Re: For review: seccomp_user_notif(2) manual page - Michael Kerrisk (man-pages)

From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
To: Tycho Andersen <tycho@tycho.pizza>
Cc: linux-man <linux-man@vger.kernel.org>,
	Song Liu <songliubraving@fb.com>,
	wad@chromium.org, Kees Cook <keescook@chromium.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Jann Horn <jannh@google.com>, Robert Sesek <rsesek@google.com>,
	Linux Containers <containers@lists.linux-foundation.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	mtk.manpages@gmail.com, Giuseppe Scrivano <gscrivan@redhat.com>,
	bpf@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>,
	Christian Brauner <christian@brauner.io>
Subject: Re: For review: seccomp_user_notif(2) manual page
Date: Thu, 1 Oct 2020 09:45:19 +0200	[thread overview]
Message-ID: <8f20d586-9609-ef83-c85a-272e37e684d8@gmail.com> (raw)
In-Reply-To: <20200930230327.GA1260245@cisco>

On 10/1/20 1:03 AM, Tycho Andersen wrote:
> On Wed, Sep 30, 2020 at 10:34:51PM +0200, Michael Kerrisk (man-pages) wrote:
>> Hi Tycho,
>>
>> Thanks for taking time to look at the page!
>>
>> On 9/30/20 5:03 PM, Tycho Andersen wrote:
>>> On Wed, Sep 30, 2020 at 01:07:38PM +0200, Michael Kerrisk (man-pages) wrote:

[...]

>>>>        ┌─────────────────────────────────────────────────────┐
>>>>        │FIXME                                                │
>>>>        ├─────────────────────────────────────────────────────┤
>>>>        │Interestingly, after the event  had  been  received, │
>>>>        │the  file descriptor indicates as writable (verified │
>>>>        │from the source code and by experiment). How is this │
>>>>        │useful?                                              │
>>>
>>> You're saying it should just do EPOLLOUT and not EPOLLWRNORM? Seems
>>> reasonable.
>>
>> No, I'm saying something more fundamental: why is the FD indicating as
>> writable? Can you write something to it? If yes, what? If not, then
>> why do these APIs want to say that the FD is writable?
> 
> You can't via read(2) or write(2), but conceptually NOTIFY_RECV and
> NOTIFY_SEND are reading and writing events from the fd. I don't know
> that much about the poll interface though -- is it possible to
> indicate "here's a pseudo-read event"? It didn't look like it, so I
> just (ab-)used POLLIN and POLLOUT, but probably that's wrong.

I think the POLLIN thing is fine.

So, I think maybe I now understand what you intended with setting
POLLOUT: the notification has been received ("read") and now the
FD can be used to NOTIFY_SEND ("write") a response. Right?

If that's correct, I don't have a problem with it. I just wonder:
is it useful? IOW: are there situations where the process doing the
NOTIFY_SEND might want to test for POLLOUT because the it doesn't
know whether a NOTIFY_RECV has occurred? 

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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