Tip: Use gdbserver to debug qemu running under libguestfs

If qemu crashes or fails when run under libguestfs, it can be a bit hard to debug things. However a small qemu wrapper and gdbserver can help.

Create a file called qemu-wrapper chmod +x and containing:

#!/bin/bash -

if ! echo "$@" | grep -sqE -- '-help|-version|-device \?' ; then
  gdbserver="gdbserver :1234"
fi

exec $gdbserver /usr/bin/qemu-system-x86_64 "$@"

Set your environment variables so libguestfs will use the qemu wrapper instead of running qemu directly:

$ export LIBGUESTFS_BACKEND=direct
$ export LIBGUESTFS_HV=/path/to/qemu-wrapper

Now we run guestfish or another virt tool as normal:

$ guestfish -a /dev/null -v -x run

When qemu starts up, gdbserver will run and halt the process, printing:

Listening on port 1234

At this point you can connect gdb:

$ gdb
(gdb) file /usr/bin/qemu-system-x86_64
(gdb) target remote tcp::1234
set breakpoints etc here
(gdb) cont