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DEBUGGING.md

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Overview

This debugging guide continues at the point where EXAMPLE.md stopped. At this point bluebox created an archive called initramfs.cpio that we want debug further.

Required software

In this guide the following software is used.

Extract files from the initramfs archive

As an optional step one can extract embedded files from the archive.

$ cpio -iv < /tmp/initramfs.cpio

This command will extract at least two executables, init and bluebox-init, from initramfs.cpio that is dynamically created when using bluebox. If bluebox was instructed to embedd more executables or files into the archive, these will be extracted from the archive as well.

Prepare a debugger for a remote debugging session

In a first shell start gdb

$ gdb
      # Load an executable into the debugger session. gdb will read symbols and offsets from this
      # executable. This is needed to later be able to set breakpoints.
      # The executable here can either be init executable that was extracted in the optional
      # previous step or is an executable that is triggered by init and placed alongside in the
      # archive.
(gdb) file init
(gdb) file bluebox-init
      # Breakpoints depend on the executable(s) that were loaded into the debugging session. Both
      # dynamically generated Go executables will have the symbol main.main as they are regular
      # Go executables.
      # To debug the setups that set up the minimal environment for the executables set a breakpoint
      # in the init executable that is called by the Kernel.
(gdb) break init:main.main
      # To debug the execution of the additional embedded executables set a breakpoint at the
      # executable that will run these embedded executables in a sequential order.
(gdb) break bluebox-init:main.main
      # Instruct the debugger to connect to gdbserver.
(gdb) target remote localhost:1234

Start the process that should be debugged

In a second shell run qemu with all the arguments as in EXAMPLE.md and add -s -S as argument. -s is shorthand for -gdb tcp::1234 and will start a gdbserver on TCP port 1234 so the first shell in the previous step can connect to it. -S will instruct qemu to not start CPU at startup and wait for a continue instruction from gdb.

$ qemu-system-x86_64 -s -S -nographic  -append "console=ttyS0" -m 4G -kernel /tmp/ci-kernels/linux-4.14.264.bz -initrd /tmp/initramfs.cpio