do_debug is a bit of a red herring here. ptrace should not be able
put a breakpoint on a kernel address, period. I would just pick a
fixed address that's in the kernel text range or even just in the
pre-KASLR text range and make sure it gets rejected. Maybe try a few
different addresses for good measure.
I've looked at the code and it seems like this would be a bit more
complicated since the breakpoint is set by an accident in a race and the
call still fails. Which is why the test triggers the breakpoint and
causes infinite loop in the kernel...
I guess that we could instead read back the address with
PTRACE_PEEKUSER, so something as:
break_addr = ptrace(PTRACE_PEEKUSER, child_pid,
(void *)offsetof(struct user, u_debugreg),
if (break_addr == kernel_addr)
tst_res(TFAIL, "ptrace() set break on a kernel address");