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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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

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