ash,hush: fix $RANDOM in children being repeated

function                                             old     new   delta
next_random                                           46      68     +22
forkshell                                            248     263     +15
expand_vars_to_list                                 2118    2131     +13
run_pipe                                            1775    1782      +7
popstring                                            134     140      +6
builtin_umask                                        123     121      -2
ash_main                                            1356    1336     -20
get_local_var_value                                  125     104     -21
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 5/3 up/down: 63/-43)             Total: 20 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash.c

@@ -4725,10 +4725,12 @@ forkshell(struct job *jp, union node *n, int mode)
 			freejob(jp);
 		ash_msg_and_raise_error("can't fork");
 	}
-	if (pid == 0)
+	if (pid == 0) {
+		CLEAR_RANDOM_T(&random_gen); /* or else $RANDOM repeats in child */
 		forkchild(jp, n, mode);
-	else
+	} else {
 		forkparent(jp, n, mode, pid);
+	}
 	return pid;
 }
 
@@ -10079,12 +10081,6 @@ setcmd(int argc UNUSED_PARAM, char **argv UNUSED_PARAM)
 static void FAST_FUNC
 change_random(const char *value)
 {
-	/* Galois LFSR parameter */
-	/* Taps at 32 31 29 1: */
-	enum { MASK = 0x8000000b };
-	/* Another example - taps at 32 31 30 10: */
-	/* MASK = 0x00400007 */
-
 	uint32_t t;
 
 	if (value == NULL) {
@@ -13268,11 +13264,6 @@ int ash_main(int argc UNUSED_PARAM, char **argv)
 #endif
 	rootpid = getpid();
 
-#if ENABLE_ASH_RANDOM_SUPPORT
-	/* Can use monotonic_ns() for better randomness but for now it is
-	 * not used anywhere else in busybox... so avoid bloat */
-	INIT_RANDOM_T(&random_gen, rootpid, monotonic_us());
-#endif
 	init();
 	setstackmark(&smark);
 	procargs(argv);

shell/hush.c

@@ -88,6 +88,8 @@
 #include "match.h"
 #if ENABLE_HUSH_RANDOM_SUPPORT
 # include "random.h"
+#else
+# define CLEAR_RANDOM_T(rnd) ((void)0)
 #endif
 #ifndef PIPE_BUF
 # define PIPE_BUF 4096  /* amount of buffering in a pipe */
@@ -1319,8 +1321,6 @@ static const char *get_local_var_value(const char *name)
 	// bash compat: UID? EUID?
 #if ENABLE_HUSH_RANDOM_SUPPORT
 	if (strcmp(name, "RANDOM") == 0) {
-		if (G.random_gen.galois_LFSR == 0)
-			INIT_RANDOM_T(&G.random_gen, G.root_pid, monotonic_us());
 		return utoa(next_random(&G.random_gen));
 	}
 #endif
@@ -4000,6 +4000,7 @@ static NOINLINE int run_pipe(struct pipe *pi)
 		if (!command->pid) { /* child */
 #if ENABLE_HUSH_JOB
 			disable_restore_tty_pgrp_on_exit();
+			CLEAR_RANDOM_T(&G.random_gen); /* or else $RANDOM repeats in child */
 
 			/* Every child adds itself to new process group
 			 * with pgid == pid_of_first_child_in_pipe */
@@ -5207,6 +5208,7 @@ static FILE *generate_stream_from_string(const char *s)
 			+ (1 << SIGTTIN)
 			+ (1 << SIGTTOU)
 			, SIG_IGN);
+		CLEAR_RANDOM_T(&G.random_gen); /* or else $RANDOM repeats in child */
 		close(channel[0]); /* NB: close _first_, then move fd! */
 		xmove_fd(channel[1], 1);
 		/* Prevent it from trying to handle ctrl-z etc */

shell/math.c

@@ -258,7 +258,7 @@ static int
 arith_lookup_val(v_n_t *t, a_e_h_t *math_hooks)
 {
 	if (t->var) {
-		const char * p = lookupvar(t->var);
+		const char *p = lookupvar(t->var);
 
 		if (p) {
 			int errcode;

shell/random.c

@@ -20,6 +20,13 @@ next_random(random_t *rnd)
 
 	uint32_t t;
 
+	if (UNINITED_RANDOM_T(rnd)) {
+		/* Can use monotonic_ns() for better randomness but for now
+		 * it is not used anywhere else in busybox... so avoid bloat
+		 */
+		INIT_RANDOM_T(rnd, getpid(), monotonic_us());
+	}
+
 	/* LCG has period of 2^32 and alternating lowest bit */
 	rnd->LCG = 1664525 * rnd->LCG + 1013904223;
 	/* Galois LFSR has period of 2^32-1 = 3 * 5 * 17 * 257 * 65537 */

shell/random.h

@@ -13,7 +13,13 @@ typedef struct random_t {
 	uint32_t LCG;        /* LCG (fast but weak) */
 } random_t;
 
+#define UNINITED_RANDOM_T(rnd) \
+	((rnd)->galois_LFSR == 0)
+
 #define INIT_RANDOM_T(rnd, nonzero, v) \
 	((rnd)->galois_LFSR = (nonzero), (rnd)->LCG = (v))
 
+#define CLEAR_RANDOM_T(rnd) \
+	((rnd)->galois_LFSR = 0)
+
 uint32_t next_random(random_t *rnd) FAST_FUNC;