ptrace: Call arch_ptrace_attach() when request=PTRACE_TRACEME

arch_ptrace_attach() is a hook that allows the architecture to do
book-keeping after a ptrace attach. This patch adds a call to this
hook when handling a PTRACE_TRACEME request as well.

Currently only one architecture, m32r, implements this hook. When
called, it initializes a number of debug trap slots in the ptraced
task's thread struct, and it looks to me like this is the right thing
to do after a PTRACE_TRACEME request as well, not only after
PTRACE_ATTACH. Please correct me if I'm wrong.

I want to use this hook on AVR32 to turn the debugging hardware on
when a process is actually being debugged and keep it off otherwise.
To be able to do this, I need to intercept PTRACE_TRACEME and
PTRACE_ATTACH, as well as PTRACE_DETACH and thread exit. The latter
two can be handled by existing hooks.
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>

kernel/ptrace.c

@@ -470,6 +470,8 @@ asmlinkage long sys_ptrace(long request, long pid, long addr, long data)
 	lock_kernel();
 	if (request == PTRACE_TRACEME) {
 		ret = ptrace_traceme();
+		if (!ret)
+			arch_ptrace_attach(current);
 		goto out;
 	}