linux-kernel - Re: [PATCH v4] signal: add taskfd_send

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Message-ID: <CAKOZuet4ayDu-54u2tO9tRDCgxa9JQ4avAiYS8xEtReN4MMhAA@mail.gmail.com>
Date:   Thu, 6 Dec 2018 08:17:18 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Christian Brauner <christian@...uner.io>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Serge E. Hallyn" <serge@...lyn.com>, Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        linux-man <linux-man@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Florian Weimer <fweimer@...hat.com>, tglx@...utronix.de,
        x86@...nel.org
Subject: Re: [PATCH v4] signal: add taskfd_send_signal() syscall

On Thu, Dec 6, 2018 at 7:02 AM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
> Christian Brauner <christian@...uner.io> writes:
>
> > The kill() syscall operates on process identifiers (pid). After a process
> > has exited its pid can be reused by another process. If a caller sends a
> > signal to a reused pid it will end up signaling the wrong process. This
> > issue has often surfaced and there has been a push [1] to address this
> > problem.
> >
> > This patch uses file descriptors (fd) from proc/<pid> as stable handles on
> > struct pid. Even if a pid is recycled the handle will not change. The fd
> > can be used to send signals to the process it refers to.
> > Thus, the new syscall taskfd_send_signal() is introduced to solve this
> > problem. Instead of pids it operates on process fds (taskfd).
>
> I am not yet thrilled with the taskfd naming.

Both the old and new names were fine. Do you want to suggest a name at
this point? You can't just say "I don't like this. Guess again"
forever.

> Is there any plan to support sesssions and process groups?

Such a thing could be added with flags in the future. Why complicate this patch?

> I am concerned about using kill_pid_info.  It does this:
>
>
>         rcu_read_lock();
>         p = pid_task(pid, PIDTYPE_PID);
>         if (p)
>                 error = group_send_sig_info(sig, info, p, PIDTYPE_TGID);
>         rcu_read_unlock();
>
> That pid_task(PIDTYPE_PID) is fine for existing callers that need bug
> compatibility.  For new interfaces I would strongly prefer pid_task(PIDTYPE_TGID).

What is the bug that PIDTYPE_PID preserves?