Date:   Tue, 4 Dec 2018 14:26:06 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     ebiederm@...ssion.com, linux-kernel@...r.kernel.org,
        serge@...lyn.com, jannh@...gle.com, luto@...nel.org,
        akpm@...ux-foundation.org, oleg@...hat.com, cyphar@...har.com,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, dancol@...gle.com, timmurray@...gle.com,
        linux-man@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] signal: add procfd_signal() syscall

On Tue, Dec 04, 2018 at 01:55:10PM +0100, Florian Weimer wrote:
> * Christian Brauner:
> 
> > On Mon, Dec 03, 2018 at 05:57:51PM +0100, Florian Weimer wrote:
> >> * Christian Brauner:
> >> 
> >> > Ok, I finally have access to source code again. Scratch what I said above!
> >> > I looked at the code and tested it. If the process has exited but not
> >> > yet waited upon aka is a zombie procfd_send_signal() will return 0. This
> >> > is identical to kill(2) behavior. It should've been sort-of obvious
> >> > since when a process is in zombie state /proc/<pid> will still be around
> >> > which means that struct pid must still be around.
> >> 
> >> Should we make this state more accessible, by providing a different
> >> error code?
> >
> > No, I don't think we want that. Imho, It's not really helpful. Signals
> > are still delivered to zombies. If zombie state were to always mean that
> > no-one is going to wait on this thread anymore then it would make sense
> > to me. But given that zombie can also mean that someone put a
> > sleep(1000) right before their wait() call in the parent it seems odd to
> > report back that it is a zombie.
> 
> It allows for error checking that the recipient of a signal is still
> running.  It's obviously not reliable, but I think it could be helpful
> in the context of closely cooperating processes.
> 
> >> Will the system call ever return ESRCH, given that you have a handle for
> >> the process?
> >
> > Yes, whenever you signal a process that has already been waited upon:
> > - get procfd handle referring to <proc>
> > - <proc> exits and is waited upon
> > - procfd_send_signal(procfd, ...) returns -1 with errno == ESRCH
> 
> I see, thanks.
> 
> >> Do you want to land all this in one kernel release?  I wonder how
> >> applications are supposed to discover kernel support if functionality is
> >> split across several kernel releases.  If you get EINVAL or EBADF, it
> >> may not be obvious what is going on.
> >
> > Sigh, I get that but I really don't want to have to land this in one big
> > chunk. I want this syscall to go in in a as soon as we can to fulfill
> > the most basic need: having a way that guarantees us that we signal the
> > process that we intended to signal.
> >
> > The thread case is easy to implement on top of it. But I suspect we will
> > quibble about the exact semantics for a long time. Even now we have been
> > on multiple - justified - detrous. That's all pefectly fine and
> > expected. But if we have the basic functionality in we have time to do
> > all of that. We might even land it in the same kernel release still. I
> > really don't want to come of as tea-party-kernel-conservative here but I
> > have time-and-time again seen that making something fancy and cover ever
> > interesting feature in one patchset takes a very very long time.
> >
> > If you care about userspace being able to detect that case I can return
> > EOPNOTSUPP when a tid descriptor is passed.
> 
> I suppose that's fine.  Or alternatively, when thread group support is
> added, introduce a flag that applications have to use to enable it, so
> that they can probe for support by checking support for the flag.
> 
> I wouldn't be opposed to a new system call like this either:
> 
>   int procfd_open (pid_t thread_group, pid_t thread_id, unsigned flags);
> 
> But I think this is frowned upon on the kernel side.

If this is purely about getting a procfd then I think this isn't really
necessary since you can get it from /proc/<pid> and
/proc/<pid>/task/<tid> so a syscall just for that is likely overkill.
However, I started to pick up the CLONE_FD patchset but ideally I would
like it to be way simpler to what was proposed back in the day (which is
not a critique, I just don't feel comfortable with bringing massive
patches to the table that I can barely judge wrt to their correctness.
:)). I have toyed around with this a little and I'm tempted to simply
have the syscall always return an fd for the process and not require a
separate flag for this. But I need to work through the details and this
is really far out into the (kernel) future.

> 
> >> What happens if you use the new interface with an O_PATH descriptor?
> >
> > You get EINVAL. When an O_PATH file descriptor is created the kernel
> > will set file->f_op = &empty_fops at which point the check I added 
> >         if (!proc_is_tgid_procfd(f.file))
> >                 goto err;
> > will fail. Imho this is correct behavior since technically signaling a
> > struct pid is the equivalent of writing to a file and hence doesn't
> > purely operate on the file descriptor level.
> 
> Yes, that's quite reasonable.  Thanks.
> 
> Florian

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