From 608889ebe255266b621bd65a999b88dc9ad36dc0 Mon Sep 17 00:00:00 2001
From: cheloha <cheloha@openbsd.org>
Date: Sat, 23 Jul 2022 22:10:58 +0000
Subject: [PATCH] kernel: remove global "randompid" toggle

Apparently, we used to created several kthreads before the kernel
random number generator was up and running.  A toggle, "randompid",
was needed to tell allocpid() whether it made sense to attempt to
allocate random PIDs.

However, these days we get e.g. arc4random(9) into a working state
before any kthreads are spawned, so the toggle is no longer needed.

Thread: https://marc.info/?l=openbsd-tech&m=165541052614453&w=2

Very nice historical context provided by miod@.

probably ok miod@ deraadt@
---
 sys/kern/init_main.c |  4 +---
 sys/kern/kern_fork.c | 25 +++++++++++++------------
 sys/sys/proc.h       |  3 +--
 3 files changed, 15 insertions(+), 17 deletions(-)

diff --git a/sys/kern/init_main.c b/sys/kern/init_main.c
index bfc324c3ae5..a7413dc59ab 100644
--- a/sys/kern/init_main.c
+++ b/sys/kern/init_main.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: init_main.c,v 1.315 2022/02/22 01:15:01 guenther Exp $	*/
+/*	$OpenBSD: init_main.c,v 1.316 2022/07/23 22:10:58 cheloha Exp $	*/
 /*	$NetBSD: init_main.c,v 1.84.4.1 1996/06/02 09:08:06 mrg Exp $	*/
 
 /*
@@ -431,8 +431,6 @@ main(void *framep)
 		initprocess = initproc->p_p;
 	}
 
-	randompid = 1;
-
 	/*
 	 * Create any kernel threads whose creation was deferred because
 	 * initprocess had not yet been created.
diff --git a/sys/kern/kern_fork.c b/sys/kern/kern_fork.c
index 8b4b4206058..e5bdcc7e9bc 100644
--- a/sys/kern/kern_fork.c
+++ b/sys/kern/kern_fork.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_fork.c,v 1.240 2022/05/13 15:32:00 claudio Exp $	*/
+/*	$OpenBSD: kern_fork.c,v 1.241 2022/07/23 22:10:58 cheloha Exp $	*/
 /*	$NetBSD: kern_fork.c,v 1.29 1996/02/09 18:59:34 christos Exp $	*/
 
 /*
@@ -67,7 +67,6 @@
 
 int	nprocesses = 1;		/* process 0 */
 int	nthreads = 1;		/* proc 0 */
-int	randompid;		/* when set to 1, pid's go random */
 struct	forkstat forkstat;
 
 void fork_return(void *);
@@ -638,20 +637,22 @@ ispidtaken(pid_t pid)
 pid_t
 allocpid(void)
 {
-	static pid_t lastpid;
+	static int first = 1;
 	pid_t pid;
 
-	if (!randompid) {
-		/* only used early on for system processes */
-		pid = ++lastpid;
-	} else {
-		/* Find an unused pid satisfying lastpid < pid <= PID_MAX */
-		do {
-			pid = arc4random_uniform(PID_MAX - lastpid) + 1 +
-			    lastpid;
-		} while (ispidtaken(pid));
+	/* The first PID allocated is always 1. */
+	if (first) {
+		first = 0;
+		return 1;
 	}
 
+	/*
+	 * All subsequent PIDs are chosen randomly.  We need to
+	 * find an unused PID in the range [2, PID_MAX].
+	 */
+	do {
+		pid = 2 + arc4random_uniform(PID_MAX - 1);
+	} while (ispidtaken(pid));
 	return pid;
 }
 
diff --git a/sys/sys/proc.h b/sys/sys/proc.h
index ec2fd803476..01fa10baf70 100644
--- a/sys/sys/proc.h
+++ b/sys/sys/proc.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: proc.h,v 1.333 2022/07/05 15:06:16 visa Exp $	*/
+/*	$OpenBSD: proc.h,v 1.334 2022/07/23 22:10:59 cheloha Exp $	*/
 /*	$NetBSD: proc.h,v 1.44 1996/04/22 01:23:21 christos Exp $	*/
 
 /*-
@@ -498,7 +498,6 @@ extern struct proc proc0;		/* Process slot for swapper. */
 extern struct process process0;		/* Process slot for kernel threads. */
 extern int nprocesses, maxprocess;	/* Cur and max number of processes. */
 extern int nthreads, maxthread;		/* Cur and max number of threads. */
-extern int randompid;			/* fork() should create random pid's */
 
 LIST_HEAD(proclist, proc);
 LIST_HEAD(processlist, process);
-- 
2.20.1