Re: [PATCH v7 2/2] selftests: add tests for pidfd_send_signal()

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:

quoted

On Tue, Jan 08, 2019 at 10:58:43AM -0700, Tycho Andersen wrote:

quoted

On Tue, Jan 08, 2019 at 11:54:15AM -0600, Serge E. Hallyn wrote:

quoted

On Tue, Jan 08, 2019 at 10:53:06AM -0700, Tycho Andersen wrote:

quoted

On Wed, Jan 02, 2019 at 05:16:54PM +0100, Christian Brauner wrote:

quoted

+			/*
+			 * 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?

Oh yeah that would work :)

I was stuck on the idea of just limiting the range of pids.