linux-kernel - Re: [PATCH v7 2/2] selftests: add tests for pidfd_send

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Date:   Tue, 8 Jan 2019 11:25:54 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     Christian Brauner <christian@...uner.io>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, luto@...nel.org, arnd@...db.de,
        keescook@...omium.org, akpm@...ux-foundation.org, jannh@...gle.com,
        oleg@...hat.com, cyphar@...har.com, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, dancol@...gle.com,
        timmurray@...gle.com, fweimer@...hat.com, tglx@...utronix.de,
        x86@...nel.org, ebiederm@...ssion.com
Subject: Re: [PATCH v7 2/2] selftests: add tests for pidfd_send_signal()

On Tue, Jan 08, 2019 at 07:24:46PM +0100, Christian Brauner wrote:
> On Tue, Jan 08, 2019 at 11:20:23AM -0700, Tycho Andersen wrote:
> > On Tue, Jan 08, 2019 at 12:17:42PM -0600, Serge E. Hallyn wrote:
> > > On Tue, Jan 08, 2019 at 10:58:43AM -0700, Tycho Andersen wrote:
> > > > On Tue, Jan 08, 2019 at 11:54:15AM -0600, Serge E. Hallyn wrote:
> > > > > On Tue, Jan 08, 2019 at 10:53:06AM -0700, Tycho Andersen wrote:
> > > > > > On Wed, Jan 02, 2019 at 05:16:54PM +0100, Christian Brauner wrote:
> > > > > > > +			/*
> > > > > > > +			 * Stop the child so we can inspect whether we have
> > > > > > > +			 * recycled pid PID_RECYCLE.
> > > > > > > +			 */
> > > > > > > +			close(pipe_fds[0]);
> > > > > > > +			ret = kill(recycled_pid, SIGSTOP);
> > > > > > > +			close(pipe_fds[1]);
> > > > > > > +			if (ret) {
> > > > > > > +				(void)wait_for_pid(recycled_pid);
> > > > > > > +				_exit(PIDFD_ERROR);
> > > > > > > +			}
> > > > > > 
> > > > > > Sorry for being late to the party, but I wonder if this whole thing
> > > > > > couldn't be simplified with /proc/sys/kenrel/ns_last_pid?
> > > > > 
> > > > > no, bc it's not namespaced :)
> > > > 
> > > > Huh? It looks like it is...
> > > > 
> > > > static int pid_ns_ctl_handler(struct ctl_table *table, int write,
> > > >                 void __user *buffer, size_t *lenp, loff_t *ppos)
> > > > {
> > > >         struct pid_namespace *pid_ns = task_active_pid_ns(current);
> > > >         struct ctl_table tmp = *table;
> > > >         int ret, next;
> > > > 
> > > >         if (write && !ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN))
> > > >                 return -EPERM;
> > > > 
> > > >         ...
> > > 
> > > Oh - hah, but that's ns_last_pid.  You'd want pid_max.  And that one
> > > is not namespaced.
> > 
> > Perhaps I'm misunderstanding, but isn't the point of all this code to
> > get the same pid again? So can't we just fork(), kill(), then set
> > ns_last_pid to pid-1, and fork() again to re-use?
> 
> Maybe. It's just a selftest that works reliably as it is so unless
> there's a technical issue with the patch I'm not going to do another
> version just because of that unless people feel super strongly about
> this.
> Another advantage is that the code we have right now works even when
> CONFIG_CHECKPOINT_RESTORE is not selected.

No, it's fine as is. Just a lot less code if we do it the other way.

Cheers,

Tycho