorials with GDB and KGDB debug and minimal QEMU educational models. Tested in x86, ARM and MIPS guests, Ubuntu 14.04 - 17.04 hosts.

cd kernel_module
./make-host.sh

If the compilation of any of the C files fails, e.g. because of kernel or toolchain differences that we don't control on the host, just rename it to remove the .c extension and try again:

mv broken.c broken.c~
./build_host

Once you manage to compile, try it out with:

sudo insmod hello.ko

# Our module is there.
sudo lsmod | grep hello

# Last message should be: hello init
dmest -T

sudo rmmod hello

# Last message should be: hello exit
dmesg -T

# Not present anymore
sudo lsmod | grep hello

Why this is very bad and you should be ashamed:

• bugs can easily break you system. E.g.:
  • segfaults can trivially lead to a kernel crash, and require a reboot
  • your disk could get erased. Yes, this can also happen with sudo from userland. But you should not use sudo when developing newbie programs. And for the kernel you don't have the choice not to use sudo
  • even more subtle system corruption such as not being able to rmmod
• can't control which kernel version and build options to use. So some of the modules may simply not compile because of kernel API changes, since the Linux kernel does not have a stable kernel module API.
• can't control which hardware is used, notably the CPU architecture
• can't step debug it with GDB easily

The only advantage of using your host machine, is that you don't have to wait 2 hours and use up 8 Gigs for the build. But you will soon find out that this is a very reasonable price to pay.

sudo apt-get build-dep qemu
./run

First build will take a while (GCC, Linux kernel).

QEMU opens up, and you can run:

root
insmod /hello.ko
insmod /hello2.ko
rmmod hello
rmmod hello2

This should print to the screen:

hello init
hello2 init
hello cleanup
hello2 cleanup

which are printk messages from init and cleanup methods of those modules.

Each module comes from a C file under kernel_module/. For module usage see:

head kernel_module/modulename.c

Good bets inside guest are:

/modulename.sh
/modulename.out

After the first build, you can also run just:

./runqemu

to save a few seconds. ./run wouldn't rebuild everything, but checking timestamps takes a few moments.

If you make changes to the kernel modules or most configurations, you can just use again:

./run

and they will updated.

But if you change any package besides kernel_module, you must also request those packages to be reconfigured or rebuilt with extra targets, e.g.:

./run -t linux-reconfigure -t host-qemu-rebuild

Those aren't turned on by default because they take quite a few seconds.

If you are feeling fancy, you can also insert modules with:

modprobe hello

This method also deals with module dependencies, which we almost don't use to make examples simpler:

• https://askubuntu.com/questions/20070/whats-the-difference-between-insmod-and-modprobe
• https://stackoverflow.com/questions/22891705/whats-the-difference-between-insmod-and-modprobe

modprobe searches for modules under:

ls /lib/modules/*/extra/

Kernel modules built from the Linux mainline tree with CONFIG_SOME_MOD=m, are automatically available with modprobe, e.g.:

modprobe dummy-irq

If you are feeling raw, you can use our own minimal:

/myinsmod.out /hello.ko

which demonstrates the C module API: https://stackoverflow.com/questions/5947286/how-can-linux-kernel-modules-be-loaded-from-c-code/38606527#38606527

We use printk a lot, and it shows on the QEMU terminal by default. If that annoys you (e.g. you want to see stdout separately), do:

dmesg -n 1

See also: https://superuser.com/questions/351387/how-to-stop-kernel-messages-from-flooding-my-console

You can scroll up a bit on the default TTY with:

Shift + PgUp

but I never managed to increase that buffer:

• https://askubuntu.com/questions/709697/how-to-increase-scrollback-lines-in-ubuntu14-04-2-server-edition
• https://unix.stackexchange.com/questions/346018/how-to-increase-the-scrollback-buffer-size-for-tty

We use Buildroot's default kernel version with small educational patches on top, you can confirm it after build with:

grep BR2_LINUX_KERNEL_VERSION buildroot/output.*~/.config

or in QEMU:

cat /proc/version

or in the source:

cd linux
git log | grep -E '    Linux [0-9]+\.' | head

Sometimes in Ubuntu 14.04, after the QEMU SDL GUI starts, it does not get updated after keyboard strokes, and there are artifacts like disappearing text.

We have not managed to track this problem down yet, but the following workaround always works:

Ctrl + Shift + U
Ctrl + C
root

This started happening when we switched to building QEMU through Buildroot, and has not been observed on later Ubuntu.

Using text mode is another workaround if you don't need GUI features.

The root filesystem is persistent across:

./runqemu
date >f
sync

then:

./runqemu
cat f

This is particularly useful to re-run shell commands from the history of a previous session with Ctrl + R.

When you do:

./run

the disk image gets overwritten by a fresh filesystem and you lose all changes.

Remember that if you forcibly turn QEMU off without sync or poweroff from inside the VM, disk changes may not be saved.

Show serial output of QEMU directly on the current terminal, without opening a QEMU window:

./runqemu -n

To exit, just do a regular:

poweroff

This mode is very useful to:

• get full panic traces when you start making the kernel crash :-) See also: https://unix.stackexchange.com/questions/208260/how-to-scroll-up-after-a-kernel-panic
• copy and paste commands and stdout output to / from host
• have a large scroll buffer, and be able to search it, e.g. by using GNU screen on host

If the system crashes, you can't can quit QEMU with poweroff, but you can use either:

Ctrl-C X

or:

Ctrl-C A
quit

or:

./qemumonitor
quit

or:

echo quit | ./qemumonitor

See also:

• http://stackoverflow.com/questions/14165158/how-to-switch-to-qemu-monitor-console-when-running-with-curses
• https://superuser.com/questions/1087859/how-to-quit-qemu-monitor
• https://superuser.com/questions/488263/problems-switching-to-qemu-control-panel-with-nographics
• https://superuser.com/questions/1087859/how-to-quit-the-qemu-monitor-when-not-using-a-gui/1211516#1211516

Limitations:

• TODO: Ctrl + C kills the emulator, it is not sent to guest processes. See:

  • cloudius-systems/osv#49
  • https://unix.stackexchange.com/questions/167165/how-to-pass-ctrl-c-in-qemu

  This is however fortunate when running QEMU with GDB, as the Ctrl + C reaches GDB and breaks.

• Very early kernel messages such as early console in extract_kernel only show on the GUI, since at such early stages, not even the serial has been setup.

When debugging a module, it becomes tedious to wait for build and re-type:

root
/mymod.sh

Instead, you can add your test commands to:

cd rootfs_overlay/etc/init.d
cp S98 S99
vim S99

and they will be run automatically before the login prompt.

S99 is already gitignored for you.

For convenience, we also setup a symlink from S99 to rootfs_overlay/etc/init.d/S99.

Scripts under /etc/init.d are run by /etc/init.d/rcS, which gets called by the line ::sysinit:/etc/init.d/rcS in /etc/inittab.

Is the default BusyBox /init too bloated for you, minimalism freak?

No problem, just use the init kernel boot parameter:

./runqemu -e 'init=/sleep_forever.out'

Remember that shell scripts can also be used for init https://unix.stackexchange.com/questions/174062/init-as-a-shell-script/395375#395375:

./runqemu -e 'init=/count.sh'

Also remember that if your init returns, the kernel will panic, there are just two non-panic possibilities:

• run forever in a loop or long sleep
• poweroff the machine

I know, right?

Add this line to rootfs_post_build_script:

rm -f "${1}/etc/init.d/"S*network

To restore it, run:

./run -t initscripts-reconfigure

To GDB the Linux kernel, first run:

./runqemu -d

If you want to break immediately at a symbol, e.g. start_kernel of the boot sequence, run on another shell:

./rungdb start_kernel

Now QEMU will stop there, and you can use the normal GDB commands:

l
n
c

To skip the boot, run just:

./rungdb

and when you want to break, do Ctrl + C from GDB.

To have some fun, you can first run inside QEMU:

/count.sh

which counts to infinity to stdout, and then in GDB:

Ctrl + C
break sys_write
continue
continue
continue

And you now control the counting from GDB.

See also: http://stackoverflow.com/questions/11408041/how-to-debug-the-linux-kernel-with-gdb-and-qemu

O=0 is an impossible dream, O=2 being the default: https://stackoverflow.com/questions/29151235/how-to-de-optimize-the-linux-kernel-to-and-compile-it-with-o0 So get ready for some weird jumps, and <value optimized out> fun. Why, Linux, why.

Loadable kernel modules are a bit trickier since the kernel can place them at different memory locations depending on load other.

So we cannot set the breakpoints before insmod.

However, the Linux kernel GDB scripts offer the lx-symbols command, which takes care of that beautifully for us:

./runqemu -d
./rungdb

In QEMU:

insmod /fops.ko

In GDB, hit Ctrl + C, and note how it says:

scanning for modules in ../kernel_module-1.0/
loading @0xffffffffa0000000: ../kernel_module-1.0//fops.ko

That's lx-symbols working! Now simply:

b fop_write
c

In QEMU:

printf a >/sys/kernel/debug/lkmc_fops/f

and GDB now breaks at our fop_write function!

Just don't forget to remove your breakpoints after rmmod, or they will point to stale memory locations.

TODO: why does break work_func for insmod kthread.ko not break the first time I insmod, but breaks the second time?

Useless, but a good way to show how hardcore you are. From inside QEMU:

insmod /fops.ko
cat /proc/modules

This will give a line of form:

fops 2327 0 - Live 0xfffffffa00000000

And then tell GDB where the module was loaded with:

Ctrl + C
add-symbol-file ../kernel_module-1.0/fops.ko 0xfffffffa00000000

TODO: why can't we break at early startup stuff such as:

./rungdb extract_kernel
./rungdb main

See also: https://stackoverflow.com/questions/2589845/what-are-the-first-operations-that-the-linux-kernel-executes-on-boot

The portability of the kernel and toolchains is amazing: change an option and most things magically work on completely different hardware.

First build:

./run -a arm

Run without build:

./runqemu -a arm

Debug:

./runqemu -a arm -d
# On another terminal.
./rungdb -a arm

TODOs:

• only managed to run in the terminal interface (but weirdly a blank QEMU window is still opened)
• GDB not connecting to KGDB. Possibly linked to -serial stdio. See also: https://stackoverflow.com/questions/14155577/how-to-use-kgdb-on-arm

./run -a aarch64

TODOs:

• GDB gives a ton of messages:

  no module object found for ''
  

  when you connect. Ctrl + C then c works though.

• How to add devices to -M virt as we did for -M versatilepb

./run -a mips64

KGDB is kernel dark magic that allows you to GDB the kernel on real hardware without any extra hardware support.

It is useless with QEMU since we already have full system visibility with -gdb, but this is a good way to learn it.

Cheaper than JTAG (free) and easier to setup (all you need is serial), but with less visibility as it depends on the kernel working, so e.g.: dies on panic, does not see boot sequence.

Usage:

./runqemu -k
./rungdb -k

In GDB:

c

In QEMU:

/count.sh &
/kgdb.sh

In GDB:

b sys_write
c
c
c
c

And now you can count from GDB!

If you do: b sys_write immediately after ./rungdb -k, it fails with KGDB: BP remove failed: <address>. I think this is because it would break too early on the boot sequence, and KGDB is not yet ready.

See also:

• https://github.com/torvalds/linux/blob/v4.9/Documentation/DocBook/kgdb.tmpl
• https://stackoverflow.com/questions/22004616/qemu-kernel-debugging-with-kgdb/44197715#44197715

In QEMU:

/kgdb-mod.sh

In GDB:

lx-symbols ../kernel_module-1.0/
b fop_write
c
c
c

and you now control the count.

TODO: if I -ex lx-symbols to the gdb command, just like done for QEMU -gdb, the kernel oops. How to automate this step?

If you modify runqemu to use:

-append kgdboc=kbd

instead of kgdboc=ttyS0,115200, you enter a different debugging mode called KDB.

Usage: in QEMU:

[0]kdb> go

Boot finishes, then:

/kgdb.sh

And you are back in KDB. Now you can:

[0]kdb> help
[0]kdb> bp sys_write
[0]kdb> go

And you will break whenever sys_write is hit.

The other KDB commands allow you to instruction steps, view memory, registers and some higher level kernel runtime data.

But TODO I don't think you can see where you are in the kernel source code and line step as from GDB, since the kernel source is not available on guest (ah, if only debugging information supported full source).

When you start interacting with QEMU hardware, it is useful to see what is going on inside of QEMU itself.

This is of course trivial since QEMU is just an userland program on the host, but we make it a bit easier with:

./runqemu -q

Then you could:

b edu_mmio_read
c

And in QEMU:

/pci.sh

Just make sure that you never click inside the QEMU window when doing that, otherwise you mouse gets captured forever, and the only solution I can find is to go to a TTY with Ctrl + Alt + F1 and kill QEMU.

You can still send key presses to QEMU however even without the mouse capture, just either click on the title bar, or alt tab to give it focus.

Step debug userland processes to understand how they are talking to the kernel.

In guest:

/gdbserver.sh /myinsmod.out /hello.ko

In host:

./rungdbserver kernel_module-1.0/user/myinsmod.out

You can find the executable with:

find buildroot/output.x86_64~/build -name myinsmod.out

TODO: automate the path finding:

• using the executable from under buildroot/output.x86_64~/target would be easier as the path is the same as in guest, but unfortunately those executables are stripped to make the guest smaller. BR2_STRIP_none=y should disable stripping, but make the image way larger.

• outputx86_64~/staging/ would be even better than target/ as the docs say that this directory contains binaries before they were stripped. However, only a few binaries are pre-installed there by default, and it seems to be a manual per package thing.

  E.g. pciutils has for lspci:

  define PCIUTILS_INSTALL_STAGING_CMDS
      $(TARGET_MAKE_ENV) $(MAKE1) -C $(@D) $(PCIUTILS_MAKE_OPTS) \
          PREFIX=$(STAGING_DIR)/usr SBINDIR=$(STAGING_DIR)/usr/bin \
          install install-lib install-pcilib
  endef
  

  and the docs describe the *_INSTALL_STAGING per package config, which is normally set for shared library packages.

  Feature request: https://bugs.busybox.net/show_bug.cgi?id=10386

An implementation overview can be found at: https://reverseengineering.stackexchange.com/questions/8829/cross-debugging-for-mips-elf-with-qemu-toolchain/16214#16214

As usual, different archs work with:

./rungdbserver -a arm kernel_module-1.0/user/myinsmod.out

BusyBox executables are all symlinks, so if you do on guest:

/gdbserver.sh ls

on host you need:

./rungdbserver busybox-1.26.2/busybox

Our setup gives you the rare opportunity to step debug libc and other system libraries e.g. with:

b open
c

Or simply by stepping into calls:

s

This is made possible by the GDB command:

set sysroot ${buildroot_out_dir}/staging

which automatically finds unstripped shared libraries on the host for us.

See also: https://stackoverflow.com/questions/8611194/debugging-shared-libraries-with-gdbserver/45252113#45252113

GDB breakpoints are set on virtual addresses, so you can in theory debug userland processes as well.

• https://stackoverflow.com/questions/26271901/is-it-possible-to-use-gdb-and-qemu-to-debug-linux-user-space-programs-and-kernel
• https://stackoverflow.com/questions/16273614/debug-init-on-qemu-using-gdb

The only use case I can see for this is to debug the init process (and have fun), otherwise, why wouldn't you just use gdbserver? Known of direct userland debugging:

• the kernel might switch context to another process, and you would enter "garbage"

• TODO step into shared libraries. If I attempt to load them explicitly:

  (gdb) sharedlibrary ../../staging/lib/libc.so.0
  No loaded shared libraries match the pattern `../../staging/lib/libc.so.0'.
  

  since GDB does not know that libc is loaded.

Custom init process:

• Shell 1:

  ./runqemu -d -e 'init=/sleep_forever.out' -n
  

• Shell 2:

  ./rungdb-user kernel_module-1.0/user/sleep_forever.out main
  

BusyBox custom init process:

• Shell 1:

  ./runqemu -d -e 'init=/bin/ls' -n
  

• Shell 2:

  ./rungdb-user -h busybox-1.26.2/busybox ls_main
  

This follows BusyBox' convention of calling the main for each executable as <exec>_main since the busybox executable has many "mains".

BusyBox default init process:

• Shell 1:

  ./runqemu -d -n
  

• Shell 2:

  ./rungdb-user -h busybox-1.26.2/busybox init_main
  

This cannot be debugged in another way without modifying the source, or /sbin/init exits early with:

"must be run as PID 1"

Non-init process:

• Shell 1

  ./runqemu -d -n
  

• Shell 2

  ./rungdb-user kernel_module-1.0/user/sleep_forever.out
  Ctrl + C
  b main
  continue
  

• Shell 1

  /sleep_forever.out
  

This is of least reliable setup as there might be other processes that use the given virtual address.

Only tested successfully in x86_64:

startx

More details: https://unix.stackexchange.com/questions/70931/how-to-install-x11-on-my-own-linux-buildroot-system/306116#306116

Not sure how well that graphics stack represents real systems, but if it does it would be a good way to understand how it works.

On ARM, startx hangs at a message:

vgaarb: this pci device is not a vga device

and nothing shows on the screen, and:

grep EE /var/log/Xorg.0.log

says:

(EE) Failed to load module "modesetting" (module does not exist, 0)

ARM support: a friend told me but I haven't tried it yet:

• xf86-video-modesetting is likely the missing ingredient, but it does not seem possible to activate it from Buildroot currently without patching things.
• xf86-video-fbdev should work as well, but we need to make sure fbdev is enabled, and maybe add some line to the Xorg.conf

• https://www.quora.com/How-many-instructions-does-a-typical-Linux-kernel-boot-take
• https://github.com/cirosantilli/chat/issues/31
• https://rwmj.wordpress.com/2016/03/17/tracing-qemu-guest-execution/
• qemu/docs/tracing.txt and qemu/docs/replay.txt
• https://stackoverflow.com/questions/39149446/how-to-use-qemus-simple-trace-backend/46497873#46497873

Best attempt so far:

time ./runqemu -n -e 'init=/poweroff.out' -- -trace exec_tb,file=trace && \
  time ./qemu/scripts/simpletrace.py qemu/trace-events trace >trace.txt && \
  wc -l trace.txt && \
  sed '/0x1000000/q' trace.txt >trace-boot.txt && \
  wc -l trace-boot.txt

Notes:

• -n is a good idea to reduce the chances that you send unwanted non-deterministic mouse or keyboard clicks to the VM.

• -e 'init=/poweroff.out' is crucial as it reduces the instruction count from 40 million to 20 million, so most instructions were actually running on the VM.

  Without it, the bulk of the time seems to be spent in setting up the network with ifup that gets called from /etc/init.d/S40network from the default Buildroot BusyBox setup.

  And it becomes even worse if you try to -net none as recommended in the 2.7 replay.txt docs, because then ifup waits for 15 seconds before giving up as per /etc/network/interfaces line wait-delay 15.

• 0x1000000 is the address where QEMU puts the Linux kernel at with -kernel in x86.

  It can be found from:

  readelf -e buildroot/output.x86_64~/build/linux-*/vmlinux | grep Entry
  

  TODO confirm further. If I try to break there with:

  ./rungdb *0x1000000
  

  but I have no corresponding source line. Also note that this line is not actually the first line, since the kernel messages such as early console in extract_kernel have already shown on screen at that point. This does not break at all:

  ./rungdb extract_kernel
  

  It only appears once on every log I've seen so far, checked with grep 0x1000000 trace.txt

  Then when we count the instructions that run before the kernel entry point, there is only about 100k instructions, which is insignificant compared to the kernel boot itself.

• We can also discount the instructions after init runs by using readelf to get the initial address of init. One easy way to do that now is to just run:

  ./rungdb-user kernel_module-1.0/user/poweroff.out main
  

  And get that from the traces, e.g. if the address is 4003a0, then we search:

  grep -n 4003a0 trace.txt
  

  I have observed a single match for that instruction, so it must be the init, and there were only 20k instructions after it, so the impact is negligible.

This works because we have already done the following with QEMU:

• ./configure --enable-trace-backends=simple. This logs in a binary format to the trace file.

  It makes 3x execution faster than the default trace backend which logs human readable data to stdout.

  This also alters the actual execution, and reduces the instruction count by 10M TODO understand exactly why, possibly due to the All QSes seen thing.

• the simple QEMU patch mentioned at: https://rwmj.wordpress.com/2016/03/17/tracing-qemu-guest-execution/ of removing the disable from exec_tb in the trace-events template file in the QEMU source

Possible improvements:

• to disable networking. Is replacing init enough?

  • https://superuser.com/questions/181254/how-do-you-boot-linux-with-networking-disabled
  • https://superuser.com/questions/684005/how-does-one-permanently-disable-gnu-linux-networking/1255015#1255015

  CONFIG_NET=n did not significantly reduce instruction, so maybe replacing init is enough.

• logging with the default backend log greatly slows down the CPU, and in particular leads to this during kernel boot:

  All QSes seen, last rcu_sched kthread activity 5252 (4294901421-4294896169), jiffies_till_next_fqs=1, root ->qsmask 0x0
  swapper/0       R  running task        0     1      0 0x00000008
   ffff880007c03ef8 ffffffff8107aa5d ffff880007c16b40 ffffffff81a3b100
   ffff880007c03f60 ffffffff810a41d1 0000000000000000 0000000007c03f20
   fffffffffffffedc 0000000000000004 fffffffffffffedc ffffffff00000000
  Call Trace:
   <IRQ>  [<ffffffff8107aa5d>] sched_show_task+0xcd/0x130
   [<ffffffff810a41d1>] rcu_check_callbacks+0x871/0x880
   [<ffffffff810a799f>] update_process_times+0x2f/0x60
  

  in which the boot appears to hang for a considerable time.

• Confirm that the kernel enters at 0x1000000, or where it enters. Once we have this, we can exclude what comes before in the BIOS.

1. Introduction
2. Build
3. kmod
4. vermagic
5. ftrace
6. Device tree
7. GEM5
8. Bibliography
9. Examples
   1. Host
   2. QEMU Buildroot