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

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Date:   Thu, 6 Dec 2018 16:34:54 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     Christian Brauner <christian@...uner.io>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, 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 4:31 PM Serge E. Hallyn <serge@...lyn.com> wrote:
>
> On Fri, Dec 07, 2018 at 12:17:45AM +0100, Christian Brauner wrote:
> > On Thu, Dec 06, 2018 at 11:39:48PM +0100, Christian Brauner wrote:
> > > On Thu, Dec 06, 2018 at 03:46:53PM -0600, Eric W. Biederman wrote:
> > > > Christian Brauner <christian@...uner.io> writes:
> > > >
> > > > >> Your intention is to add the thread case to support pthreads once the
> > > > >> process case is sorted out.  So this is something that needs to be made
> > > > >> clear.  Did I miss how you plan to handle threads?
> > > > >
> > > > > Yeah, maybe you missed it in the commit message [2] which is based on a
> > > > > discussion with Andy [3] and Arnd [4]:
> > > >
> > > > Looking at your references I haven't missed it.  You are not deciding
> > > > anything as of yet to keep it simple.  Except you are returning
> > > > EOPNOTSUPP.  You are very much intending to do something.
> > >
> > > That was clear all along and was pointed at every occassion in the
> > > threads. I even went through the hazzle to give you all of the
> > > references when there's lore.kernel.org.
> > >
> > > >
> > > > Decide.  Do you use the flags parameter or is the width of the
> > > > target depending on the flags.
> >
> > Ok, let's try to be constructive. I understand the general concern for
> > the future so let's put a contract into the commit message stating that
> > the width of the target aka *what is signaled* will be based on a flag
> > parameter if we ever extend it:
> >
> > taskfd_send_signal(fd, SIGSTOP, NULL, TASKFD_PGID);
> > taskfd_send_signal(fd, SIGSTOP, NULL, TASKFD_TID);
> >
> > with the current default being
> >
> > taskfd_send_signal(fd, SIGSTOP, NULL, TASKFD_PID);
> >
> > This seems to me the cleanest solution as we only use one type of file
> > descriptor. Can everyone be on board with this? If so I'm going to send
> > out a new version of the patch.
> >
> > Christian
>
> I'm on board with this, but I think you need to also clarify what exactly
> the fd stands for.  I think that (a) userspace should not have to care
> about the struct pid implementation, and so (b) the procfd should stand
> for all the pids.  So when taskfd_send_signal(fd, SIGSTOP, NULL, TASKFD_PGID)
> becomes implemented, then open(/proc/5) will pin all three pids, as will
> open(/proc/5/task/6).

This change doesn't "pin" any PID, and it makes no sense to make a
process FD stand for all its threads. What does that even mean?