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Date:   Thu, 23 Jun 2022 15:29:54 -0500
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Subject: Re: [PATCH V4 00/20] The Runtime Verification (RV) interface
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To:     Song Liu <song@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Wim Van Sebroeck <wim@linux-watchdog.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Jonathan Corbet <corbet@lwn.net>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Marco Elver <elver@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Gabriele Paoloni <gpaoloni@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Clark Williams <williams@redhat.com>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
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References: <cover.1655368610.git.bristot@kernel.org>
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 <CAPhsuW67decxVH4n4YitkW40OmUqq2LRX7Ry4jqdjrgXSAgknA@mail.gmail.com>
From:   Daniel Bristot de Oliveira <bristot@kernel.org>
In-Reply-To: <CAPhsuW67decxVH4n4YitkW40OmUqq2LRX7Ry4jqdjrgXSAgknA@mail.gmail.com>
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On 6/23/22 12:52, Song Liu wrote:
> On Thu, Jun 23, 2022 at 9:42 AM Daniel Bristot de Oliveira
> <bristot@kernel.org> wrote:
>>
>> On 6/22/22 09:24, Song Liu wrote:
>>> This is interesting work!
>>>
>>> I applied the series on top of commit 78ca55889a549a9a194c6ec666836329b774ab6d
>>> in upstream. Then, I got some compile/link error for CONFIG_RV_MON_WIP and
>>> CONFIG_RV_MON_SAFE_WTD. I was able to compile the kernel with these two
>>> configs disabled.
>>
>> I rebased the code and... it compiled. Maybe it was missing some
>> config options that I forgot to set as "depends on" in the Kconfig.
>>
>> Can you check if it was the same problem automatically reported?
>>
>> Any further information here would help. I will revisit this.
> 
> Here are the error messages I got: https://pastebin.com/zJxMA6RK , and
> attached is the config file I used.
> 
>>
>> However, I hit the some issue with monitors/wwnr/enabled :
>>>
>>>     [root@eth50-1 ~]# cd /sys/kernel/debug/tracing/rv/
>>>     [root@eth50-1 rv]# cat available_monitors
>>>     wwnr
>>>     [root@eth50-1 rv]# echo wwnr > enabled_monitors
>>>     [root@eth50-1 rv]# cd monitors/
>>>     [root@eth50-1 monitors]# cd wwnr/
>>>     [root@eth50-1 wwnr]# ls
>>>     desc  enable  reactors
>>>     [root@eth50-1 wwnr]# cat enable
>>>     1
>>>     [root@eth50-1 wwnr]# echo 0 > enable   <<< hangs
>>>
>>> The last echo command hangs forever on a qemu vm. I haven't figured out why
>>> this happens though.
>>
>> I could reproduce it. It is an error in the return code of monitor_enable_write_data(),
>> I fixed it locally (return retval ? retval : count; // needs more test), and
>> will add it to the next version. Thanks!
>>
>>> I also have a more general question: can we do RV with BPF and simplify the
>>> work? AFAICT, the idea of RV is to maintain a state machine based on events.
>>> If something unexpected happens, call the reactor.
>>>
>>> IIUC, BPF has most of these building blocks ready for use. With BPF, we
>>> can ship many RV monitors without much kernel changes.
>>
>> I am aware of bpftrace and bpf + libbpf, and I have a PoC tool doing most of the
>> work I do in C/kernel in C/bpf.
>>
>> From the cover letter:
>>
>> "Things kept for a second moment (after this patchset):
>> [...]
>>         - dot2bpf"
>>
>> The point is that there are use-cases in which the users need the code in
>> C. One of those is the work being done in the Linux Foundation Elisa group.
>> There will be more formalism, like timed automata... which will require
>> infra-structure that is easily accessible in C... including synchronization,
>> and reactors that are available only in C on "per use-cases" basis - for
>> example on embedded devices.
> 
> Where can I find more information about the constraints of these use cases?

Check the LF elisa workgroup.

> I am asking because there are multiple ways to load a BPF program to the
> system. If the constraint is that we cannot have bpftrace or bcc in the system,
> maybe it is ok to run a standalone binary (written in C, compiled on a different
> system).

as I said... *I am aware of that*. I do like BPF! I was already convinced I will having
things in BPF :-)

dot2bpf does stand alone application, C + libbpf (and I did it this way to
have the most of flexibility), it works (for the things that are possible in BPF).
It shares most of the work in C/kernel, I will add it in the second patch series.

Or maybe we can load BPF programs in a kernel module, or compile
> the BPF programs into the kernel? (Yes, we can do it now, check
> kernel/bpf/preload). If any of these works, we can benefit from the good
> properties of BPF.

RV will take all these benefits, it is in the todo list as I said in this thread.
But the in kernel version also has its facilities.

For example, we can update the RV models without
> rebooting the system; and we can reuse various BPF maps, so we don't
> need to add union rv_task_monitor to task_struct.
> 
> Of course, we are out of luck if these systems cannot enable CONFIG_BPF
> at all. But I guess this is not common for modern embedded systems?

I understand your motivations, and I agree with the benefits of BPF, but I also
see benefits of having it in kernel as well.

So, RV will go with both, they are not mutually exclusive.

Thanks!
-- Daniel
> Thanks,
> Song