x86, bts: add fork and exit handling

Impact: introduce new ptrace facility

Add arch_ptrace_untrace() function that is called when the tracer
detaches (either voluntarily or when the tracing task dies);
ptrace_disable() is only called on a voluntary detach.

Add ptrace_fork() and arch_ptrace_fork(). They are called when a
traced task is forked.

Clear DS and BTS related fields on fork.

Release DS resources and reclaim memory in ptrace_untrace(). This
releases resources already when the tracing task dies. We used to do
that when the traced task dies.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/fork.c

@@ -1096,6 +1096,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 #ifdef CONFIG_DEBUG_MUTEXES
 	p->blocked_on = NULL; /* not blocked yet */
 #endif
+	if (unlikely(ptrace_reparented(current)))
+		ptrace_fork(p, clone_flags);
 
 	/* Perform scheduler related setup. Assign this task to a CPU. */
 	sched_fork(p, clone_flags);