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

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Date:   Thu, 6 Dec 2018 18:31:24 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        luto@...nel.org, arnd@...db.de, serge@...lyn.com, jannh@...gle.com,
        akpm@...ux-foundation.org, oleg@...hat.com, cyphar@...har.com,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        dancol@...gle.com, timmurray@...gle.com, linux-man@...r.kernel.org,
        keescook@...omium.org, fweimer@...hat.com, tglx@...utronix.de,
        x86@...nel.org
Subject: Re: [PATCH v4] signal: add taskfd_send_signal() syscall

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).

-serge