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Date:   Tue, 25 Dec 2018 23:07:57 +1100
From:   Aleksa Sarai <cyphar@cyphar.com>
To:     Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Cc:     Christian Brauner <christian@brauner.io>,
        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>,
        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>
Subject: Re: [PATCH v2] signal: add procfd_signal() syscall
Message-ID: <20181225120757.37w37lmyhxo6kekl@yavin>
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On 2018-12-25, Lai Jiangshan <jiangshanlai+lkml@gmail.com> wrote:
> On Tue, Dec 25, 2018 at 1:32 PM Lai Jiangshan
> <jiangshanlai+lkml@gmail.com> wrote:
> >
> > 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 ) ca=
n 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/$pid/stat */
> >   /* don't need to keep any fd after save or keep it for 1 ticket at mo=
st */
> > }
> >
> > safe_kill(pid, starttime, sig) {
> >     fd =3D open(/proc/$pid/stat); /* also hold the pid until close() if
> > open() successes */
> >     if (open successes and read starttime from fd and if it equals to s=
tarttime)
> >         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.
> >
>=20
> Oh, Sorry, I was wrong, the pid isn't reserved even when
> the fd is kept in the user space. And I'm sorry that I had
> replied to an "old" email thread.

Don't worry, this was a common point of confusion during this (and
sister) threads. All the fd ensures is that access through that fd will
give you -ESRCH if the process is gone (and if the PID is reused it will
still give you -ESRCH).


--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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