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 <elver@google.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20240411102158.1272267-1-elver@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/fs/exec.c b/fs/exec.c
index cf1df7f16e55..b3c40fbb325f 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1268,6 +1268,14 @@ int begin_new_exec(struct linux_binprm * bprm)
 		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;