linux-kernel - Re: [PATCH] tracing: Add new

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Date: Tue, 9 Apr 2024 16:45:47 +0200
From: Marco Elver <elver@...gle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Eric Biederman <ebiederm@...ssion.com>, Kees Cook <keescook@...omium.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, linux-mm@...ck.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH] tracing: Add new_exec tracepoint

On Tue, 9 Apr 2024 at 16:31, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Mon,  8 Apr 2024 11:01:54 +0200
> Marco Elver <elver@...gle.com> wrote:
>
> > Add "new_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 "new_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 ("new_exec" to "sched_process_exec").
> >
> > Example of tracing output ("new_exec" and "sched_process_exec"):
>
> How common is this? And can't you just do the same with adding a kprobe?

Our main use case would be to use this in BPF programs to become
exec-aware, where using the sched_process_exec hook is too late. This
is particularly important where the BPF program must stop inspecting
the user space's VM when the task does exec to become a new process.

kprobe (or BPF's fentry) is brittle here, because begin_new_exec()'s
permission check can still return an error which returns to the
original task without crashing. Only at the point of no return are we
guaranteed that the exec either succeeds, or the task is terminated on
failure.

I don't know if "common" is the right question here, because it's a
chicken-egg problem: no tracepoint, we give up; we have the
tracepoint, it unlocks a range of new use cases (that require robust
solution to make BPF programs exec-aware, and a tracepoint is the only
option IMHO).

Thanks,
-- Marco