linux-kernel - Re: [PATCH] signal: don't silently convert SI

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Date:   Wed, 17 Apr 2019 16:13:09 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Jann Horn <jannh@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] signal: don't silently convert SI_USER signals to
 non-current pidfd

On Wed, Apr 17, 2019 at 03:50:03PM +0200, Oleg Nesterov wrote:
> On 04/17, Christian Brauner wrote:
> >
> > On Wed, Apr 17, 2019 at 03:16:03PM +0200, Oleg Nesterov wrote:
> > > On 04/17, Christian Brauner wrote:
> > > >
> > > > On Mon, Apr 08, 2019 at 03:13:16PM +0200, Oleg Nesterov wrote:
> > > > >
> > > > > but perhaps it should always fail, even if task_pid(current) == pid.
> > > > >
> > > > > sys_rt_sigqueueinfo() allows to send any siginfo to yourself, but this is only needed
> > > > > for checkpoint/restart.
> > > >
> > > > Yes, that's why this was added. I would leave it in exactly because of
> > > > checkpoint/restart.
> > >
> > > I don't understand...
> > >
> > > c/r doesn't need this "feature" in pidfd_send_signal(), so it can be removed.
> > > But,
> >
> > Just out of curiosity: in what sense? They don't need it since they have
> > other ways of doing this
> 
> Yes. The restarting process needs to "restore" the pending signals, including the
> signals with si_code >= 0. It does this using tgsigqueueinfo() and that is why we
> allow this if the signal sent to itself.
> 
> So criu simply doesn't need pidfd_send_signal() to do this. And at the same time,
> 
> > or they *can't* use it for some other reason
> 
> Yes again. pidfd_send_signal() does kill_pid_info(), so it can't be used to restore
> the "per-thread" task->pending signals.

In the future pidfd_send_signal() will gain the flag
PIDFD_SIGNAL_THREAD. Jann and I already have a patch for this but we're
holding off until the need arises.

Christian