haiku/docs/develop/kernel/boot/Debugging_Bootloaders_GEF.rst

Bootloader debugging with GEF
=============================

When Haiku’s early boot process is experiencing unknown crashes or
faults, it can be extremely difficult to troubleshoot (especially when
serial, video, or other i/o devices are non-functional)

It **is** possible to step through the boot of any architecture of Haiku
in a debugger if the system boots and the issue can be reproduced in
qemu.

   This works for any architecture and is *extremely* helpful to trouble
   early platforms. Linux or Mac OS are requirements. You need a full
   POSIX environment.

Building Haiku
--------------

On most non-x86 platforms, you will need a “kernel” (haiku_loader) and
an “initrd” (haiku_floppyboot).

For arm/arm64: ``jam -q @minimum-mmc``

Launching Haiku in QEMU
-----------------------

In the example below, we will prepare Haiku arm in QEMU for debugging.

::

   qemu-system-arm -M raspi2 -kernel haiku_loader.u-boot -initrd haiku-floppyboot.tgz.u-boot -serial stdio -m 2G -dtb rpi2.dtb -s -S

**Key Flags:**

-  **-s**

   -  Shorthand for -gdb tcp::1234, i.e. open a gdbserver on TCP port
      1234.

-  **-S**

   -  Do not start CPU at startup (you must type ‘c’ in the monitor).

These simple flags will make qemu listen for a debugger connection on
localhost:1234 and have the VM not start until you tell it to.

   In the example above, we are Emulating a Raspberry Pi 2, and using
   our Raspberry Pi 2 dtb. If you don’t have a dtb for the machine
   you’re emulating, you can dump qemu’s internal dtb by adding
   ``-M dumpdtb=myboard.dtb`` to the end of your qemu command.

Attaching GEF
-------------

`GEF <https://github.com/hugsy/gef>`__ is an enhanced debugger which
works extremely well for debugging code running in virtual machines. It
piggy-backs on gdb and offers a lot of valueable insight at a glance
without requiring to know every gdb command.

Once GEF is installed, we can step through the process to attach gdb to
qemu.

Open gdb with our symbols.
~~~~~~~~~~~~~~~~~~~~~~~~~~

First we run gdb pointed at our boot loader. We use the native ELF
binary as that seems to give gdb/gef the most accurate knowledge of our
symbols. (the haiku_loader.u-boot is wrapped by u-boot’s mkimage, your
milage may vary based on platform)

``gdb objects/haiku/arm/release/system/boot/u-boot/boot_loader_u-boot``

Set the architecture
~~~~~~~~~~~~~~~~~~~~

This may not be required, but re-enforces to gef/gdb that we’re working
on arm.

``set architecture arm``

Connect to QEMU
~~~~~~~~~~~~~~~

Now we tell gdb/gef about out running (but paused) QEMU instance.

``gef-remote -q localhost:1234``

A successful connection should occur.

Step into debugging
~~~~~~~~~~~~~~~~~~~

Before you begin execution, it’s handy to set a *breakpoint*. A
*breakpoint* tells gdb/gef where it should pause execution to begin the
debugging process. All of our bootloaders start in a ``start_gen``
function, so this is a good place to start.

``breakpoint start_gen``

Now that a breakpoint is defined, lets run the virtual machine.

In gef, type ``continue``.

If everything is working as expected, you should now be “paused” at the
``start_gen`` function (hopefully showing the C/C++ code).

Now, you have a few commands to leverage:

-  **step**

   -  Take a single step forward and execute the code listed.
   -  Does **not** step “into” functions, just over them getting the
      return from the code.
   -  Alias: s

-  **stepi**

   -  step forward “into” the next code.
   -  If you’re on a function it will enter the function and show the
      code executed.

-  **break**

   -  add additional “breakpoints” where you can step through the code
      execution.

-  **continue**

   -  Resume execution.
   -  If you have no additional breakpoints the code will “go do what
      it’s supposed to”
   -  Alias: c

-  **next**

   - Resume execution until it reaches the next line of code.
   - Useful for example to run until a loop is completed, and stop at the first line after that loop.