Enabling Linux Dynamic Debug Statements when Scripting QEMU

Trying to debug intermittent data corruption under QEMU can be fairly painful, especially if you’re trying to reproduce with stock boot images. I happened to be in this dark corner recently and wanted to enable some extra debug output in the provided kernel. Thankfully Linux supports enabling dynamic debug statements on the kernel commandline, though depending on which statements you want and how you’re trying to enable them this can be a straightforward or like trying to run a Rube Goldberg machine in reverse.

There were several hurdles in my case:

1. I was trying to automate reproduction of the bug and so was scripting the guest boot process under QEMU using expect(1)
2. The expect script was driving the u-boot shell to indirectly set bootargs via a u-boot script
3. I needed to enable dynamic debug statements for a pattern containing spaces

As it turns out there are several relevant points that fall out as a consequence:

1. u-boot does not sensibly handle escaping of nested quotes. Also it will eat all your single-quotes nested inside double-qoutes.
2. As it turns out, dyndbg will handle octal representation of characters just fine, so you can encode your spaces as \040 to avoid badly split strings arguments on the kernel commandline.
3. You’ll still have to do a lot of escaping to wedge this all into an expect script that does the right thing.

In my case the result ended up looking like:

send "setenv bootargs \$bootargs debug systemd.log_target=null dyndbg=\"\\\"format UBI\\\\040DBG\\\\040wl +p\\\"\"\n"