linux-kernel - Re: [PATCH v2 1/2] tracing: Add task_prctl

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Message-ID: <CANpmjNPWLOfXBMYV0_Eon6NgKPyDorTxwS4b67ZKz7hyz5i13A@mail.gmail.com>
Date: Thu, 7 Nov 2024 16:46:47 +0100
From: Marco Elver <elver@...gle.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Steven Rostedt <rostedt@...dmis.org>, Kees Cook <keescook@...omium.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>, 
	kasan-dev@...glegroups.com
Subject: Re: [PATCH v2 1/2] tracing: Add task_prctl_unknown tracepoint

On Thu, 7 Nov 2024 at 16:45, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> On 2024-11-07 07:25, Marco Elver 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:
> >
> >    test-484     [000] .....   631.748104: task_prctl_unknown: comm=test option=1234 arg2=101 arg3=102 arg4=103 arg5=104
> >
>
> My concern is that we start adding tons of special-case
> tracepoints to the implementation of system calls which
> are redundant with the sys_enter/exit tracepoints.
>
> Why favor this approach rather than hooking on sys_enter/exit ?

It's __extremely__ expensive when deployed at scale. See note in
commit description above.