Re: [PATCH v2 1/2] tracing: Add task_prctl_unknown tracepoint

[Mathieu Desnoyers]

On 2024-11-07 10:46, Marco Elver wrote:
> On Thu, 7 Nov 2024 at 16:45, Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> 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.

I suspect you base the overhead analysis on the x86-64 implementation
of sys_enter/exit tracepoint and especially the overhead caused by
the SYSCALL_WORK_SYSCALL_TRACEPOINT thread flag, am I correct ?

If that is causing a too large overhead, we should investigate if
those can be improved instead of adding tracepoints in the
implementation of system calls.

Thanks,

Mathieu


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com