Re: [PATCH] tracing: Add task_prctl_unknown tracepoint

[Marco Elver]

On Tue, 5 Nov 2024 at 17:31, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Tue,  5 Nov 2024 14:34:05 +0100
> Marco Elver <elver@xxxxxxxxxx> wrote:
>
> > prctl() is a complex syscall which multiplexes its functionality based
> > on a large set of PR_* options. Currently we count 64 such options. The
> > return value of unknown options is -EINVAL, and doesn't distinguish from
> > known options that were passed invalid args that also return -EINVAL.
> >
> > To understand if programs are attempting to use prctl() options not yet
> > available on the running kernel, provide the task_prctl_unknown
> > tracepoint.
> >
> > Note, this tracepoint is in an unlikely cold path, and would therefore
> > be suitable for continuous monitoring (e.g. via perf_event_open).
> >
> > While the above is likely the simplest usecase, additionally this
> > tracepoint can help unlock some testing scenarios (where probing
> > sys_enter or sys_exit causes undesirable performance overheads):
> >
> >   a. unprivileged triggering of a test module: test modules may register a
> >      probe to be called back on task_prctl_unknown, and pick a very large
> >      unknown prctl() option upon which they perform a test function for an
> >      unprivileged user;
> >
> >   b. unprivileged triggering of an eBPF program function: similar
> >      as idea (a).
> >
> > Example trace_pipe output:
> >
> >   <...>-366     [004] .....   146.439400: task_prctl_unknown: pid=366 comm=a.out option=1234 arg2=101 arg3=102 arg4=103 arg5=104
>
>           ^^^                                                       ^^^
>
> >
> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> > ---
> >  include/trace/events/task.h | 43 +++++++++++++++++++++++++++++++++++++
> >  kernel/sys.c                |  3 +++
> >  2 files changed, 46 insertions(+)
> >
> > diff --git a/include/trace/events/task.h b/include/trace/events/task.h
> > index 47b527464d1a..ab711e581094 100644
> > --- a/include/trace/events/task.h
> > +++ b/include/trace/events/task.h
> > @@ -56,6 +56,49 @@ TRACE_EVENT(task_rename,
> >               __entry->newcomm, __entry->oom_score_adj)
> >  );
> >
> > +/**
> > + * task_prctl_unknown - called on unknown prctl() option
> > + * @task:    pointer to the current task
> > + * @option:  option passed
> > + * @arg2:    arg2 passed
> > + * @arg3:    arg3 passed
> > + * @arg4:    arg4 passed
> > + * @arg5:    arg5 passed
> > + *
> > + * Called on an unknown prctl() option.
> > + */
> > +TRACE_EVENT(task_prctl_unknown,
> > +
> > +     TP_PROTO(struct task_struct *task, int option, unsigned long arg2, unsigned long arg3,
> > +              unsigned long arg4, unsigned long arg5),
> > +
> > +     TP_ARGS(task, option, arg2, arg3, arg4, arg5),
> > +
> > +     TP_STRUCT__entry(
> > +             __field(        pid_t,          pid             )
>
> Why record the pid that is already recorded by the event header?

To keep in style with the other "task" tracepoints above. I can
certainly do without - it does seem unnecessary.

To cleanup, do we want to remove "pid=" from the other tracepoints in
this file as well (in another patch). Or does this potentially break
existing users?

> > +             __string(       comm,           task->comm      )
>
> I'm also surprised that the comm didn't show in the trace_pipe.

Any config options or tweaks needed to get it to show more reliably?

> I've
> updated the code so that it should usually find it. But saving it here may
> not be a big deal.

Thanks,
-- Marco