tracing: Add sched_prepare_exec tracepoint

Add "sched_prepare_exec" tracepoint, which is run right after the point
of no return but before the current task assumes its new exec identity.

Unlike the tracepoint "sched_process_exec", the "sched_prepare_exec"
tracepoint runs before flushing the old exec, i.e. while the task still
has the original state (such as original MM), but when the new exec
either succeeds or crashes (but never returns to the original exec).

Being able to trace this event can be helpful in a number of use cases:

* allowing tracing eBPF programs access to the original MM on exec,
before current->mm is replaced;
* counting exec in the original task (via perf event);
* profiling flush time ("sched_prepare_exec" to "sched_process_exec").

Example of tracing output:

$ cat /sys/kernel/debug/tracing/trace_pipe
<...>-379 [003] ..... 179.626921: sched_prepare_exec: interp=/usr/bin/sshd filename=/usr/bin/sshd pid=379 comm=sshd
<...>-381 [002] ..... 180.048580: sched_prepare_exec: interp=/bin/bash filename=/bin/bash pid=381 comm=sshd
<...>-385 [001] ..... 180.068277: sched_prepare_exec: interp=/usr/bin/tty filename=/usr/bin/tty pid=385 comm=bash
<...>-389 [006] ..... 192.020147: sched_prepare_exec: interp=/usr/bin/dmesg filename=/usr/bin/dmesg pid=389 comm=bash

Signed-off-by: Marco Elver <[e-mail]>
Acked-by: Steven Rostedt (Google) <[e-mail]>
Reviewed-by: Masami Hiramatsu (Google) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>

fs/exec.c

···
 		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_sched_prepare_exec(current, bprm);
+
+	/*
 	 * Ensure all future errors are fatal.
 	 */
 	bprm->point_of_no_return = true;

include/trace/events/sched.h

···
 		  __entry->pid, __entry->old_pid)
 );
 
+/**
+ * sched_prepare_exec - called before setting up new exec
+ * @task:	pointer to the current task
+ * @bprm:	pointer to linux_binprm used for 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(sched_prepare_exec,
+
+	TP_PROTO(struct task_struct *task, struct linux_binprm *bprm),
+
+	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("interp=%s filename=%s pid=%d comm=%s",
+		  __get_str(interp), __get_str(filename),
+		  __entry->pid, __get_str(comm))
+);
 
 #ifdef CONFIG_SCHEDSTATS
 #define DEFINE_EVENT_SCHEDSTAT DEFINE_EVENT