Re: [PATCH v2] signal: add procfd_signal() syscall

From: Arnd Bergmann <arnd@arndb.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: christian@brauner.io, Andy Lutomirski <luto@kernel.org>,
	Florian Weimer <fweimer@redhat.com>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	Linux Kernel Mailing List <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>,
	cyphar@cyphar.com, Al Viro <viro@zeniv.linux.org.uk>,
	Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Daniel Colascione <dancol@google.com>,
	Tim Murray <timmurray@google.com>,
	linux-man@vger.kernel.org, Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v2] signal: add procfd_signal() syscall
Date: Thu, 29 Nov 2018 22:02:13 +0100	[thread overview]
Message-ID: <CAK8P3a2i9DFaS9-3A9V_pKwFfwVR0bDqgLosmivge3Cx2VyiuQ@mail.gmail.com> (raw)
In-Reply-To: <6E21165F-2C76-4877-ABD9-0C86D55FD6AA@amacapital.net>

On Thu, Nov 29, 2018 at 9:14 PM Andy Lutomirski <luto@amacapital.net> wrote:
> > On Nov 29, 2018, at 11:55 AM, Christian Brauner <christian@brauner.io> wrote:
> >> On Thu, Nov 29, 2018 at 11:22:58AM -0800, Andy Lutomirski wrote:
> >>> On Thu, Nov 29, 2018 at 11:17 AM Christian Brauner <christian@brauner.io> wrote:
> >>>> On November 30, 2018 5:54:18 AM GMT+13:00, Andy Lutomirski <luto@amacapital.net> wrote:
> >>
> >> The #1 fix would add a copy_siginfo_from_user64() or similar.
> >
> > Thanks very much! That all helped a bunch already! I'll try to go the
> > copy_siginfo_from_user64() way first and see if I can make this work. If
> > we do this I would however only want to use it for the new syscall first
> > and not change all other signal syscalls over to it too. I'd rather keep
> > this patchset focussed and small and do such conversions caused by the
> > new approach later. Does that sound reasonable?
>
> Absolutely. I don’t think we can change old syscalls — the ABI is set in stone.
> But for new syscalls, I think the always-64-bit behavior makes sense.

It looks like we already have a 'struct signalfd_siginfo' that is defined in a
sane architecture-independent way, so I'd suggest we use that.

We may then also want to make sure that any system call that takes a
siginfo has a replacement that takes a signalfd_siginfo, and that this
replacement can be used to implement the old version purely in
user space.

Is the current procfd_signal() proposal (under whichever name) sufficient
to correctly implement both sys_rt_sigqueueinfo() and sys_rt_tgsigqueueinfo()?
Can we implement sys_rt_sigtimedwait() based on signalfd()?
If yes, that would leave waitid(), which already needs a replacement
for y2038, and that should then also return a signalfd_siginfo.
My current preference for waitid() would be to do a version that
closely resembles the current interface, but takes a signalfd_siginfo
and a __kernel_timespec based rusage replacement (possibly
two of them to let us map wait6), but does not operate on procfd or
take a signal mask. That would require yet another syscall, but I
don't think I can do that before we want to have the set of y2038
safe syscalls.

       Arnd