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From:   Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Date:   Tue, 25 Dec 2018 13:32:51 +0800
Message-ID: <CAJhGHyC=Zc=iG+iL+Ny8GsMd-5xypxdk4Et_1ZViMao5aXreCw@mail.gmail.com>
Subject: Re: [PATCH v2] signal: add procfd_signal() syscall
To:     Christian Brauner <christian@brauner.io>
Cc:     Florian Weimer <fweimer@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Daniel Colascione <dancol@google.com>,
        Tim Murray <timmurray@google.com>,
        linux-man <linux-man@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>
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Is it possible to avoid adding any syscall?

Since holding /proc/pid/reg_file can also hold the pid.
With this guarantee, /proc/pid/uuid (universally unique identifier ) can be
introduced to identify tasks, the kernel generates
a uuid for every task when created.

save_pid_uuid_pair_for_later_kill(int pid) {
  /* save via /proc/$pid/uuid */
  /* don't need to keep any fd after save */
}

safe_kill(pid, uuid, sig) {
    fd =3D open(/proc/$pid/uuid); /* also hold the pid until close() if
open() successes */
    if (open successes and read uuid from fd and if it equals to uuid)
        kill(pid, sig)
    close(fd)
}

All things needed to be done is to implement /proc/pid/uuid. And if pid can=
't
be recycled within 1 ticket, or the user can ensure it. The user can use
starttime(in /proc/pid/stat) instead.

save_pid_starttime_pair_for_later_kill(int pid) {
  /* save via /proc/$pid/stat */
  /* don't need to keep any fd after save or keep it for 1 ticket at most *=
/
}

safe_kill(pid, starttime, sig) {
    fd =3D open(/proc/$pid/stat); /* also hold the pid until close() if
open() successes */
    if (open successes and read starttime from fd and if it equals to start=
time)
        kill(pid, sig)
    close(fd)
}

In this case, zero LOC is added in the kernel. All of it depends on
the guarantee that holding /proc/pid/reg_file also holds the pid,
one of which I haven't checked carefully either.

On Fri, Dec 7, 2018 at 3:05 AM Christian Brauner <christian@brauner.io> wro=
te:
>
> On December 7, 2018 7:56:44 AM GMT+13:00, Florian Weimer <fweimer@redhat.=
com> wrote:
> >* Andy Lutomirski:
> >
> >>> I suppose that's fine.  Or alternatively, when thread group support
> >is
> >>> added, introduce a flag that applications have to use to enable it,
> >so
> >>> that they can probe for support by checking support for the flag.
> >>>
> >>> I wouldn't be opposed to a new system call like this either:
> >>>
> >>>  int procfd_open (pid_t thread_group, pid_t thread_id, unsigned
> >flags);
> >>>
> >>> But I think this is frowned upon on the kernel side.
> >>
> >> I have no problem with it, except that I think it shouldn=E2=80=99t re=
turn an
> >> fd that can be used for proc filesystem access.
> >
> >Oh no, my intention was that it would just be used with  *_send_signal
> >and related functions.
>
> Let's postpone that discussion a little.
> I think we don't need a syscall to base this off of pids.
> As I said I rather send my revived version of CLONE_NEWFD that would serv=
e the same task.
> The same way we could also just add a new open() flag that blocks fs acce=
ss completely.
> I just pitched that idea to Serge a few days back: O_NOCHDIR or similar.
> That could even be part of Aleksa's path resolution patchset.
>
> >
> >Thanks,
> >Florian
>