Re: For review: seccomp_user_notif(2) manual page [v2] - Michael Kerrisk (man-pages)

From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
To: Tycho Andersen <tycho@tycho.pizza>
Cc: Sargun Dhillon <sargun@sargun.me>,
	Christian Brauner <christian@brauner.io>,
	Kees Cook <keescook@chromium.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Giuseppe Scrivano <gscrivan@redhat.com>,
	Song Liu <songliubraving@fb.com>,
	Robert Sesek <rsesek@google.com>,
	Containers <containers@lists.linux-foundation.org>,
	linux-man <linux-man@vger.kernel.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Aleksa Sarai <cyphar@cyphar.com>, Jann Horn <jannh@google.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Will Drewry <wad@chromium.org>, bpf <bpf@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>
Subject: Re: For review: seccomp_user_notif(2) manual page [v2]
Date: Mon, 26 Oct 2020 15:30:29 +0100	[thread overview]
Message-ID: <CAKgNAkgbvuEJ0rkLrZGgCf0OTC8YH2vxemNic8SsDxjh=Z22uw@mail.gmail.com> (raw)
In-Reply-To: <20201026135418.GN1884107@cisco>

Hi Tycho,

Thanks for getting back to me.

On Mon, 26 Oct 2020 at 14:54, Tycho Andersen <tycho@tycho.pizza> wrote:
>
> On Mon, Oct 26, 2020 at 10:55:04AM +0100, Michael Kerrisk (man-pages) wrote:
> > Hi all (and especially Tycho and Sargun),
> >
> > Following review comments on the first draft (thanks to Jann, Kees,
> > Christian and Tycho), I've made a lot of changes to this page.
> > I've also added a few FIXMEs relating to outstanding API issues.
> > I'd like a second pass review of the page before I release it.
> > But also, this mail serves as a way of noting the outstanding API
> > issues.
> >
> > Tycho: I still have an outstanding question for you at [2].
> > [2] https://lore.kernel.org/linux-man/8f20d586-9609-ef83-c85a-272e37e684d8@gmail.com/
>
> I don't have that thread in my inbox any more, but I can reply here:
> no, I don't know any users of this info, but I also don't anticipate
> knowing how people will all use this feature :)

Yes, but my questions were:

[[
[1] 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?

[2] 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?
]]

So, do I understand right in [1]? (The implication from your reply is
yes, but I want to be sure...)

For [2], my question was not about users, but *use cases*. The
question I asked myself is: why does the feature exist? Hence my
question [2] reworded: "when you designed this, did you have in mind
scenarios here the process doing the NOTIFY_SEND might need to test
for POLLOUT because 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/