Introduce OpenBSD-like Random PIDs. Controlled by a sysctl knob

(kern.randompid), which is currently defaulted off.  Use ARC4 (RC4) for our
random number generation, which will not get me executed for violating
crypto laws; a Good Thing(tm).

Reviewed and Approved by: bde, imp

sys/conf/files.i386

@@ -373,6 +373,7 @@ isa/sio.c			optional	sio
 isa/syscons_isa.c		optional	sc
 isa/vga_isa.c			optional	vga
 kern/subr_diskmbr.c		standard
+libkern/arc4random.c		standard
 libkern/bcd.c			standard
 libkern/divdi3.c		standard
 libkern/index.c			standard

sys/i386/conf/files.i386

@@ -373,6 +373,7 @@ isa/sio.c			optional	sio
 isa/syscons_isa.c		optional	sc
 isa/vga_isa.c			optional	vga
 kern/subr_diskmbr.c		standard
+libkern/arc4random.c		standard
 libkern/bcd.c			standard
 libkern/divdi3.c		standard
 libkern/index.c			standard

sys/kern/kern_fork.c

@@ -142,6 +142,9 @@ rfork(p, uap)
 int	nprocs = 1;		/* process 0 */
 static int nextpid = 0;
 
+static int randompid = 0;
+SYSCTL_INT(_kern, OID_AUTO, randompid, CTLFLAG_RW, &randompid, 0, "");
+
 int
 fork1(p1, flags, procp)
 	struct proc *p1;
@@ -262,8 +265,8 @@ fork1(p1, flags, procp)
 	 * restart somewhat above 0, as the low-numbered procs
 	 * tend to include daemons that don't exit.
 	 */
-	if (nextpid >= PID_MAX) {
-		nextpid = 100;
+	if (nextpid >= PID_MAX || randompid) {
+		nextpid = (randompid) ? arc4random() % PID_MAX : 100;
 		pidchecked = 0;
 	}
 	if (nextpid >= pidchecked) {

sys/libkern/arc4random.c

@@ -0,0 +1,89 @@
+/*-
+ * THE BEER-WARE LICENSE
+ *
+ * <dan@FreeBSD.ORG> wrote this file.  As long as you retain this notice you
+ * can do whatever you want with this stuff.  If we meet some day, and you
+ * think this stuff is worth it, you can buy me a beer in return.
+ *
+ * Dan Moschuk
+ *
+ * $FreeBSD$
+ */
+
+#include <sys/libkern.h>
+#include <sys/time.h>
+
+static u_int8_t arc4_i, arc4_j;
+static int arc4_initialized = 0;
+static u_int8_t arc4_sbox[256];
+
+static __inline void
+arc4_swap(u_int8_t *a, u_int8_t *b)
+{
+	u_int8_t c;
+
+	c = *a;
+	*a = *b;
+	*b = c;
+}	
+
+/*
+ * Initialize our S-box to its beginning defaults.
+ */
+static void
+arc4_init(void)
+{
+	struct timespec ts;
+	u_int8_t key[256];
+	int n;
+
+	for (n = 0; n < 256; n++)
+		arc4_sbox[n] = (u_int8_t) n;
+
+	nanotime(&ts);
+	srandom(ts.tv_sec ^ ts.tv_nsec);
+	for (n = 0; n < 256; n++)
+		key[n] = random() % 256;
+
+	arc4_i = arc4_j = 0;
+	for (n = 0; n < 256; n++)
+	{
+		arc4_j = arc4_j + arc4_sbox[n] + key[n];
+		arc4_swap(&arc4_sbox[n], &arc4_sbox[arc4_j]);
+	}
+	arc4_initialized = 1;
+}
+
+/*
+ * Generate a random byte.
+ */
+static u_int8_t
+arc4_randbyte(void)
+{
+	u_int8_t arc4_t;
+
+	arc4_i = (arc4_i + 1) % 256;
+	arc4_j = (arc4_j + arc4_sbox[arc4_i]) % 256;
+
+	arc4_swap(&arc4_sbox[arc4_i], &arc4_sbox[arc4_j]);
+
+	arc4_t = (arc4_sbox[arc4_i] + arc4_sbox[arc4_j]) % 256;
+	return arc4_sbox[arc4_t];
+}
+
+u_int32_t
+arc4random(void)
+{
+	u_int32_t ret;
+
+	/* Initialize array if needed. */
+	if (!arc4_initialized)
+		arc4_init();
+
+	ret = arc4_randbyte();
+	ret |= arc4_randbyte() << 8;
+	ret |= arc4_randbyte() << 16;
+	ret |= arc4_randbyte() << 24;
+
+	return ret;
+}

sys/sys/libkern.h

@@ -61,6 +61,7 @@ static __inline u_long ulmax(u_long a, u_long b) { return (a > b ? a : b); }
 static __inline u_long ulmin(u_long a, u_long b) { return (a < b ? a : b); }
 
 /* Prototypes for non-quad routines. */
+u_int32_t arc4random __P((void));
 int	 bcmp __P((const void *, const void *, size_t));
 #ifndef HAVE_INLINE_FFS
 int	 ffs __P((int));