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

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Message-ID: <20190108182023.GA28330@cisco>
Date:   Tue, 8 Jan 2019 11:20:23 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     Christian Brauner <christian@...uner.io>,
        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 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?

Tycho