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From:   Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Date:   Tue, 25 Dec 2018 15:11:05 +0800
Message-ID: <CAJhGHyAHKm0q4FtHBgbbfaB3g4oix76vJWgjqLYoaHvKCD3gvA@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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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 ) 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/$pid/uuid */
>   /* don't need to keep any fd after save */
> }
>
> safe_kill(pid, uuid, sig) {
>     fd = open(/proc/$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/$pid/stat */
>   /* don't need to keep any fd after save or keep it for 1 ticket at most */
> }
>
> safe_kill(pid, starttime, sig) {
>     fd = open(/proc/$pid/stat); /* also hold the pid until close() if
> open() successes */
>     if (open successes and read starttime from fd and if it equals to starttime)
>         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.
>

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.