libbb: shrink monotonic_XXX functions, introduce monotonic_ns

(unused for now)

function                                             old     new   delta
get_mono                                               -      31     +31
sv_main                                             1228    1234      +6
expand                                              1693    1697      +4
get_address                                          178     181      +3
utoa_to_buf                                          108     110      +2
builtin_exit                                          46      48      +2
qrealloc                                              36      33      -3
qgravechar                                           109     106      -3
ash_main                                            1383    1380      -3
grep_file                                            850     846      -4
popstring                                            140     134      -6
monotonic_us                                          85      60     -25
monotonic_sec                                         41      16     -25
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 5/7 up/down: 48/-69)            Total: -21 bytes

1 files changed, 7 insertions, 5 deletions

diff --git a/shell/ash.c b/shell/ash.c
index 9c3216243..85064d5af 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -9615,7 +9615,6 @@ setcmd(int argc ATTRIBUTE_UNUSED, char **argv ATTRIBUTE_UNUSED)
 }
 
 #if ENABLE_ASH_RANDOM_SUPPORT
-/* Roughly copied from bash.. */
 static void
 change_random(const char *value)
 {
@@ -9636,10 +9635,11 @@ change_random(const char *value)
                 if (random_galois_LFSR < 0) /* if we just shifted 1 out of msb... */
                         t ^= MASK;
                 random_galois_LFSR = t;
-                /* Both are weak, xoring them gives better randomness
+                /* Both are weak, combining them gives better randomness
                  * and ~2^64 period. & 0x7fff is probably bash compat
-                 * for $RANDOM range. */
-                t = (t ^ random_LCG) & 0x7fff;
+                 * for $RANDOM range. Combining with subtraction is
+                 * just for fun. + and ^ would work equally well. */
+                t = (t - random_LCG) & 0x7fff;
                 /* set without recursion */
                 setvar(vrandom.text, utoa(t), VNOFUNC);
                 vrandom.flags &= ~VNOFUNC;
@@ -13432,7 +13432,9 @@ int ash_main(int argc ATTRIBUTE_UNUSED, char **argv)
         rootpid = getpid();
 
 #if ENABLE_ASH_RANDOM_SUPPORT
-        random_galois_LFSR = random_LCG = rootpid + time(NULL);
+        /* Can use monotonic_ns() for better randomness but for now it is
+         * not used anywhere else in busybox... so avoid bloat */
+        random_galois_LFSR = random_LCG = rootpid + monotonic_us();
 #endif
         init();
         setstackmark(&smark);