ftrace: Create set_ftrace_notrace_pid to not trace tasks

There's currently a way to select a task that should only be traced by
functions, but there's no way to select a task not to be traced by the
function tracer. Add a set_ftrace_notrace_pid file that acts the same as
set_ftrace_pid (and is also affected by function-fork), but the task pids in
this file will not be traced even if they are listed in the set_ftrace_pid
file. This makes it easy for tools like trace-cmd to "hide" itself from the
function tracer when it is recording other tasks.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

1 files changed, 14 insertions, 6 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 5e634b9c1e0a..6519b7afc499 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -386,16 +386,22 @@ trace_find_filtered_pid(struct trace_pid_list *filtered_pids, pid_t search_pid)
  * Returns false if @task should be traced.
  */
 bool
-trace_ignore_this_task(struct trace_pid_list *filtered_pids, struct task_struct *task)
+trace_ignore_this_task(struct trace_pid_list *filtered_pids,
+		       struct trace_pid_list *filtered_no_pids,
+		       struct task_struct *task)
 {
 	/*
-	 * Return false, because if filtered_pids does not exist,
-	 * all pids are good to trace.
+	 * If filterd_no_pids is not empty, and the task's pid is listed
+	 * in filtered_no_pids, then return true.
+	 * Otherwise, if filtered_pids is empty, that means we can
+	 * trace all tasks. If it has content, then only trace pids
+	 * within filtered_pids.
 	 */
-	if (!filtered_pids)
-		return false;
 
-	return !trace_find_filtered_pid(filtered_pids, task->pid);
+	return (filtered_pids &&
+		!trace_find_filtered_pid(filtered_pids, task->pid)) ||
+		(filtered_no_pids &&
+		 trace_find_filtered_pid(filtered_no_pids, task->pid));
 }
 
 /**
@@ -5013,6 +5019,8 @@ static const char readme_msg[] =
 #ifdef CONFIG_FUNCTION_TRACER
 	"  set_ftrace_pid\t- Write pid(s) to only function trace those pids\n"
 	"\t\t    (function)\n"
+	"  set_ftrace_notrace_pid\t- Write pid(s) to not function trace those pids\n"
+	"\t\t    (function)\n"
 #endif
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	"  set_graph_function\t- Trace the nested calls of a function (function_graph)\n"