Re: [PATCH] tracing: Add new_exec tracepoint

[Marco Elver <elver@google.com>]

On Tue, Apr 09, 2024 at 08:46AM -0700, Kees Cook wrote:
[...]
> > +	trace_new_exec(current, bprm);
> > +
> 
> All other steps in this function have explicit comments about
> what/why/etc. Please add some kind of comment describing why the
> tracepoint is where it is, etc.

I beefed up the tracepoint documentation, and wrote a little paragraph
above where it's called to reinforce what we want.

[...]
> What about binfmt_misc, and binfmt_script? You may want bprm->interp
> too?

Good points. I'll make the below changes for v2:

diff --git a/fs/exec.c b/fs/exec.c
index ab778ae1fc06..472b9f7b40e8 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1268,6 +1268,12 @@ int begin_new_exec(struct linux_binprm * bprm)
 	if (retval)
 		return retval;
 
+	/*
+	 * This tracepoint marks the point before flushing the old exec where
+	 * the current task is still unchanged, but errors are fatal (point of
+	 * no return). The later "sched_process_exec" tracepoint is called after
+	 * the current task has successfully switched to the new exec.
+	 */
 	trace_new_exec(current, bprm);
 
 	/*
diff --git a/include/trace/events/task.h b/include/trace/events/task.h
index 8853dc44783d..623d9af777c1 100644
--- a/include/trace/events/task.h
+++ b/include/trace/events/task.h
@@ -61,8 +61,11 @@ TRACE_EVENT(task_rename,
  * @task:	pointer to the current task
  * @bprm:	pointer to linux_binprm used for new exec
  *
- * Called before flushing the old exec, but at the point of no return during
- * switching to the new exec.
+ * Called before flushing the old exec, where @task is still unchanged, but at
+ * the point of no return during switching to the new exec. At the point it is
+ * called the exec will either succeed, or on failure terminate the task. Also
+ * see the "sched_process_exec" tracepoint, which is called right after @task
+ * has successfully switched to the new exec.
  */
 TRACE_EVENT(new_exec,
 
@@ -71,19 +74,22 @@ TRACE_EVENT(new_exec,
 	TP_ARGS(task, bprm),
 
 	TP_STRUCT__entry(
+		__string(	interp,		bprm->interp	)
 		__string(	filename,	bprm->filename	)
 		__field(	pid_t,		pid		)
 		__string(	comm,		task->comm	)
 	),
 
 	TP_fast_assign(
+		__assign_str(interp, bprm->interp);
 		__assign_str(filename, bprm->filename);
 		__entry->pid = task->pid;
 		__assign_str(comm, task->comm);
 	),
 
-	TP_printk("filename=%s pid=%d comm=%s",
-		  __get_str(filename), __entry->pid, __get_str(comm))
+	TP_printk("interp=%s filename=%s pid=%d comm=%s",
+		  __get_str(interp), __get_str(filename),
+		  __entry->pid, __get_str(comm))
 );
 
 #endif