tracehook: tracehook_tracer_task

This adds the tracehook_tracer_task() hook to consolidate all forms of
"Who is using ptrace on me?" logic.  This is used for "TracerPid:" in
/proc and for permission checks.  We also clean up the selinux code the
called an identical accessor.

Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Reviewed-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/tracehook.h

@@ -72,6 +72,24 @@ static inline int tracehook_unsafe_exec(struct task_struct *task)
 	return unsafe;
 }
 
+/**
+ * tracehook_tracer_task - return the task that is tracing the given task
+ * @tsk:		task to consider
+ *
+ * Returns NULL if noone is tracing @task, or the &struct task_struct
+ * pointer to its tracer.
+ *
+ * Must called under rcu_read_lock().  The pointer returned might be kept
+ * live only by RCU.  During exec, this may be called with task_lock()
+ * held on @task, still held from when tracehook_unsafe_exec() was called.
+ */
+static inline struct task_struct *tracehook_tracer_task(struct task_struct *tsk)
+{
+	if (task_ptrace(tsk) & PT_PTRACED)
+		return rcu_dereference(tsk->parent);
+	return NULL;
+}
+
 /**
  * tracehook_report_exec - a successful exec was completed
  * @fmt:		&struct linux_binfmt that performed the exec